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INTRODUCTION

rrOMOLOGY:

rs

)F INSECTS:

h l^iytni iflai

isQ. F.L.S.

LONGMAN, REE8, OKME, BROWN, AND GREEN, PATERNOSTER ROW.

y

V

rRINTED BT RICHARD TAYLOR, SHOK UkNEy LONDON.

/

« > « * k

CONTENTS OF VOL. IV.

Letter. XXXVII.

XXXVIII.

XXXIX.

XL.

XLI.

XLIL

XLIIL

XLIV.

XLV.

XL VI.

XLVIL

XLVJIL

XLIX.

Page. Interna] Anatomy and Physiology of Insects. Sensation 1 — 3S

Internal Anatomy and Physiology of Insects continued. Respiration . . S4r— 79

Internal Anatomy and Physiology of Insects continued. Ciradation, . . . 80 — 95

Internal Anatomy and Physiology of Insects continued Digestion .... 96 — 120

Internal Anatomy and Physiology of Insects continued. Secretion .... 121 — 145

Internal Anatomy and Physiology of Insects continued. Reproduction. . 146 — 166

Internal Anatomy and Physiology of Insects concluded. Motion 167 — 196

Diseases of Insects 197 — 232

Senses of Insects ^S—256

Orismology, or Explanation of Terms 257 — 354

System of Insects 355 — *18

History of Entomology 419—473

Geographical Distribution of Insects ; their Stations and Haunts ; Seasons ; Times of Action and Repose 474—514

\ O, A ^^ ^? r)

iv CONTENTS.

Letter. Page.

L. On Entomological Instruments ; and the best Methods of collecting, breeding, and preserving Insects. . 515 — 54^

^LL InveAtigation of Insects 547—560

Appendix 561—572

Authors quoted 573—589

Explanation of the Plates 590—602

Indexes 603—634

ERRATA.

Page. Line, Tjbxt.

58 12, for Semblis read ISialis. 78 antepenult, fir Case! read puttis. 97 21, put a commu tjfter longitudinaHy, and dde thai after

transversely. 107 2, o^er crop insert and.

289 _ 23, t^ier Menelaus insert Plate XIV. Fio. I. a. 590 3, for d. The Bronchiaa connected with the Trachea, read

d. The Nerves &c.

Page. Note.

10 20 37 44 97 149

176

Notes.

dele 1.

for creadb. for ^ read A", dele n'\

after Fio. add 3.

after 109 add Plats XXX. Fia. 12. a. dele Plate XXX. Fic. 12. a. for a read a.

AN

INTRODUCTION

TO

ENTOMOLOGY.

LETTER XXXVII.

INTERNAL ANATOMY AND PHYSIOLOGY

OF INSECTS.

SEN1SATION..

Having given you this fidl account of the external parts of insects, and their most remarkable variations ; I must next direct your attention to such discoveries as have been made with regard to their Internal Anatomy and Physiology t a subject still more fertile, if possible^ than the former in wonderful 'manifestations of the POWER, WISDOM and GOODNESS of the Creator.

The vital system of these little creatures, in all its great features, is perfectly analogous to that of the verr tebrate animals. Sensation and perception are by the means of nerves and a common sensorium / the respiration of air is evident, being received and expelled by a par*

VOL. IV. B

£ INTERNAL ANATOMY OF INSECTS.

ticular apparatus ; nutrition is effected through a stomach and intestines ; the analogue of the blood prepared by these organs pervades every part of the body, and from it are secreted various peculiar substances; generation takes place, and an intercourse between the sexes, by means of appropriate organs ; and lastly, motion is the result of the action otmnscles* Some of these functions are, hbwever, exercised in a mode apparently so dissimi- lar from what obtains in the higher animals, that upon a first view we are inclined to pronounce them the effect of processes altogether peculiar. Thus, though insects re- spire a/r, they do not receive it by the mouthy but through little orifices in the sides of the body ; and instead of lungSf they are fiimished with a system of air vessels, ramified ad infinitiwij and penetrating to every part and organ of their fraone ; and though they are nourished by a fluid prepared from the food received into the stomachy this fluid, unlike the blood of vertebrate animals, is white^ and the mode in which it is distributed to the different parts of the system, except in the case of the true Arach- nidoj in which a circulation in the ordinary way has been detected, is altogether obscure.

In order that you may more clearly understand the variations that occur in insects, and in what respects they differ amongst themselves, and from the higher ani- mals, in the vital functions and their organs, I shall con- sider them as to their organs of sensation^ respiratiouy qir- culationy nutrition^ generation^ secretion^ and muscular mctionm

Organs of Sensation. — The nervous system of animals is <Mie of the most wonderful and mysterious works of

INTERNAL ANATOMY OF INSECTS. 3

the Creator. Its pulpy substance is the visible medium by which the governing principle* transmits its com- mands to the various organs of the body, and they move instantaneously — ^yet this appears to be but the conduct- or of some higher principle, which can be more imme- diately acted upon by the mind and by the wilL This principle, however, whatever it be, whether we call it the nervous ^id, or the nervous jHnoer^ has not been de- tected, and is known only by its effects. The system of which we are Speaking may therefore be deemed the foundation and root of the animal, the centre from which emanate all its powers and functions*

Comparative anatomists have considered the nervous isystem of animals as formed upon three primary types, which may be called the molecular^ the ganglionic^ and cerebrO'^nal^. The Jirsl is where invisible nervous molecules are dispersed in a gelatinous body, the exist- ence of which has only been ascertained by the nervous irritability of such bodies, their fine sense of touch, their perceiving the movements of the waters in which they reside, and from their perfect sense of the degrees of light and heat^. Of this description are the infusory animals, the Polypi^ the star-fish and sea-urchins. The nervous molecules in these are conjectured to constitute so many ganglions, or centers of sensation and vitality^* The second^ the ganglionic, is where the nervous system

* To 'Hyf/MOf/»oy.

*> See Hooper's Medical DictUmary, under Nervous FluH and Mr. Sandwith's useful Introduction to Anatomy and Physiology, 83.

« K. Diet. d'Hist. Nat. xvi. 305-,

' Cuv. Anat. Comp. ii. 36^. Compare MacLeay Bor. Entomolog. 215—. ' « N met. d^Hist. Nat. ubi supr.

B 2

4f INTERNAL ANATOMY OF INSECTS.

consists of a series of ganglions connected by nervous threads or a medullary chord, placed, except the first ganglion, below the intestines, from which proceed nerves to the various parts of the body*. This prevails in the Classes /ns^^a, Crustacea, Arachnida, Mollusca, Annelida^ &c. In the third, the cerebro-spinal, the nervous tree may be siud to be double, or to consist of ttw systems— the first taking its origin in a brain formed of two hemi- spheres contained in the cavity of the head, from which posteriorly proceeds a spinal marrow, included in a dor- sal vertebral column. These send forth numerous nerves to the organs' of the senses and the muscles of the limbs. The second consists of two principal ventral chords, which by their ganglions, hut without any direct communica- tion^ anastomose with the spinal nerves and some of those of the brain, and run one on each side from the base of the skull to the extremity of the sacrum. This system consists of an assemblage of nervous filaments bearing numerous ganglions, from which nervous threads are distributed to the organs of nutrition and reproduc- tion*. Its chords are called the great sympathetic, the intercostal, or trisplanchnic nerves*^. While the first of these two systems is the messenger of the will, by means of the organs of the senses connects us with the external world, and is subject to have its agency interrupted by sleep or disease^ ; the latter is altogether independent of

■ N. Diet. (THiit. Nat. xvi. 306. ^

»> Jifid. 307. The great sympathetic nerves in Jishes are said to have no ganglions. Cuv. idn supr. 297.

c They arc called trisplanchnic hecause they render to the three cavities of the viscera : — ^viz. the thoracic, the abdominal and the pelvic, y. Diet. d'Hist. Nat. xxii. 524. 527-

^ In Hemiplegia, &c.

INTERNAL ANATOMY OF INSECTS. &

the will and of the intellect, is confined to the internal orga^ nic life, its agency continues uninterrupted during sleep, and is subject to no paralysis. While the former is the seat of the intellectual powers, the latter has no relation to them, but is the focus firom whence instincts exclu- sively emanate : fi*om it proceed spontaneous impulses and sympathies, and those passions and affections that excite the agent to acts in which the will and the judge- ment have no concern*.

. It is probable, though the above appear to exhibit the primary types of nervous systems, that -others ex- ist 0f an intermediate nature, with which future investi-' gators may render us better acquainted^ : but as our bu- siness is solely with that upon which insects in this re- spect have been modelled, without expatiatii]^ further in this interesting field, I shall therefore now confine myself to them*

We have before seen*^ that the nervous system -of in- sects belongs to the ganglionic type : but it requires a iQore fuU description, and this is the place for it. It ori- ginates in a small brain placed in the head, and consist* ing almost universally of two lobes, sometimes extremely distinct It is placed over or upon the oesophagus or gul*- let, ahd frcw its posterior part proceed^ a double ner- vous chord, which embracing that organ as a collar dips below the intestines, and proceeds towards the anus, form- ing knots or ganglions at intervals, in many cases cor-

• N. Diet. d^HuL Nat. xvi. 307.

^ Thus in the MoUuiC4S there must be a great difference in this respect, since in some of these the brain or cerebral ganglion has ^)«en cut off with the head, and another reproduced. Ibid. xvL 306. Comp. V, 391, * Vol. III. p. :29.

Q INTERNAL ANATOMT OF INSECTS.

responding in number with the segments of the body, and sending forth nerves in pairs, the ramifications of which are distributed to every part of the frame. This may be considered more particularly with respect to its substance and colour ; its tunics ; and parts.

I. Substance and Colour. — The nervous apparatus of insects is stated by those who have examined it mostnar- rowly, though consisting of a cortical and medullary part, the latter more delicate and transparent than the former, to be less tender and less easy to separate than the hu- man brain K It has a degree of tenacity, and does not break without considerable tension; in general, it i? clammy and flabby, and under a microscope a number of minute grains are discoverable in it, and when left to dry upon glass, it appears to contain a good deal of oil, which does not dry with the rest**. That of thp gan- glions differs from the substance of the rest of the spinal chord, in being filled with very fine aerial vessels, which are not discoverable in the latter*^. With regard to co- lour^ Lyonnet states that the chords of the spinal mar- row in the larva of the great goat-moth are of a blueish gray, and have some transparence^ ; M alpighi and Swam- merdam observed that the cortical part of the ganglions of that of the silk-worm and the hive-bee had a reddish hue,

" Lyonnet Anatcm. 100. ^ Ibid. 101.

^ Ibid, 1 00. In man and the vertebrate animals, the medullary pulp is every where homogeneous ; under the microscope it appears to con- sist of a number of minute conglomerated globules. M« Vauquelin has analysed it, and found it to contain, of water 80 parts, of albumen in a state of demicoagulation 7*0; of phosphorus 1*50; of osraazone 11^; of a white and transparent oily matter 4*53; of a similar red do. 0*75; of a little sulphur and some salts 5*15. N,Dict. d*Hi9i, Nat. xxii. 531-^. ^ Anat. 99«

« I

INTERNAL ANATOMY OF INSECTS. 7

while the medullary part was white^ ; Cu^ier relates that the brain and the third gan^on in Htffwgymna di^ar^ with us a scarce moth, differed in colour irom all the resl^ being quite white, while the others were more <»r less tinted, and examined under a lens appeared vari&* gated by reddish sinuous markings, resembling blood- vessels as they axe seen in injected glands^

IL Tunics. — Thecoats that inclose thevarious brandies of the nervous systeiQ in insects seem analogous to those of vertebrate animals. The first thing that strikes the eye, when these parts in a recent subject are submitted to a microscope, is a tissue of very delicate vessels, which ramify beyond the reach of the assisted sight; these are merely air-vessels or bronchus derived originally from the trpcheic of the animal : but besides these is an exterior and ay interior tunic ; the first corresponding with the Asra mater of anatomists ; and the other, which is the jtnost delicate and incloses the cortical and medullary parts, with ihtpia mater ^.

JII. Parts. — The nervous system of insects consists of the brain s the spinal marrow and its ganglions / and the nerves.

L Brain^. Linne denied the (existence of a brain in insects, and most modern physiologists seem to be of the same opinion. A part however, analogous to this impor- tant organ — at least in its situation, and in its emission of nerves to the principal organs of the senses^ in which re- spect it certainly differs very materially from the upper

"" Malpigh. de Bombifc. 30. Swamm. JBibl. Nat. i. 224. a.

^ Anat. Comp. ii. •348.

f Lyonnet Anat, 100. t. iv. /. 6.. Sandwith Introd. 59—.

^ Plate XXI. Fig. 1. 7. 8. a.

8' INTERNA'L ANATOMY OF INSECTS.

cervical ganglion, which Dn Virey regards as its ana- logue *-^is certainly to be found in them ; and as Messrs. Cuvier and Lamarck distinguish this part by tibe name c/l'brainy we may continue to call it by that name with- out impropriety. The brain of insects, then, is distin- gidshed from the succeeding ganglions of the spinal chord by its sittiation in the head, the middle of the internal cavity of which it occupies, and by being the only gan- glion' cSove the oesophagus. It is usually small^ though in some cases larger than they are**. It consists of two lobes, more or less distinct and generally of a spherical form.' In Oryctes nasicomts and Pieris Brassiae the lobes are separated both before and behind*^ ; wliile in the iBTy&KsfJDytiscus marginalise but not in the imago, in which tfiere ate two large hemispheres -separated by a furrowj flie brdn is undivided^. Cuvier meiitions the larva of a Tenthredolj. in whichthispartisformed of j^r nearly equal' spherical bulbs * : in the Scorpion (to judge by the figiu'e of Treviranus*^) the two lobes represent an" equi- lateral triai^le, the exterior angle of which terminates ito Several lesser spherical bulbs ^ in Acrida viridissimay Nepa cinerea^ Clnbiona atrox^ and the common' Louse, the lobes are pe^-shaped^* ' ' " ■

' ii. 754^ spinal marram and its ganglionsK From the posterior part of the brain of insects, but in Carahus and DytiscUs L. from its sides below V issue two chords whieh

> N. Diet. d'Hist, Nat. xxii. 6^. ^ Ibid. v. 591.

c Cuv. Anat. Comp. ii. 318. Swamm. BiM. Kai. t. xxix./ 7. Hc- rold Afhmetterl. t ilf. I—IO. a. «» Cuv. Ibid. 322. 337.

e Ibid. 324. f Arachnid, t. If. ll m.m,

' Cuv. ubi supr. 343.346. Treviranus Arachnid, i. v. / 45. a. Plate XXI. Fig. 8. a. ^ Ibid. Fig. 1. bJf. * Cuv. iSi supr. 337.

IKTSBNAL ANATOMY OF INJSECTS. 9

diY^ging embrace the (Bsophagusi and dipping below it and the intestines, — a situation they maintain to the end of their course, — and in their further progress unitilig at intervals and dilating into several knots or gsmglions, compose their spinal marrow* This part is so named^ from a supposed analogy to the spinal marrow of verte- brate animals, which however admits o£ some degree of dcmbt; yet, since", it mixe& the iuncticms of that organ with those of the great sympathetic nerves, thedenomi- nation is not wholly improper, and^may be retmned. Though this ehoid is usually double when it first pi*o« ceeds from the bltdn, and surioundi^ the cesophdgus like acdllar, yet in some insects k may^be. cidled a sit^le chord. This is the case with that of the common louse, in which Swammerdam could perceiv^^ no opening for the transmission of th^ part just named* ; if he was not i^staken in this, the brain, a:» well^as the rest of the spi« nal marrow in that animd, would be below the intestines ; from the figures of Trevii^anus^ it i^ould seem that die spiders, at least Clubkma (drox^ are similarly circum- stanced^ ; in the cheese-maggot, which^ turns to a two- Winged fly {Tyraphaga putris K.)* the chord is also sin- gle, but it has a small orifice through' which the gullet passes^. At the union of the ch<»rd9 in other cases be- low that organ, a knot or ganglion is usually formed, and an alternate succession of intemodes and ganglions com- monly follows to the end. The intemodes ako may ge- nerally be stated to consist of a dmtble chord, though in many cases the two chords unite and become one, qr

* Plate XXI. Fig. 8. Swamm. BUd. Nat. i. 36. b.

* AruchnkL t v./. 45. « Swamm. M supr. I. xUii./. J.

10 INTERNAL ANATOMY OF INfiSCTS.

are distinguished mly by a Icoig^tudinal furrow, and evtfti where they are really distinct and separable^ in the body of the insect they lie close together*. In Oryctes nasu- carnis and Acrida viridissima &c. all the internodes con-* sist of a double chord ^ ; but in many other insects nume- rous variations in this respect occur. — Thus in the stag-- beetle the last intemode is single^ ; in the caterpillar of the cabbage butterfly {Pieri$ Brassica) the Jive Jlrst are double, and the six last single^ ; in that of the great goat^ moth {Cossus ligniperda) the three first only are double, but the others teiminate in a fork^ ; in the cock-roaches (Blatta) the Jour first, in Hydraphilm jnceus the three first, and in Elophilus tenax the two first only are double^ the rest being all single^. A singular variation takes place in Hypogymna disparj aU the internodes are single, except the second^ the chords of which at first are sepa* rate, and afterwards united^ ; and, to name no more^ in Cbdnona atrox there is only one internode^ which is sin- gle, with a longitudinal furrow N In some, as in the louse, the grub of Chyctes nasicomis, and the cheese-maggot, there are no internodes, the spinal marrow being formed of knots separated only by slight or deep constrictions '• I must next say something of the ganglionsK Lyon- net has observed that, in the caterpillar of the great goat-

* Swamm. M ntpr, 11^. a. ^ Cuv. Anat. Comp. ii, 337. 343—.

^ Ibid. 336. ^ Herold SchmeUerl. t, iL/. 1.

' Lyonnet Anat. 98.

^ Cxiy,'ubiiwpr.Z42. Gaede iV. ^c/. ^corf. CVr^. XL. ii. 3^. Cuv. JIM. 351. « Ibid. 348.

*> Treviranus Arachnid, t. v./. 45. » Plate XXL Fig. 7. 8. Swamm. BiM. Nat. t. xKiL/. 7. » Plate XXL Fig. L 7. 8. c.

INTERNAL ANATOMY OF lNaECT& 11

moth, these in ^ne respect diffi^ remarkably from the chords that connect them ; in the latter the air-vessels or bronchise only cover the outside of the tmiic, while in the former they enter the substance of the ganglion, which is quite filled with their delicateand numberless branches^. Every ganglion may be regarded in some degree as a cenr tre of vitality or little brain ^, and ia many cases, as wdl as the brain, they are formed of two lobes ^. I shall now consider them more particularly as to their station^ nunP" heTj and shape.

!• With regard to the first head, their station^ they are most commonly divided between the trunk and ab- domen ; but in some cases, as in Hydrophilus piceus and Acrida virtdissima, ihejirst ganglion is in the head^ ; in others, as in the louse, the water-scorpion, and the grub of the rhinoceros-beetle (Oryctes nasicomis\ they are confined to the trunks their fimctions in the abdomen be- ing supplied by numerous radiating nerves^; in others again, as in the scorpion, they are all abdominal. The ganglions vary also in their situation with respect to each other. Thus in some, as in the larva of the Chamaekon- fly {Stratyomis Chanueleon\ they are so near as to appear like a string of beads ^ ; in that of the ant-lion {Myrme^ lean) the two ganglions of the trunk are separated by an interval bom those of the abdomen, which are so conti-

• Lyoimet AmU. 100. ^ N. Diet. d*Hut. Nat. xxii. 5^^—.

^ Lyonnet M supr, U ix./. 1 — 4.

<» Cuv. Anai, Comp. ii. 339. 343. * Platb XXI. Fio. 7-

f Swamm. ubi supr. U xl. /. 5. Cuvier (ii. 33^.) accuses Swam- merdam of representing the spinal marrow in this grub as producing nerves only on one side; whereas he expressly states (iL 50. b.) that a considerable number spring on each side from the eleven ganglions^ but that to avoid confbsion he had omitted some.

lit IHTEaVAL ANATOMY Of IN8XCT&

guous as to resemble the rattle of the rattle-snake *• In odiers the intemodes are longer, and the ganglions occur at nearly equal intervals, as in the larva of the Ephe- . mera>^ ; but in the majority they, are unequal in length : thus in the scorpion the three first gai3glions are the most distant^ ; in the hive-bee the third and fourth^ ; and in the* spider the last ^

2« The ganglions also in difierent species, and often in ike same, insect in its different states, vary in their number. Thus in the grub of the rhinoQeros-beetle.lhi^ whole spmal marrpw appears Uke a single gangUo^ di- vided. only by transverse furrows ^ ; in the water scoipion there are two^ ; in the louse there are three^ ; in the rhi<« nooeros-beede there are jftmr^; fioe in the stag-beetle^ ; fffoen in the hive-bee and some Lepidoptera ^ ; eight in the grub of the stag^-beetle"^; nine in the great Hydrophilm^ ; ten in Dytiscus^ ; eleven in the grub of the great Hydro^ pkilus^ ; twelve in the grub of Dytiscus and the caterpil- lars of Lepidoptera^ ; thirteen in the larva of j^khna^ ; and tmenty-four in Scolopendra morsitans*. You must observe that, generally speaking, the number of ganglions

* Cttv. vM supr. 325. » Swamm. BiU. Nat, t. xv./. 6. c Trcviran. Arachnid, tl.f. 13, 1—4.

** Bwamm. ubi supr, t xxii./. 7.

* Tireviran. uM tupr. t. v./. 45. ' Plate XXI. Fig. 7. « Cuv. Afua. Comp. ii. 346. * Plate XXI. Fig. 8.

Cuv. vH supr. 337. ^ Ihid. 335- . » Ibid. 348.

«JWrf.320— . » /Aid 340— . «^/«if.338— .

* * P Gaede vbi supr.

fl Cuv. M tupr. 323—. 327—. Mr. Bauer (PAtf. Trans. 1824. t. ii, /. 1.) has figured only seven^ excluding the brain, in that* of the silk- wormy and Malpighi {De Bombyc, t. vi. /• 2.) ten, — S^mtnerdam {BQd. Nat. t xxviii./. 3.) however has twelve.

' Ibid. 326. • Ibid. 352.

INTERNAL ANATOMY OF INSECTS. 1]3

isl less in the imago than in the larva. With regard^ to the distribution of these knots to the different primary parts; of the body, the following table irill exhibit it^ as far as 1 am acquainted with it, at one ^dSew, I omit thofee in which the ganglions are only in one of these parts.

Head. Trubk. AbdSanain.

Acrida viridissima 1 ••••••••• S ••^'••••&« 6*

Hydrophilus piceus 1 •••• 6 2

Clubiona atrox ••• 0 • 2 •••• 1

Qryllotalpa vulgaris .... 0 2 *••••••••>?'*

Myrmeleon Larva ••«••• 0 2 ••;*k.*.« 8^

Elophilm tenax 0 S ••••••••• 2^

Apis domestica *•• 0 •••• 3 •*•••••••• 4

Ephemera Larva • 0 3 ••• 7

^^hna Larva .•••• 6 ••••••;•• 6 •••• 7

3. I am next to say a few words upon the sAflJ^^'of the ganglions. Most commonly it approaches to a spherical figure, but in many instances, as I said before, theyj as well as the brain, consist of two lobes : they are, however, seldom all precisely of the same shape. Iii the Dytisci^ and Carabif the last is marked with a transverse furrow, which seems to indicate the reunion of two^ ; in the stag- beetle, the first ganglion is oval or elliptical, the second hexagonal ; the third and fourth shaped like a crescent, and the last like an olive '^ ; in the caterpillar of the great goat-moth the first is oblong and constricted in the mid- dle, and the seven last are rhomboidal^; in the great Hydrophilus the second, and in the silk-worm all the gan-

• Cuv. vjfn supr. 343—. <» IHd. 345. « Ibid. 9^5—.

^ Ibid. 351. • Ibid. 839. ' Ibid. 335—.

' Lyonnet Anat. 190.

14 INTERNAL ANATOMY OF INSECTS.

glionst aire quadrangular*; in Hypogymna dispar the third is heart-shaped^ ; the great ganglion which forms the spmal marrow of the cheese-maggot is pear-shaped ^ ; that of the grub of the rhinoceros-beetle is fusiform^ ; and in the scorpion all the ganglions are lenticular ^ But the most remarkable in this respect are those of a spider {dubiona airox) i in this insect the brain sits upon a iHlobed ganglicm of the ordinary form, which is imme- diately followed without any intemode by another bi- lobM one, terminating on each side in four pear-shaped processes or fingers^ whidi give it a very singular ap- pearance^*

ilL The nerves^ of insects, as of other animals, are white filaments runnmg from the brain and spinal marrow to every part of the body which they are destined to ani- mate; and their numerous ramifications, when delineated, form no unpleasing picture \ In the caterpillar of Cos-^ sus ligniperda the accurate Lyonnet coxmXj&A forty-five pairs of them,' and t(X30 single ones, making in all ninety- two nerves; whereas in the human body anatomists count only seventy-eight^. From the brain issue several pairs, which go to the eyes^ antennce^ palpi, and other parts of the mouth ; sometimes those that render to the mandibles issue from the first ganglion, as in the larva o{ Dytiscus marginalis, the stag-beetle, &c.^; those bodi

■ Cuv. Anat Cmnp, ii 340. Malpigh. de Bombyc, L vi./. 2.

«» Cuv. Ibid. 348.' « Swamm. JBiM. Nat. t. xlviii./. 7.

^ Cuv. IM. 319. « N. met. d*HUt. Nat. xxx. 420.

f Treviran. Arachnid, t. v./. 45. m.

» Plate XXI. Fig. 1. ?. 8. d.

«» Lyonnet tdn supr. t x.f. 5. 6. * Ihid.l92.

^ Cuv. nbi supr. 323. 335.

INTERNAL ANATOMY OF INSECTS. 15

of Tuandibles and palpi in the great Hydrephtlus^ ; and in Blatta some which act also apon the antennce^.

The optic are usually the most conspicuous and re- markable of the nerves. In some insects with large eyes, afr many Neuropteroy Hymenoptera^ and DipterOy their size is considerable ; in the hive-bee th^ present the ap> pearance of a pair of kidney-shaped lobes, larger than the brain ^ i in the dragon-flies, whose brain consists of two very minute lobes, these nerves dilate into two large plates of a similar shape, which line all the iniieF-4sm^e erf the eyes ^ ; in the stag-beetle they are pear-shaped, and terminate in a bulb, from which issue an infinity of mi- nute nerves^ ; it is probable that this takes pla^e in all cases, and that a separate nerve renders to every separate lens in a compound eye^ ; the optic nerve in DytiscusKoA Carabus is pyramidal, with the base of the pyramid at the eye and the summit at the brain ^; in Ehphilus tenax it is very large, cylindrical, and of a diameter equal to the length of the last^mentioned part, upon the side of which it is supported; it terminates in a very laige bulb corresponding to the eye^ ; in Scolopendra morsitans the optic nerves divide into four branches long before they arrive at the eyes, and in this insect the nerves which render to the fuitennse are so thick as to appear portions of the brain, which they equal in diameter ^ Swammer- dam discovered in the grub of the rhinoceros-beetle and in the caterpillar of the silk-worm, a pair of nerves which

• Cuv, Anat. Comp, n. 339. ^ IM. 342.

* Swamm. Bibl, Nat, i, laaLf, 6. m.m.

^ Cuv. vbi supr, 350. ' IM. 335.

' Vol. III. p. 497. Lyonnet AtuU. 581.

« Cuv, uhi supr. 337. ** If^id. 351. ' Ihid. 362.

16 INTERNAL ANATOMY OF INSECTS.

be regarded as analogous to the recurrent nerves in the human subject, and therefore he distinguishes them by the $am& name * : they issue from the lower surface, of the brain, or that which rests on thecesojAagusj and at first go towards the mputhy but afterwards turn back, and uniting form a smaU ganglion; this produces a single nerve, which passing below the brain follows the eesofdia- gpis to the stomach, where it, swells into another gan^- gUon, from which issue some small nerves that lender to the stomach, and one more considerable which accom- panies the intestinal canal, producing at intervals lateral filaments whidx lose themselves m the. tunics of that tube ^. Lyonnet aflerwards discovered these nerv^ in the cater* {Hilar of the goat-moth % $nd Cuvier in oth^r insects^.

The other nerves which issue, from the brain eachibit no remarkable features. Those whidi originnte in the spinal marrow are mostly derived from the ganglions^ and are sometimes mterwoven with the muscles, as the woof with the warp in a piece of dpth^ ; those from the three or four first commonly rendering to the muscles of the leg&f wings, and other parts of the-^n^^, and those from the remainder to the abdomen, Afler their origin diey oflen divide and subdivide, and terminate in ntiroe-^ rous ramifications that connect every part of the body with the sensorium commune, A pair of nerves is the most usual number that proceeds from each i^de of a ganglion^ ; but this i^ by no means constant, since in

^ Cuvier {vbi supr, 319.) seems not to have been aware that Swam- merdam was the tet discoverer of these nerves^ since he attribntes their name to Lyonnet.

b Bi&l. Nat. i. 138* b. t. xxviii./. 2. a, b, c,f. 3. g.

c miiupr. 57S. ^ Ulfi supr. S20. 33&, &c.

* Cuv. M tupr. 349. ' Lyonnet Anat. t. ix. x.

INTERNAL ANATOMY OF INSECTS; 17

the louse, the hive-bee^ and several other insects, only a 5f ftgfZenerve thus proeeeds ^ ; and in the larva ofEphemeraSy while two pairs issue from the six Jirst ganglions^ oiily a single one is emitted by thejivelaist^. In the spinal mar- row of the rhinoceros<l>eetle, both larva and imago, the nerves consist of simple filaments which diverge like rays in all (fitections ^ : the same drcumstance distinguishes the dieese-maggot, only some of the nerves appear to branch at tlie end^ : in the louse, the last ganglion isends forth pos-* teriorly three purs of nerves which render to the abdo- men ^ Sometimes, though rarely, nerves originate in the ifttemodes of the spinal marrow. Cuvier indeed has asserted that in invertebrate animals aU the n^es spring from a»e ganglions, and never immediately from the spf nal marrow; but Swammerdam, in describing those of the silk-worm, mentions and figures four pairs as pro, ceeding from the four anterior intemodes, excluding the first ^ I and at the same time he gives it as his opinion^ that all the nerves in insects really originate firom the marrow itself, and not fi'om the ganglions, which he as- serts are of a different substance, and are inclosed in the

• Plate XXL Fig. 8. Swamm. BibL Nat. t, xxii./ 6.

»» Ibid, t. xv./. 6. "^ Plate XXI. Fig. 7.

<* Swamm. ubi supr. t. xliii./. 7* ^> ^*

« Plate XXI. Fig. 8.

f In Mr. Bauer's figure (Philos, Trans. 1824. t ii./. 1.) no less than eighteen pairs of nerves are represented as issuing fi'om the inter- nodes; but it should seem as if in the specimefi from which his figiu'e was taken, several ofthe ganglions, perhaps from some injury received in the diasection, had become obliterated, while their nerves remain- ed : yet still, even making allowance for these, many pairs will appear to take thdr origin from the spinal chord.

VOL. IV. C .

IS INTERNAL ANATOMY OF INSECTS.

marrow for the sake of giving it greater firmness*. In this opinion, however, he seems singular^. Those re- jnarkable nerves described by Lyonnet under the name of spinal bridle {bride epiniere) also take their origin, not from the gangUons, but from a bifurcation of the spinal marrow. Of these, in the caterpillar of the goat^moth there are ten, the first issuing from the bifurcation of the internode between the fourth and fifth ganglions, and the remainder from the succeeding ones. After approach- ing the succeeding ganglion, these nerves form a pair of branches that diverge nearly at right angles from the 1>ridle, and producing several lesser branches, lose them-, selves in the sides of the animal^. Besides the nerves above-mentioned, two generally issue from the poste- rior part of the last gangUon, diverging in opposite and oblique directions : some of these render to the parts of generation; and in the silk-worm, and probably other species, the innermost pair is perforated for the passage of the Vdsa deferential.

After duly considering this general outline of the ner- vous system of insects, the question will continually oc- cur to you,"— is then what you have called the brain the sensorium commune of these animals, in the same manner as it is in those with warm blood ? To this query a ne- gative must be returned. In the latter, the brain is the common centre to which, by means of the nerves and

• Comp. Cuv. AndL Comp. ii. 102—123.; with Swainin. Expl. of Plates xxxii. t xxviii./. 3. k,

•» Malpighi seems, however, to agree with him. De Bombi/c, t. vi. /.I. ^ Lyoimetvbi supr. 201. /. ix./. li 2. n. 1, 2. &Ci

** Swamm. M gupr, 1. 139. a. t. xxviii./. 3. s,s.

INTERNAL ANATOMY OF INSECTS. 19

spinal marrow, all the sensations of the animal are con- veyed, and in which all its perceptions terminate. The nerves and spinal marrow are merely the roads by which the sensations travel; and if their communication with the brain, by any means be cut off at the neck, the whole trunk of the animal becomes paralytic, evidently proving that the organ by which it feels is the brain. This, how'^ ever, is so far from being the case in insects, that in them, if the head be cut o£^ the remainder of the body will con- tinue to give proofs of life and sensation longer than the head : both portions will live after the separation, some- times for a considerable period; but the largest will sur- vive the longest, and will move^ walkj and occasionally even ^/^, at first almost as actively without the head, as when united to it. Lyonnet informs us, that he has seen motion in the body of a wasp three days after it had been separated from the head; and that a caterpillar even waited some days after that operation; and when touched, the headless animal made the same movements as when intire*. Dr. Shaw has observed — ^an observation con- firmed in Unzer's Kkine Schreifterty — that if Scolopendra eUctrica {Geophilm Leach) be cut in two, the halves will live and appear vigorous even for 2i fortnight afterwards ; and what is more remarkable, that the tail part always sur- vives the head two or three days^. The sensorium com^ mun^ of insects, therefore, does not, as in the warm-blooded animals, reside in the brain alone, but in the spinal mar- row also. It was on this account probably that Linn^

* In Lesser Insecto4heol, ii. 84. note *,

•» Linn, Trans, ii. 8. Aristotle bad observed this vitality of insects, and that that of the myriapods is greatest. Hist, Animal, /. Iv. c, 7. De Mespiratione, c, 3. Reptiles have also this faculty. N. Diet* iPHist. Nat. }^xix. 161.

C2

20 INTERNAL ANATOMY OF INSECTS.

denied the existence of a brain in insects, regarding it merely as the first ganglion of the spine.

Cuvier and other modern physiologists, from the gan- glionic structure of this organ, are of opinion that it is not the analogue of the cerebrch-spinal system of verte- brate animals, but rather of their gr^^a/ sympathetic nerves. Indeed, considering solely the extetmal structure of the nervous system of insects, a great resemblance strikes us between it and. these nerves ; for besides its general gan- glionic structure, there is also in them an upper ganglion in the neck, seemingly corresponding with, what we have named the brain of insects, from which the nervous chord dips to the lower part of the neck, where it forms a se* cond ganglion, which appears to correspond with what we have considered as their second ganglion^. We may observe, however, that at least in one respect there is even bxl external lesemblance between the brain of in- sects and that of vertebrate animals :-*-it most commonly consists^ as has been stated, like, them, of two lobes, often very distinct ; a circumstance which not unfrequeiitly di- stinguishes the other gahgHons^, and is not borrowed from the ganglions of thegreat sympatheiics. With re- spect to the internal structure of the ganglions and spinal marrow of insects, we know little to build any theory upon, except that the internal substance of the fi>rmer is filled with air-vessels ; at least so Lyonnet, as has been already observed, found in the Cossusj while only the tunics of the.latter ape covered by them, — -a circumstance which I shall again have occasion to advert to^ Taking

* Cuv. Anat Comp. ii. 283 — . These are named ** the upper and lower cervical ganglions."

*» Lyonnet Anat. t. ix. x. Plate XXI. Fig. 1. a. c.

INTERNAL ANATOMY OF INSECTS. 21

the above resemblance to the brain of vertebrates into consideration, there appears ground for thinking that the nervous system of insects, like some of their arti- culations *5 is of a mixed kind, combining in it both the ce- rebro-spinal and the ganglionic systems ; and this will appear further if we consid^ iX&Junctions,

That learned and acute physiologist Dr. Virey, assum- ing as an hypothesis, that the structure of the system in question is simply ganglionic, and merely analogous to the sympathetic system of vertebrate animals, has built a theory upon the assumption, which appears evidently contradicted by facts. Because, as he conceives afler Cuvier, insects are not gifted with a real brain and spinal marrow, he would make it a necessary consequence that they have no degree of intellect^ no memory, judgement or free will ; but are guided in every respect by instinct ahd i^>ontaneous impulses,-^that they are incapable of instruction, and can superadd no acquired habits to those which are instinctive and inbred^. This consequence would certainly necessarily follow, was their nervous system perfectiy analogous to the sympathetic of warm- blooded animals. But when we come to take into conside- ration ikefunctimis that in insects this system confessedly discharges, we are led to doubt very strongly the correct- ness of the assumption. Now in these animals tiie system in question not only renders to the nutritive and repro- ductive organs, which is the principal function of the great sympadietic nerves in the vertebrates ; but by the com- mon organs maintains a connexion witii the external

» Vol. III. p. 664. 671.

»» N. Diet, d'Hist. Nat. ii. 47—. v. 692. xvi. 308—.

52 INTERNAL ANATOMY OF INSBCTS.

world, and acquires ideas of things without, which in them is a function of the cerebral system : from the same centre also issue those powers which at the bidding qC the will put the limbs in action, which also belongs to the cerebral system. That insects have memory, and consequently a real brain, has been before largely proved, as also that they have that degi'ee of intellect and judge- ment which enables them to profit by the notices fur- nished by their senses*. What can be the use of eyes,-— of the senses of hearing, smelling, feeling &c. if they are not instructed by them what to choose and what to avoid? And if they ar<? thus instructed — they must have sufficient intellect to apprehend it, and a portion of free will to en- able them to act according to it. With regard to the assertion that they are incapable of instruction, or of ac- quiring new habits; few or no experiments have been tried with the express purpose of ascertaining this point : but some well authenticated facts are related, from which it seems to result that insects may be taught some things, and acquire habits not instinctive. They could scarcely be brought from their wild state, and domesticated, as bees have been so universally, and both ants and wasps occasionally**, without some departure from the habits of their wild state ; and the fact of the corsair-bees, that ac- quire predatory habits before described ^, shows this more evidently : but one of the most remarkable stories to our purpose upon record, is that of M. Pelisson, who, when he was confined in the Bastile, tamed a spider, and taught it to come for food at the sound of an instrument. A

• Vol. II. p. 525—. 613—.

*» Huber Fotirmis, 260—. Reaum. vi. 172 — .

*= Vol. II. p. 207.

INTERNAL ANATOMY OF INSECTS. 23

manu&cturer also in Paris, fed 800 spiders in an apart- ment, which became so tame that whenever he entered it, which he usually did bringing a dish filled with flies but not always, they immediately came down to him to receive their food*.

All these circumstances having their due consideration and weight, it seems, I think, most probable, that as insects have their communication with the external world by means of certain organs in connexion with their ner- vous system, and appear to have some degree of intellect, memory, and free will, all of which in the higher animals are functions of a cerebral system, and at the same tim^ in other respects manifest those which are peculiar to the sympathetic system, — ^it is most probable, I say, as was above hinted, that in their system both are united.

I nuist bespeak your attention to a circumstance con- nected with the subject of this letter, which merits parti- cular consideration: I mean the gradual change that takes place in the nervous system when insects undergo their metamorphoses ; so that, except in the Orthoptera^ Hemipteray and Newx^tera Orders, in which no change is itndergone, the number of ganglions of the spinal chord is less in the imago than in the larva« There seems an exception indeed to this rule in the case of the rhinocerbs- beetle {Oryctes nastcorftis), in the larva of which there is only one ganglion, while in the imago there ore Jour ^^ But as this one ganglion occupies the wh^le spinal mar- row, it is resdly of greater extent than the four of the imago; so that even in this case there is a concentration

» K. Diet d^HuU NaL ii. 279— »» Cuv. Anat, Comp, ii. 319. 337,

24 INTERNAL ANATOMY OF INSECTS.

of the cerebral pulp. In some cases, as in Dytiscus mar- ginaltSj and Hydrophilus piceus^y the imago has only one ganglion less than the larva, but more generally it loses Jimr oxjhe. Dr. Herold has traced the gradual changes that take place in the spinal marrow of the common cab- bage-butterfly {Pierts Brassi€a:\ from the time that it has attained its full size to its assumption of the imago. Of these I shall now give you some account.

In the full-grown caterpillar, besides the brain there are eleven ganglions, the chords of the four first inter- nodes being double, arid the rest single: from each gan- glion proceed two pairs of nerves, one from each side. In diis the lobes of the brain form an angle with each other ^ In two days the double chords mutually recede, so as to diminish the interval between the ganglions, and the single ones have becoitie curved : thus the length of the spinal marrow is shortened about a fonrfh^ and the fourth and fifth ganglions have made an approach to each other ^. On the eighth day, when the insect has assumed the pupa but remains still in the skin of the caterpillar^ the flexure of the internodes is much increased ; the first gangUon is now united to the brain, ^and the fourth and fifth have joined each other, though they are still distinct ; th^ spindi marrow has now lost considerably more than a third of its length^. On the fourteenth day, the in- ternodes, except the double ones, have become nearly straight a^ain ; the fourth and fifth ganglions have coa- lesced so as to form one, and the sixth and seventh have each lost their pairs of nerves ^. Shordy after this, these

* Cuv. Anat. Cmnp. u. 322, 323^; 338, 339—.

»» Plate XXX. Fig. 1. ^ Ibid. Fig. 2.

*> Ibid. Fig. 3. . « Herold Schmett. L ii./. 6.

INTERNAL ANATOMY OF INSECTS. 25

last gan^UoDs have nearly disappeared^ and the chords of the three first intemodes have again approached each other^. The next change ^Jiibited is the absorption of tiie first ganglion by the brain, the union of the chords of the first intemode, which is now straight, the approxi- mation of the second and third ganglions, and the en<» largemiKit of the (Hie formed by the miion of the fourth and fifth, at the expense perhaps of the sixth and sevaitb,. which have liow entirely disappeared, and in their place is a very long intemode* These united ganglions retain j the pairs of nerres they had when separate^. Just be- I fi>re the assms^tion of the imago^ the direction of the I lobes of the brain becomes horizontal, the second and third ganglions unite^ and the intemode between the tlurd and fourdi is shortened ^. Lastly, when the animal is become a butterfiy, the second and third ganglions have coalesced, and are joined to that formed by the union of the fourth and fifth ; a short isthmus or rather ccmstric^ tion, .with, an orifice, being their only separation : eadbi of these united gaoiglions send forth laterally fi>ur pairs of nerves^. In his figure. Dr. Herold has not repre- sented the orifice for the passage of the gullet, but doubt- less one eidsts, which for an animal that imbibes only fluid food is piobaUy very minute. In Hypogyfmna d&- for^ we learn fium Cuvier, this orifice Is of that desorip-^ tion, and of a triangular shaped

It can admit of no reasonaUe doubt that one cf the principal intentions of these changes is to accommo- date the nervous system to the altered functions of the

' Herold SchmeU. t, ii./. 7- •* Plate XXX. Fig. 4,

« Ibid, Fig. 5. ' Ibid. Fig. 6.

* Antd, Comp, Ii. 348.

VOL. IV.

S$ INTERNAL ANATOMY OF INSECTS.

animal in its new stage of existence, in which th^ an- tennae, eyes, and other organs of the senses, as well as Ae limbs and muscles moving them, and the sexual or- gans, being very different Irbm those of the larva, and if not whdlly new, yet expanded from minute germs to their itiU size, may well demand corresponding changes in the stiticture of the n«*vous system by whichfthey are acted

upon.

But are these changes also concerned, as Dr. Virey Conjectures, in producing that remarkable alteration which usually takes place between the instincts of the larva and imago? In order to answer this question, it will be requisite first to xjuote the ingenious illustration with which this able physiologist elucidates his ideas on this pointl " The more readily," he observes, ." to com- prehend the action of instinct, let us compare the insect to one of those hand-organs in which a revolving cylin- der presents different tunes noted at its surface, and pressing the keys of the pipes of the organ, gives birth to all the tones of a song : if the tune is to be changed, the eylinder must be pulled out or pushed in one or more notches, to present other notes: to the. keys. In the same manner let us suppose that nature has impressed or en- graved certain determinations or notes of action, fixed in a determinate series in the nervous system and the gan- glions of the caterpillar^ by which alone she lives, she will act according to a certain sequence of operations ; and, so to speak, she will sing the air engraven within h^:. When she undergoes her metamorphosis into a butterfly, her nervous system being, if I may so express myseli^ pulled out a notch, like the cylinder, will present the notes of another tune, another series of instinctive ope-

INTERNAL ANATOMSC OF INSGSCTS. 27

rations ; and the animal will even find itself as perfectly instructed suid as capable of employing its new organs, as it was to use the old ones. The relations will be the 8atne; it will always be the play of the instrument^." .

This illustration is doubtless at the.first glance very striking and plausible : but a closer examination will^ I think, show, that, as in so many other, instances in meta* physical reasoning, when fanciful analogies are substi* tuted for a rigid adherence to stubborn facts, it is satis* &ctory only on a superficial vieir, and will not stand the test of investigation ; and as this is a question intimately connected with what I have advanced on the subject of instinct in a former letter, I must be permitted to go somewhat into detail in considering iu

To prove his position. Dr. Virey ought at least to be able to show that, whenever a change takes place in the instincts of insects in their difierent states of larva and imago, a corresponding change takes place in the exter- nal structure of the nervous chord. But what are the &cts? In three whole orders, viz. Orthoptera^ Hemi-^ ptera^ arid Neuraptera, as mentioned above^, the struc- ture of the nervous chord is not changed ; -and yet we know that many tribes of these <»rders acquire instincts in their imago state altogether difierent fi'om those which directed them in their state of larvae. A perfect Locust^ for instance, acquires the new instincts of using its wings ; of undertaking those distant migrations of which so many remarkable instances were laid before you in a former letter^; and, if a female, of depositing its eggs in an

■ 3r; IHct. (tmst. Kat, xvi. 313. Comp. i. 420. *• See above, p. 23. ' Vol. I. 4th Ed. 220—.

28 IHl^RNAL ANATOMY OF INSECTS.

appropriate situation* But if such striking changes, in the instinct of these tribes can be efieoted without any perc^tihle alteration in the structure of the ner¥ous chord, it is contrary to the received rules of {^osophi- cal induction to refer to this alteration the changes in the instincts of other tribes where it is found* Is it not far more probable that this alteration has in fact no con- nexion with the changes of instinct, but is solely con- cerned with those remarkable changes in the organs of sense aad motion, which occur in the krva and imago states of the orders in which it is observed ? In a com- mon caterpillar, the form of the body, the legs, the eyes, and other organs of the s^ses, all strikingly differ from those of the imago; whereas, with the excepti<m of the acquisition. of new wings, a perfect locust di&rs little £rom its larva: so that we may reasonably expect a corresponding change, sach as we find it, in the structure <xf the nervous chord of the Jepidopterous insect, not called for in that of the neuropteiDus species, in wlnidi -accordin^y it does not take place.

This reasonir^, in oppoaition to Dr. Virej^s theory, that the changes of instinct depend on the altered struc- tmre of the nervous system, becomes greatly strengthened when we advert to the higher classes of animals, which surely in any investigatix»i of the nature of instinct ought tq be dosdy kept in view; for the faculty, though often less perfectin them than in insects, is still of the same kmd^ and may consequently be expected to follow the same general laws* In a young swallow, for example, all its instincts are not developed at once any more than in an insect. The instinct which leads it to migrate does not appear for some months after its birth, and that

INTfiRNAt ANATOMY OF INSECTS. 29

flf bmlding a nest still later. But we have not the filighteist ground for believing that these new instincts are preceded by any change in the structure of the great sympathetic nerve, or of any other portion of the nervous system : and the same may be said as to the sexual in- stincts developed in quadrupeds some years subsequent to their birth. If, then, these remarkable changes in the instinct of the higher dasses of animals can take place iid€|)endently of any visible change in the nerve^, what sttbstasitial reason can be assigned why they may not also in the class of insects?

On the whole, I think you will agree with me, that there is nothing in Dr. Virey's hj^othesis which should lead me to alter the opinion I have already so strongly expressed in a former letter^, as to the insufficiency of the mechanical theories of instinct hitherto promulgated, adequately to explain all the phenomena; and unless they do this they are evidently of small value. Such theories as I have there adverted to may often seem to be supported by a few insulated facts, but with others, far more numerous, they are utterly at variance; and, to 'Omk many other inisHances, I am strongly inclined to •doubt the possibility of satisfactorily explaining the t^- riety of instincts exereised by a bee^, or the esftraordi" nary devdl^ment of new ones in particular circum- stances only^, on any merely mechanical groulids.

And after all, even suppose it could be demonstratively shown that every kisthict is as clearly dependent <m se- condai^ causes, as I have formeriy admitted that jsome doubdess seem to be, yet what would this teach us as te the essential nidxire of instinct? We have advanced

• Vol. ir. 4th Ed. p. 467. * Ibid. p. 499. ^ Ibid. p. 509.

so INTERNAL ANATOMY OF INSECTS.

indeed a step; but stfll, as I have before observed in i^ ferring to the theories of Brown and Tucker, we have only placed the world upon the tortoise, and instinct, as to its essence^ which is what we want to detect, is as my- sterious as ever: just as, though we can clearly prove that the mind is acted upon by the senses, yet this throws no light upon the essential nature of the mind, which we are forced to admit is inscrutable, as if to teach us humility, and prevent our vainly fancying, that thou^ allowed to discover some of the arcana of nature, we shall ever be able to penetrate into her inmost sanctuaries.

That Dr. Virey should regard instinct in insects as purely mechanical was the natural consequence of his denying them any portion of intellect ; but his opinion cannot I think be consistently assented to^ if it be the fact, as I have just shown*, that they are not wholly de- void of the inteUectual principle. Whatever is merely mechanical, must, under similar circumstances, always act precisely in the same way. An automaton once con- structed, whilst its machinery remains in order, will in- variably perform the same actions ; and Des Cartes, when he had constructed his celebrated female automaton, imagined that he had irrefragably proved his principle, that brutes are mere machines. But if^ instead of losing himself in the wilds of metaphysical speculation, he had soberly attended to facts, he would have seen that the instinct of animals can be modified and counteracted by their intellect, and consequently cannot be regarded as simply mechanical. Though the instinctive impulse; pf an

empty stomach polverfuUy impel a dog to gratify his ap- petite, yet, if he be well tutored, the fear of correction

* See above, p. 21.

1NTEBNAL. ANATOMY, OF INS£CtS, 51

will make him abstain from the most tempting dainties: aad in like manner a bee will quit the nectary of a flower, however, amply replenished with sweets, if alarmed by any^interroptioja. The ants on which Buonaparte amused himself with experiments at St Helena, though they storined his sugar-bason when defended by a fosse of water, controlled their instinct and desisted when it was surrounded with vinegar^: and in 4he. remarkable in* stance communicated to Dr. Leach by Sir Joseph Banks, the instinct of a crippled spider so completely' changed, that from a sedentary web-weaver it became a hunter^. There is evidendy, therefore, no analogy between ac- tions strictly mechanical and instincts, which, though they may often seemto be excited by mechanical causes, are liable to be restrained or modified by the connexion of the instinctive and inteUectual faculties^; and while we are ignorant how this connexion takes place, it is ob- viously impossible to reason logically on the subject.

In thus denying that any existing meckanical theory of instinct is satisfactory, I by no means intend to assert that instinct is purely intellectuals I have already given you ray opinion^, that it is not the effect of any immediate agency of the Deity ; nor am I prepared to assent to the doctrine of a writer, who has in some respects written ably on the subject in question, who says, that ^^ the IHvine Energy does in reality act not immediately^ but mediatehjy or through the medium of moral and intelleo tuaL influences upon the nature or consciousness of the creature, in the production of the various, and in many instances truly wonderful, actions which they perform V

^ Antommarchi'd Last Days of Napoleon,

^ Linn, Trans, xi. 393. * Vol. IT. 4th Ed. p. 515.

■* Ibid. p. 469. • Zoological Journal^ n®. i. 5,

SS INTERHAL ANATOMY OF IMSXCTfi.

The same objectiOB ^plies to diis as to so many otfa^ metaphysical theories, that it is not adequately supported hy facts t and all theories not bo supported are injuiious to science in [ooportion as tfaeir plautibiljty is greater, by leading the student to relax in that observation of nature and attentive study of the instincts of animal^ on whidi alone sound hypothesu on this subject can be ulti- mately founded.

I shall conclude these remarks on the nature of in- stinct with a fev observations as to the circnmstances in which ioiiects may be supposed to be guided hy this &- culty, and those in which intellect seems to direct them. The bee, when it takes its flight to a field where flowers abound, is governed by intellect in the use of its senses ; for these are given to it as guides t and when it arrives there, they direct it to the flowers, and enable it to as< certMH wbioh contains the treasures it is in seardi <^; but having made this discovery, its instinct teaches it to imbibe the nectar and load its hiod legs with pollen. — Again : its senses, luded by memory, enable it to retrace its way to the hive, where instina once more impels it m its various c^erations. So that when we asci^je a certain degree of intellect to these animals, we do not place them upon a par witli man; smoe all the most wMiderful parts of then* econtooy, and those ma- nipulations that Qcceed all our powers, we admit not to be the contrivance of the animals themselves, but the necessary results of &culties implanted in their ctHistitulioQ at the flist creation by their II^Iakek. 1 may further repeat, that the mere &ct of being en-

j J ^th the external organs of sense, proves a cer-

l^ree of intellect in insects. For if in all their IS they were directed merely by their instinct.

INTERNAL ANATOMY OF INSECTS. SS

they might do as well without sight, hearing, smell, touch, &c. but having these senses and their organs, it seems to me a necessary consequence, that they must have a suf- ficient degree of intellect, memory, and judgement, to en- able them advantageously to employ them/

There is this difference between intellect in man, and the rest of the animal creation. Their intellect teaches them to follow the lead of their senses, and make such use of the external world as their appetites or instincts incline them to, — and this is their wisdom ; while the in- tellect of man, being associated with an immortal princi- ple, and being in connexion with a world above that which his senses reveal to him, can, by aid derived from heaven, control those senses, and bring under his instinc- tive appetites, so as to render them obedient to the to ^ye- fbovixov, or governing power of his nature : and this is

HIS WISDOM.

I am, &c.

VOL. iv< D

LETTER XXXVIII.

INTEBNAL ANATOMY AND PHYSIOLOGY OF INSECTS CONTINUED.

RESPIRATION.

" Life and flame have this in common/' says Cuvier, *< that neither the one nor the other can subsist without air ; all living beings, from man to the most minute ve* getable, perish when they are utterly deprived of that fluid*." The ancients, however, not perceiving insects to be furnished with any thing resembling lungs^ took it for granted that they did not breathe ; though Pliny seems to hesitate on the subject^. But the microscopic and anatomical observations of Malpighi, Swammerdam and Lyonnet, and the experiments of more modem phy siologists, have incontestably proved that insects are pro- vided with respiratory organs, and that the respiration of air is as necessary to them as to other animals. They can exist indeed for a time in irrespirable air; and im- mersion in hydrogen or carbonic acid gases is not, as I have often ascertained, so instantly fatal to them as it would be to vertebrate animals; but like them, thiey

* Anat, Conipar. iv. ^6.

^ Plin. Hist. Nat. L xi. c. 3. Even Aristotle seems to have given into the common opinion. De Bespirat, c, Z, 9. &c.

INTERNAL ANATOMY OF INSECTS. 35

q^eedily perish in air altogether deprived of its oxyg^en, or placed in situations to which all access to this essential element is excluded. Their respiration too of atmo* spheric air produces the same change in it with that of the vertebrate animals, the oxygen disappearing, and carbonic acid gas being produced in its place* Bayle had long since ascertained, that when bees, flies, and other insects were placed under an exhausted receiver, they often perished^ : and the same effect was even o]> served by the ancioits to ensue, when. their bodies were by any means covered with oil or grease, which necessa- rily closed the orifices of their respiratory organs^*

But for the first series of experiments ascertaining the necessity of a supply of air to insects, and their conver* ston of it into carbonic acid, we are indebted to the illus- trious Scheele^ ; and his experiments have been repeated and confirmed by Spallanzani, Vauquelin, and other chemists. The former found, that when caterpillars and maggots were confined in vessels containing only about eleven cubic inches of atmospheric air^ though furnished with sufficient food, they soon died, and sooner when the space was more confined^. He ascertained too, that a larva weighing only a few grains consumed, in a given time, as much oxygen as an amphibious animal a thou- sand times as voluminous *. A male grasshopper {Acrida viridissima K.) in six cubic inches of oxygen lived but eighteen hours, and the female placed in eight cubic inches of atmospheric air, only thirty-six hours. The

* PhUos. Trans, v. 2011. Works, 4to. i, 79, 112.

^ Aristot. HisL AninuU, /. viiL e. ^,

^ On Air and Fire, 148, 155. ^ TracU, 208.

' Mem. on Eespirat, 75.

d2

36 INTERNAL ANATOMY OF INSECTS.

usual tests in both instances detected the conversion of the oxygen present into carbonic acid*. Precisely the same result was obtained by Sorg and Ellis, who, having placed a number of flies in nine cubic inches of atmo- spheric air, found them all dead by the third day, the oxygen entirely vanished, and a quantity of carbonic acid nearly equal in bulk produced^.

It is ascertained too, that insects like otlier animals require in the process of respiration not merely oxygen, but such a mixture of it with nitrogen or azote as com- poses atmospheric air : for Vauquelin found that a grass- hopper placed in six cubic inches of oxygen lived only half as long (eighteen hours) as another placed in eight inches of atmospheric air ; its breathing was much more laborious, and it died when not more than one-twentieth of the oxygen had been converted into carbonic acid^. That a large quantity of oxygen penetrates all parts of insects, is evident also from the acid prevalent in the fluids of most of them, as likewise from the wonderful power of their muscles. That azote is alsp received, seems probable from the ammonia which has been ex- tracted from the fluids of many, and from the rapid pu- trescence of these animals^.

The mode, however, in which the respiration of insects is carried on, difiers greatly from that which obtains in the higher animals. They have no lungs, no organs confined to a particular part of the body, by means of which the whole of the blood is regularly exposed to the

' Ann, de Chimie, xii. 273.

'' F. L. A. Sorg, Respirat, Insect, et Vemt, Ellis, Inquiry ifUo Chang, prod, on Atmosph. Air by Bespirat. &c. 69. «= Ann. de Ckimie, xii. 273. «* Sprengel, Commentar. &c. 27— .

INTERNAL ANATOMY OF INSECTS. 37

action of the inspired air. They do not breathe through the mouthy but through- numerous orifices called spiracles^ and the respiratory vessels connected \nth theise are con- ducted to every part of the body. In some indeed, that we have included under the denomination of insects, as the Arachnida^ an approach is made to the branchial respiration of fishes.

The respiratory apparatus of insects may be consi- dered under two principal heads : — viz. the orifices or spiracles, and other external organs by which the air is alternately received and expelled ; and the internal ones, by which it is distributed. Each of these is well worthy of your attention.

I. The external respiratory organs of insects may be divided into three kinds. Spiracles ; Respiratory plates s and branch^form and other pneumatic appendages.

i. Spiracles^ (iS^/raa//a), or breathing pores, are small orifices in the trunk or abdomen of insects, opening into the trache(3e^ by which the air enters the body, or is ex- pelled from it**. They may be considered principally as to their composition and substance; shape ; colour; maS" nitude; situation; and number.

1. Composition and substance. Perhaps you may not be aware that the structure of these minute apertures is not so simple as at the first view it may seem ; but when you recollect that by them the insect breathes^ you will suspect that provision may be made for their opening and shutting. A spiracle therefore, speaking an£^lpgi-i>

• Plate XXIII. Fig. 2. and Plates VIII. IX. XVI. XXIX. c\ H\

^ Moldenhawers {Anat, de Pflanz. 314 — .) affirms that the spira-* cles of most insects are quite closed : but Sprengel {CovmientaV' ^ H,) has satisfactorily refuted that opinion.

S8 INTERNAL ANATOMY OF INSECTS.

cally, mBy be re^rded in numerous cases as a Tooulfi closed by lips. In caterpillars and many other insects, the substance of the crust where it surrounds the spiracle, is elevated so as to form a ring round it. The lips, pro- perly speaking are formed of a single cartilaginous piece or platform, with a central lon^tudinal cleft or opening wfaen closed oflen extending the whole length of the piece*; but in some appearing always open and circu- lar ; of the former description are those covered by the elytra in the common cockchafer ; and of the latter, those that are not so covered : in some, as in the antepectoral pMr of the mole-cricket, there ^pear to be no lips, the orifice being merely closed with hairs". Though the aperture is usually in the middle of the platform, in the female of Ih/tiscus marginalis, it is nearer the posterior ude, the anterior or upper lip being the longest. In the majority, the mouth or cleft is nearly as long as the spi- racle; yet in the puss-moth (Centra Vinula) it is shorter'. Some spiracles, however, are unilabiate, or have only one lip. This is the case with Gonyleptes K. and perhaps others'. The lips are usually horizontal, but sometimes they dip so as to make the spiracle appear open.

With regard to the substance of these organs, it is more or less cartilaginous, and probably elastic; the surface frequendy appears to be corrugate or platted ; this is very distincdy seen in the stag-beetle and the cockchafer : in the last insect, under a powerful magnifier, we are told that the lips appear to consist of parallel cartilaginons processes, s^uurated by a cellular web*. In some species

' PuTR XXIII. Fig. S. <• Sprengel, Camtenlar. § 7.

aid. f. m.f. 80. " Plat« XXIX. Fjq. «3.

' IM, 8.

INTERNAL ANATOMY OF INSECTS. $9

of Copris the corrugations form a perplexed labyrinth ; in the caterpillar of the puss-moth the plaits are so nar- row as to look like rays^ ; and in some Dynastida the lips approach to a lamellated structure* Again, in Hy^ dropkilus carabotdes the upper lip, and in Dytiscus ctT" eyp^iexusj both lips seem formed of elegaikt {dumes^ : a similar ornament distinguishes the inner edge of the lips in the caterpillar of the great goat-moth {Cossus lig- nipercUi) and others ^. In the grub of the rhinocero$<- beetle (Oryctes nasicomis) the margin of the lower or in- ner lip is decorated by pinnated rays, which enter the cellular membrane that covers the upper lip^ ) in this krra, and that likewise of the cockchafer, the two lips are formed of different substances ; in the last the upper or outer one consists of a perforated cellular membranis, through which the air can pass, while the lower or inner one is a cartilaginous valve that closes the orifice^: in the former this valve is surmounted by a boss ^ In the pupa of Smerinthus PopuH, a hawk-moth not uncommon, and of some dragon-^ies {LibeUtda depressa\ the margin of the two lips is crenated, probably with notches which alternate, that the mouth of the spiracle may shut more accurately <^« The substance is unusually thick In the spinose caterpillars of butterflies ; and in the pupa of one^ Hesperia Proteus^ it is villose.

Under the present head I may observe, that in some cases, as in the puss-moth, and the larva of the common

> Sprengel 7. U iii./ 30. *» im. t. il/ 92. i m.f. S9

« Plavb XXIX. Fig. 29.

^ Ibid. Fio. 16. SprcDgeU Ilnd. 9. L 1;/. 4-6.

« Ibid. 9. /. L/. 9. f Plate XXIX. Fio. 16. a.

s Sprengel, Ibid. L iii./ 27-

40 INTERNAL ANATOMY OF INSECTS.

water-beetle (JDy^^scu^ margtnalis\the spiracles arexslbsed by a semifluid substance, which however, according to Sprengel, is permeable to the air*. The animal, where these organs are furnished with lips, has doubtless, by means of a muscular apparatus, the power of opening and shtUting them : this is done, we are told, by elevating and depressing, or rather by contracting and relaxing them. Sorg counted in one case {Oryctes nasicomis) twenty^ and in another {Acrida viridissima) Jlfty^ of these motions to take place in little more than two minutes^ : but the quickness and force of this motion is not always uniform ; for the same physiologist observed, that in Carabus au- ratus^ when feeding or moving its body rapidly, the con- traction of the spiracles took place at very short intervals ; but when it was fasting, and its motions were slow, the intervals were longer^ : it is probable also, that the tem- perature may accelerate or retard the motion. In the summer I examined a specimen of MeloUmtha hirticola^ that had indeed been somewhat injured, with this view : the pulses of the abdomen, which alternately rose and fell, were at about the rate of the pulse of a man in health, six;ty in a minute, and the spiracles appeared to me to keep pace with this motion : later in the year, whta the temperature was lower, as I was walking, I took a spe- cimen of some grasshopper {Locusta Leach). Upon

* Sprongel, Commentar, 7 — •

^ Sprengel, from whom I have borrowed this quotation, expresses

*the time by " scripuh hora.** This word is of uncertain meaning,

being scarcdy ever applied to Hrne ; but as it means the twenty-fourth

part of an ounce, Faher conjectures it may mean the same portion

of an hour.

"^ Soi^,, DtsqidsU^ circa respiraU intcct. 27, 46, 66. Sprengel uin

INTERNAL ANATOMY OF INSECTS. 41

viewing it under a lens, I observed one of the convex pec* toral spiracles open and shut, and the interval between two breathings appeared nearly half a minute.

2. With regard to their shape^ spiracles vary consi- derably. In general we may observe that the abdomi- nal ones are usually flat, while those of the trunk are often convex*. Sometimes they are very narrow and nearly linear, as in many pupae of Lepidoptera, and those in the metathorax of the sandwasps {Ammophzla K.) and affinities; at others they are wider and nearly elliptical, as in Lucanu^ and many Lamellicom beetles : again, in Copris they ire circular ; in Calandra Palmarum ovate; in Dytiscus oblong^; in Stapkylinus olensXxxnxiisLtei in Crom/leptes nearly of the shape of a horse-shoe^ ; and probably many other forms might be traced, if a thorough investigation with this view were undertaken.

3. The colour of spiracles will not detain us long. In the caterpillars of Lepidoptera this is often so contrasted with that of the rest of the body, as to produce a strik- ing and pleasing efKsct. Thus when the body is of a dark colour, they are usually of a pale one^ ; or if the body is pale, they are dark% or surrounded with a dark ririg^. This contrast is often rendered more striking by their position with regard to the partial colours that often ornament caterpillars : in those whose sides are decorated by a longitudinal stripe, the spiracles are often planted in it « ; or just above it ** ; or between two^ :

• Chabrier iurle Vol des Ins. c. 1. 454.

»» Plate XXIX. Fig. 28. A", « Ibid. Fig. 23.

* Sepp. I. iv. L ii./ 3. • Uid. t. xiv./. 3. ' Ibid. t. v./, 6, 7. " Ibid. t. If. 7, 8. >» md. t. x.f. 6, 7. . ' I^' V. /. i./. 3.

/

42 INTERNAL ANATOMY OF INSECTS.

•

HI some hawkmoths the. intermediate ones are set in white or pale spots, which gives great life to the animal. In general, in perfect insects the most prevalent colour i^ bufi^ or reddish-yellow. In the larva of the great ursr- iat'-heede {Dytiscus Ttutrginalis) these organs resemble the iris of the eye, beii^ circular with concentric rings, alternately pale and dark^.

4. The size of spiracles varies considerably. Those in the larva last mentioned are so minute as to be scarcely visible except under a lens, while those behind the fore- l^s in GryUotalpa are a full line in length, and those in the pleura of Macropus accentifer^ a Brazilian Capricorn beetle, are more than twice as long. In the same species they are often found of different sizes ;-^thus the anal pairs in the Dytiscus lately alluded to, I mean in the per- fect insect, are much larger than the rest^, probably that the animal may imbibe a larger quantity of air when it rises to the surface of the water, where it suspends itself by the tail. In those Lamellicom beetles in which the terminal part of the abdomen is not protected by the el3rtra, the ctyoered spiracles are the largest

5. Under the next head, the situation of spiracles, I shall not only consider the part of the body in which they are situated, but likewise their position in the crust ; to which last, as it will not detain us long, I shall first call your attention. Their position in this respect is most commonly obliques but in the abdomen of the above Dytiscus they are transverse, and in a larva I possess, pro- bably of an ElateTj they are longitudinal. In spinose

* Sphinx Labruic€B Merian Surmatiu 34. «» Plate XXIX. Fig. 28. A".

INTERNAL ANATOMY OF INSECTS. 48

caterpillars these organs are generally planted between two spines, one being above and the other below. The lateral line of the body most commonly marks their si- tuation ; but in many cases they become ventral, and in others dorsal. The most important circumstance, how- ever, connected with the present head is their appropria^ tion to particular segments or parts of the body, for, like the ganglions of the spinal marrow, they are distributed to almost every segment. Let us take a summary view of their arrangement in this respect.

No insect has any spiracle in the head ; but in cater- pillars and many other larvce there is a pair in the Jirst segment of the trunk. This is also to be found in the other states, but is not easily detected in the pupa oi Lepidop' tera : in the Coleoptera order, in the grub of the Lamel- licom beetles, it is extremely conspicuous, and planted in the side of the first segment*; in other Coleopterous grubs it is not so readily found, but probably its station is some- where behind the base of the arms, where it is very visi- ble in that of Staphylinus. In the imago of insects o this order, this antepectoral spiracle has been overlooked, and indeed is not soon discovered : to see it clearly, the manitrunk should be separated from the alitrunk ; and then if you examine the l&wer side of the cavity, you will see a pair of, usually, large spiracles planted just above the arms, in the ligament that unites these two parts of the trunk to each other : in the common rove-beetle, however, {Staphylinus olens) you may easily see it without dissec- tion^. In the Orthoptera it is situated behind the arms,

• Swammerd. Bibl. Nat. L xxvii./. 5. Compare Stunn DeuUch. Fn, L t v./. r.

* Plate XXIX. Fig. 12. d.

44 INTERNAL ANATOMY OF INbECTS.

as in Gtyllotalpa : or between them and the prothorax, as in Blatta : in the Hemiptera and Neuroptera proha^ bly. the situation is not very different. In the Lepidop^ tera this pair of spiracles is planted just before the base of the upper or primary wings ^ : a similar situation, I suspect, is appropriated to it in the Trickoptera, but co- vered by a tubercle or scale. Something similar has been noticed by M. Chabrier, in. the same situation and circumstances, in the collar of Hymenoptera^, In nu- merous Diptera this breathing pore is planted on each side between the collar and the dor solum above the

«

arms ^9 and in Hippcbosca in the collar itself^.

In LepidopterouSy Coleopterous^ and some other larvae, the two segments of the body corresponding with the alitrunk in the perfect insect, are without spiracles, nei- ther have they in this state, though pneumatic organs have been discovered % any real ones in that part: but not so the remaining orders, all of which have these or- gans in that section of the trunk. To begin with the Orthoptera : — in Blatta there seems to be a long narrow one behind the intermediate leg ; in the Gtyllotalpa there is one in the posterior part of the pleura ; and in Lo^ custa Leach, above both the intermediate and hind legs/. It is probable, that in general those that have no spira- cles in the manitrunk have Jour in the alitrunk, which seems the natural number belonging to the trunk. In many of the Heteropterous Hemiptera in the parapleura

• De Geer, i. 81. /. v./. 10./. ^ Sur le Vol det Ins. c. i. 469. c Reaum. iv. 246. t. xix./. 8. *.

' In this tribe, which I forgot to remark before (see Vol. III. p. 551 — .) there seems hoxh prothorax and coUar.

• Vol. III. p. 552, 562. &c.

' Plate VIII. Fio. 14. h", n".

INTERNAL ANATOMY OF INSECTS. 45

there is an open spiracle without lips*, to which, as in that beautiful bug Sctitdlera Stockeriy a channel some- times leads. The space in which this spiracle is planted in other genera of bugs {Pentatoma &c.) is covered with a kind of membranous skin, often much corrugated**. In the aquatic insects of this section, and many terrestrial ones, as Meduvius, &c. this spiracle is obsolete. There is another circumstance, possibly connected with their respiration, relating to many of the bugs, which may be mentioned here. If you examine Pentatoma rnfipes^ a very common one, you will find between the scapula and parapleura a long orifice or chink ; this upon a closer inspection, under a good magnifier, you will see com- pletely filled with minute stiff hairs or bristles, which fi'inge the posterior margin of the scapula^. In a Bra- zilian species oi Lygdeus L. {seamaculatus K. M.S.) with incrassated posterior thighs, these hairs are. replaced by lamellae which have the aspect oigiUs. A red, vertical, convex spiracle, with its orifice towards the head, and terminating posteriorly in a kind of conical sac, is situated towards the hinder part of the pleura in the giant water- scorpion {Belostoma grandis^); this seems analogous to one lately mentioned in the mole cricket In the other section of this Order it is not easy to decypher the parts of the under side of the alitrunk. In Fulgora^ Tettigonia^ and many others of its genera, there appears to be more than one opening into the chest ; but whether they are of a pneumatic nature or not, can only be ascertained by an inspection of the living aniiriaL There is a very visible

• Plate XXIX. Fig. 14, 15. m". ^ Ibid. Fig. 15. a.

^ Ibid. Fig. 14, 15. b. ^ Ibid. Fig. 25. it".

46 INTERNAL ANATOMY OF INSECTS.

f

«piracle orer each of the four last legs of the LibeUulina^ but in the remainder of the Neuroptera Order they have eluded my search. In the Hymenoptera and DipUra they are nearly in the same situation, being placed be- hind the wings <m each side of the metathorax ; in the latter Order with the poiser near them on the inner side^ : in this also, the spiracles of the trunk are without lipi^ except in the larvae, but are often merely an orifice, sometimes fringed with hairs ; this is particularly con- spicuous in SyrphuSf in which these orifices are very large, and in some species closed by an elegant double firinge of white hairs. This is doubdess to prevent the entrance of any particles of dust or the like.

We are next to consider the situation of the spiracles of the abdomen : these which are supposed to be appro- priated exclusively to inspiration, are usually more nu- merous than those of the trunk, by which it is probable that expiration is performed, and have principally atr tracted the notice of ^tomologists : they are either dor- sal, lateral, or ventral. In DytiscuSj Copris, &c. amongst the beetles, all the j^piracles are dorsal ; in the larvae of Coleoptera and Lepidoptera they are lateral ; and in the Heteropterous Hemiptera they are usually ventral : in Dynastes M^L. they are commonly found of all three de- scriptions ; — ^the three first being dorsal^ the two next /a- teraly and the last pair ventral^. In some instances, as in Perga Kirbii Leach, and probably other Hymenoptera^ these organs are planted in that portion of the dorsal segments which turns under, as was observed in a former

* Chabrier iur le Voldes Ins, c. iii. t. vi./. 4. Sa, Sp. b Plate IX. Fig. 2h m". "^ Plate VIII. Fig. 9.

INTSRNAL ANATOMY OF INSECTS, 47

letter*, and becomes ventral. GenenJly there is a pair of spiracles to each s^ment^ and in those insects that have a hypochandriack joint^ there is often a spbrade in it. The last segment of the abdomen is always without these orifices, as is the basal one in VeliUf Banatray and scNtne other bugs. A singular anomaly distinguishes the UbeUidina : they appear to have :^o abdominal spiracles % yet I have seen the abdomen of LibeUula depressa when reposing, contract and dilate alternately, firom whence it follows that this part is concerned in respiration. Spreiv- gel says that the larvas in this tribe have seven or nine on each side^, and Reaumur speaks of them as disco-* verable in the pnpa^ I have carefully eicamined the pupa-skin of most of the genera of Ubellulina^ under a powerful magnifier, but have not succeeded in discover- ing any thing like these organs in the abdomen. The Ephemera and probably the other Netiroptet*a have ab- dominal spiracles^. M. Latreille observed one on eaeh side of the base of the scale on the footstalk of the abdo- men in ants^. Generally the abdominal spiracles may be described as planted in the crust of the insect; but in many cases their station is in the membranous fdids, which I have therefore named Xhepulmonarium^ that some- times separate the dorsal from the ventral segments: these folds allow of a considerable distenticm of the abdomen^ which is probably necessary when all the air-vessels are fiill. In a gravid Ichneumon I once saw it enlarged to more than twice its natural size by means of this mem- brane, through which the eggs were distinctly visible.—

* Vol. III. p. 706—, »» Ibid. p. 709. ° Sprengel, Ctrnvnient, 3. '^ Ilnd,

• vi. 39a ^ De Geer, ii. 635. » Fmrmis, 22.

48 INTESNAL ANATOMY OF INSECTS.

Before I bid adieu to this subject, I must say a few words upon the sitaation of the organs in question in the my- riapods. In hdus, in each segment b a pair of orifices which have usually been regarded t\& spiracles, but M. Savi found that these briiices opened into vesicles containing a fetid fluid, and upon a very close examination he dis- covered the real spiracles above the base of the legs, in connexion with tracheiE*. In some of the larger species o£ Scolopendris large open spiracles in the same situation are extremely visible''. SaUigera Lam. (Cermatia Illig.) presents a singular anomaly : — a single series of spiracles of the usual form, each planted ina cleft of the posterior margin of the dorsal scuta, runs along the back of the animal^ : unless we may suppose that, like the seeming spiracles of lulus just mentioned, these are merely ori- fices by which it covers itself widi some secretion.

6. A few words upon the number of spiracles.— If you examine the common dog-tick {Ixodes Ricimis), you will find (Hily one of these organs on each side of the abdo- men''; the Libetltdina, as we have seen, have only_^r, all in the trunk ; in the Dynastid/e, MelolontAa, and the larva of Ih/tiscus, there are /ourteen , sixteen in the Co- prida i eighteen in Dytiscus, and probably the majority of Coleoptera, both larva and imago, and Lepidoptera ,■ and a pair to each segment except the last, in the My- riapods'

ii. Respiratory plates {Respiratoria). Tlie nearest ap-

•vaz, ^c. itiUo lului fistid. 14 — .

are particularly visible in an undescribed Ea«t Indian ipecie*, K. M.S.] with Bcuta alternately black and yellow. XXIX. Fie. 20. ^. * De Geer, vn. (. vi./. 3.

INTERKAL ANATOMY OF INSECTS. 49

proach to spiracles is made by tbose remarkable plates that are found in such larvae of Diptera^ as in that state inhabit substances that might impede or altogether stop the entrance or exit of the air by the ordinary spiracles^ such as .dead or liyihg flesh/ dung, or the like. The Creator therafore, as he has seen it good for wise rea- sons^ to commission, certain insects to feed on unclean food, has fitted them for the offices that devolve upon them, ai^d has placed their orifices for breathing in plates at each extremity of the body. There are usually two of these plates at the head, and two at the tail. In the grub of the common flesh-fly {Musca 'cdrnaria\ at the junction of the first s^ment of the body withthe second, two of these plates are planted, which are coAcave and circular, with a denticulated margin ; in the cavity near the lower side is a round spiracle. These plates the animal can withdraw within the body, so as to preveogt this spiracle from being stopped up by any. greasy snb** jstance^. The posterior extremity of this grub, is trimr cated, and has a large and deep cavity surrounded by several fleshy prominences : at the bottom of this are two oval b]X>wn (daites, in each of which are thnee. oval spiracles; plaictsd obliquely : . by the contraction of the fleshy promiriences, this cavity also can be iclosed at the will of the animal^. In some.cases, several stiff rays or spines replace the prominences^. In Echinomjfia gvQssa and others the "anal plates appear not to be perforated, being surmounted only by a central boss 5; .but this,

* Vol. I. p. 251—. »> De Geer vi. 67i t. in.f. JO. ss. 14. « IM. 6iB. i. iii./ .13. ^ Platb XIX. Fic. 11. «.

• Reaum. iv. 375—. t. xxn.f, 7, 8.

VOL. IV. E

50 INTERNAL ANATOMY OF INSECTS.

most probably, as in the ceise of dktrus Ovis^^ is a vahe that closes the respiratory orifices. In the gad-^fiy of the ox (flS. Bovis) there are no plates at lAi€ anterior ex- tremity of the body; but those plants in the oiker end are very remarkable, attd demand particular attentic^. Each is separated by a curved line into two unequal por- tions ; the smallest of which is contiguous to the convex beily, and the largest to the concave back of the animaL This last is distinguished by two hard, brown, kidneyi- sfaaped pieces, a little elevated with the concave sides turned towards each other : in this sinus is a single, small^ white spot, which appears to be a spiracle : in the smallest poartion are eight minute circular orifices, arranged in a Kne^. . As the only communication which this grub has with the atmosphere is at its anal extremity, it has no oecasic^ for rei^iratory organs at the othe7\ The gad^ fly of tihe horse (£B. Epii, &c.) which has no communi- cation at all with external air, breathing that which ia received into ^ stomach, has these plates at both ends of the body.

iii. Respiratcrry Appendages^ ^ Th^se ntay be divide iiito ttsoo kinds ; those by which the animal has immediate e^mifmiinteation with the atmosphere, and those by which it exjtracts air fi'om water.

1. To l^egin with theirs/. These are often^fbund in in- sects which, during their two first states^ live in the waten Ko better example, nor one more easy to be examined)

■ Reaun). iv. 555. U xxxv./. 10. ss.

^ Ibid, 519—. t. xxxvii./. 3, 4,

* Plates XVI. Fig, 9. a b, XIX. Figi 9, 10, 1^, IS. a. XXIX.

Fig. 3-7.

INTERNAL ANATOMY OF INSECTS. 51

of this Structure, can be selected, than the gnat {Culex L.). You must have occasionally observed in tubs of rain>- water, numerous little wriggling worm-like animals, which frequently ascend to the surface ; there remain a while, and then bending their head under the body rapidly sink to tlie bottom again* These are the larvae of some species of the genus just named ; and if you take one out of the water and examine it^ you will perceive that it is fiirnished neur the end of its body with a singular organ, which varies in length according to the species, and forms an angle- with the last segment but one *• The mouth of this organ is tunnel-shaped, and terminates in five points like a star ; and by this it is usually suspended at the sur-* fiice of the water, and preserves its communication with the atmosphere : in its interior is a tube which is c(>n- nected widi the trachea, and terminates in several open- ings, visible undet a microscope, at the iiiotith of the or- gan* The points or rays of the mouth when the animal 18 disposed to sink in the water, are used to close it, and cut off its communication with the atmosphere. Wheri the animal is immersed, a globule of air remains at- tached to the end of the tube^ so t\M it is in fact of less specific gravity than that element, asd it is not without some effort that it descends to the bottom ; but when it wishes to rise again, it has only to unclose the tube, and it idses withcmt an effort to the surface^ and remains^ suspended for any length ef tinie< Its anal extremity is dothed with bunches of hairs, which are furnished with some repellent material which prevents their becoming wet^ : it is this repellent quality that probably causes a

• Plate XIX- Fi&. 9. a, * Ibid. d.

£ 2

52 INTERNAL ANATOMY OF INSECTS.

dimple or depresi^ion of the surface, which if you look narrowly you will discover round the mouth of the tube*.

When the gnat undergoes its first change and assumes the pupa, instead of a single respiratory appendage it is furnished with a pair, each in shape resembling a cor- nucopia, and,' what is remarkable, placed near the oppo- site extremity of the body, for they proceed from the up- per side of the trtmk**. By these tubular horns, which Reaumur compares to asses' ears *=, they respire, and are suspended at the surface.

Other respiratory, tubes or horns are more complex. The rat-tailed grub of a fly {ElopMlus pendultis) like the gnat breathes by a tube: but as if the Creator willed to show those whose delight it is to inyesttgate his works, by how many varying processed lie ciEin- accomplish the same end, this respiratory organ is of a construction to- tally different from that we have been considering. It is not fixed to the side of the tail, but is a continuation of the tail itself, and is composed of two tubes, the iiiner one, like the tube of a telescope, being retractile within the other ^. The extremity, which is very sUnder, and through which the ait finds admission by a pair of spi- racles, terminates in five diverging hairs or rays, which probably maintain it in equilibrio at its station at the surface^. As these larvae seek their food amongst the mud at the bottom of shallow pools, in which they are constantly employed, tliey require an apparatus capable of being lengthened or shortened, to ^uit the depth of

* Compare Swamm. Bibl, Nat, i. 154. L xxxi./, 5. Reaum. it. 601—. f. xliii. De Geer ti. 317—. t. xvii./. 2—8. *» Swamm. Ibid. U xxxi./. 7i 8. « Reaum. iv. 607. <* Plate XIX. Fig. \%a. * Reaum. iv. U uiAf. 2. e.

INTERNAL ANATOMY OF INSECTS. 5S

the water, that they may maintain their necessary com* munication with the atmosphere ; and for this purpose a single tube would not have been sufficient; therefore Providence has furnished them with two^ and both are extremely elastic, consisting of annular fibres, so as to admit their being stretched to an extraordinary length. Reaumur found that these animals could extend their taik to near twelve times their own length. The m^ chanism by which the terminal piece is pushed forth or retracted, is very curious, though extremely simple. Two large parallel trachea, the direction of which is firom the head of the grub to its tail, occupy a considerable por«> tion of its interior : near the origin of the tail, where they are very ample, they suddenly grow very small, so as to form a pair of. very slender tubes, but so long that, iii order to find room in a very contracted space, they form numerous zigzag folds attached to the terminal tube; when this issues from the outer tube« they conse- quently begin to unfold, and when it is entirely disen- gaged, they are become quite straight and parallel to each other. Reaumur has figured them as being united at the base of the inner tube * ; most probably, however, they do not here stop short, but, as in other instances, proceed to the end, and terminate in the two spiracles mentioned above : he conjectures that when the animal has occasion to push forth its respii'atory apparatus, it injects into these vessels part of the air contained in the body of the trachea, which of course would cause them to unfold and push forth the tube^. When this insect assumes the pupa, instead of its anal respiratory or-

» Reaum. iv. U xxx.f. 10. * Itnd. 447—.

5^ INTERNAL ANATOMY OF INSECTS.

gan it has Jbtsr respiratory horns in the trunk near the heads

The larva of the chamflBleoii^fly {Strati^omis Chamceleon) is fiirnished with a respiratory organ of a still different and more elegant structure, exhibiting some resemblance to the tentacula of what are called sea anemonies. In this larva the last joint of the body is extremely long, and terminates in an orifice to receive the air, which is surrounded by a circle of about thirty diverging rays, consisting of beautifully feathered hairs or plumes*'. This apparatus serves the same purpose with that above de- scribed of the larva of the gnat. The feathery hairs are so prepared as to repel the water, and thus to suspend the animal by its tail at the surface, and preserve a con- stant access of air. When it h^s occasion to sink, it turns these hairs in and shuts the orifice, carrying down with it an air-bubble that shines like quicksilver, and which Swammerdam conjectures enables it again to be- come buoyant when it wants to breathe*.

In the red aquatic larva of a small gnat {Chironomus plumosus) there are two anal respiratory subcylindrical horns, with the orifice fringed with hairs ^ ; and in an* other gnat( Tiptdaanntdata L. ) Reaumur discoveredyjwr*'. The larva of Tanypus maculatus^ whose remarkable legs I formerly noticed^, exhibits in the interior of its trunk two long, oval, opaque bodies, which De Geer conjec- tures may be air-reservoirs ; these, when the animal as- sumes the pupa, according to every appearance become external^ and are placed on the back, precisely where the

* Reaum. iv. 456. t, xxxi./. 1—7. ** Pl'ate XIX. Fig. 13. a, c BibL Nat ii. 44. " P|.ati: XIX. Fig. 10. a,

• Reaum. iv. /. iv./. 6. *, m. ^ Vol. II. p. 27S — .

I

INTERNAL ANATOMY OF INSECTS. &B

respiratory horns of aquatic pupae are usually situated, — > they appear to terminate in a transparent point*. The piipa of a Tipula observed by Reaumur, instead of two has only one of these respiratory organs, in the form of a very fine hair proceeding from the anterior end of the trunk, and considerably longer than the animal itself^.

It is observable that aquatic insects that come to the surface of the water for air, receive it at the anus, often carrying it down with them as a brilliant bubble of quick* silver. This is generally done by means of spiracles in perfect insects, but in the water*scorpion tribe in that state respiration is by means of a long hollow tube, con- sisting of two concavo-convex pieces which apply exactly to.each jother. This is found in both sexes, and there- fore cannot be an ovipositor, as some have thought^.

These respiratory organs, however, are not invariably confined to aquiztic larvae and pupae, for those of some jqphidivorous flies have anal ones, and the pupa of Do//- chqpus nobilitatus, or a fly nearly related to it, which is terrestrial, has likewise a pair of long sigmoidal ones on the back of the trunk ^. The pupa also of the rat^tailed larvae just noticed as having four horns, resides under the earthf the insect being only aquatic in its grub state.

2. I am next to consider those respiratory appendages by which aquatic insects, since they do not come to the surface for that purpose, i^pear to extract air for respi- raticm from the water; so that they may be looked upon in some degree as analogous to the ^f7/s of fishes : there is, however, this difference between them — in fishes, the

» Dc Geer vi. 395—. /. xxiv./. 16, 18. d. * v. /. vi./ 1, 2.

* De Geer iii. 307. /. xviii./. 1, 2, 9. •» Ibid. vi. 36. 194--. t lif. 2, 3. t.

56 INTERNAL ANATOMV OF INSKCTS.

bk>6d is conveyed in mkiute ramificaitions of the arteries to the sdr&ce of the branchial laminse, through the mem-* braiies of which they abstract the air coinbined with the water ; but as insects have no circulation^ the process in them must be different, and their branchifprm appen-^ dages may be regarded as presenting some analogy rather than any affinity to those of fishes. The firist ap- proach to this structure is exhibited by the pupa of a giiat lately mentioned {Ckironomus ptt4mosu8) ; for on each tide of the trunk this animal has a pencil consisting of five hairs el^antly feathered, which, when they diverge, form a beautiful star ; its anus also is furnished with a &n-shaped pencil of diverging hairs*; .

On most pf the abdominal segments of the larvae and pupae of the Trichaptera [Phrygariea lu.) are a number of white membranous floating threads, arranged in bun- dles, ^^oMr on each segment, two above and two below, and traversed longitudinally by several air-vessels or hrdnchine^ which run in a serpentine direction, growing more slender as they approach the extremity, and iii some places sending forth very fine ramifications, — these are their respiratory organs**. The caterpillar also of a little aquatic moth (5q^^5 stratiotalis) at first sight appears to be covered on each side with hairs, but which examined "under a microscope are found to be branching fiatti^h filaments, each furnished with tubes from the irache€e. TTiese cateipillars have also the semblance of spiracles, but apparently found in the usual situation «• The larva of a little beetle often mentioned in my letters {Gyrinus

• Plate XVI. Fig. 9. a. h,

b DeGeerii. 539—. t xi./. 12, 16, &c.

• Jbid, i. 5S6— . t, xxxvii./. 2—6,

INTEBNAL ANATOlfy OF' INSECTS* 57

Naiator\\s furnished on each side of every abdominal seginent with a long, hairy, slender, aciite, conical pro- cess,, of the substance of the segment, through each of which an air-tube meanders ; the last segme|it but one \\BsJbur of these processes, longer than the rest*.

Laminose or fbliaceous respiratory app^dages distin- guish the sides of the abdomen of the larvae and pi^ae of the Ephemera^ whose history you found so intei^esting^. In them these organs wear much the appearance oi giUs. In the different species they vary both in their number and structure. With regard to their number, some have only six pair of them, while others have seven. In their structure the variations are more numerous, and some- times present to the admiring physiologist v^^ beautiful forms ^. They usually consist of two branches, but occa- sionally are single, with one part folding over the other, as in one figured by Reaumur, which precisely resembles the leaf of some plant, the air-vessels or bronchite in con- nextcm with the trachem brainQhing and traversing it in all directions; like the yeins, of leaves**. The double ones differ in form. In the larva and pupa of Ephemera ml- gata thisre are sixoi these double false gills on each side of the abdomen, the three last segments being without them ; each branch consists of a long fusiform piece, ra- ther tumid and terminating in a poiiit, which is fringed on each side with a number of flattish filaments, blunt at the end. An air-vessel from the trdchea enters the gill at its base; is first divided into two larger branches,

* De Geer iv. 362—. U xiii. J* 16—19.

b Vol. I. p. 279—. U. 369—.

^ See Reaum. vi. ^ xlii.— xlvi. and Plate XXIX; Fig. 3 — 5.

** Reaum. Ibid, t xlv./. 2.

98 INTERNAL ANATOMY OF INSECTS.

tttch of whici) enters a branch of the false ffiH. These branches send forth on each, side numerous lesser rami- ficstions, one of which enters each of the filaments". In another species (£. vespertina) each fitlse gill presents the appearance of a pair of ovate leaves with a long acumen, and the air-vessels represent the Midrib cd* the lea^ with veins branching &om it on each side^ ; end, to name no more, in E.Jusco-grisea, one branch represents the leaf of a Begtmia, the sides not being symmetrical, with its veins, while the other consbts only of numerous branching iUaments'^. In other aquatic larvte, as in that ef'the common May-fly (SemMis lutaria Latr.), these ^>- ptsodages ctmsist of several joints''.

By the above apparatus these aquatic animals are en- ^ed to separate the air from the water, as the fish by their gills ; but how this s^aratlon is znade has not been preoisely explained. The false gills in many species are kept in continual and intense agitation. When they move briskly to one side, Reaumur conjectures they may receive the air, and when they return back they m^ emit it °. This brisk motion probably disengages it from die water. In many species, when in repose, th^ are hoA upon the back of the animal ', but in others they are not".

The larvs of the Agrionid/B appear to respire like those of the Ephemera, &c. by means of long foliaceous

• PtATE XXIX. Pig. 5. De Geer n. 624—.

" Plate XXIX. Fig. 4, De Geer Ifnd. 647—.

" Plate XXIX. Fig. 3. De Geer Ibu(. 653—.

■' Plate XXIX. Fig. 6. De Geer Ibid. 727—.

' Reaum. vi. 465. ' Ibid. I. xlii./. 4, 5. Dc Geer ii. 633.

• /6irf. 648. (. xvii./: II. 12.

INTERNAL ANATOMY OF IKSBCTS. 5^

ImJunse or &lse gills filled with aiil'-vessels ; but insteul of being ventral^ they proceed frorti tb^ unus. They are three in number, one dorsal and two lateral, perpendi- cular to the horiaon, of a lanceolate $hape$ beautiftilly veined, with a longitudinal middle nervure, bom which others diverge towards the margin, which are probably bronchice. They arc n»ed by the animal, which swims like a fish, as fins, but it does not appear to imbibe the water like the other LibeUtdin^^ nor to propel itself by ejecting it,— ^a circumstance which furnishes an additional ^argument for the moire received opinion^ that thisaelion in them is for the purpose of respiration as much aa for motion ^

The larvae and pupae of the Libelltdince^ receive the water and air that they respire by a large anal aperture, which is closed at the will of the a^^kilBal by jfive hard^ moveable, tnangular, concavo«conve:^ pieces, all very acute and fringed with hahra. These pieces are placed so that there is one above, which is the largest of all ; one on each side, which are the smallest, and two below ; when these are closedi they form together a conical point ^. Sometimes only three of these pieces are conspicuous^^ : diree other cartUagina«(s pieees> resembling the valve of a bivalve shell, <^bse the passago within the pointed pieces^. At this orifice the water is received ; and when, by an internal process to be de$qribed slberwiu'ds, it haa parted with its oxygen, is again e:$pelled»

Under this head 1 shall mention a &ct which may be connected with respiration of the insects concerned. In

» Vol. hi. p. 154. De Geer ii. 697--. t. xxi./. 4, 5, VZ, •» De Geer Ibid, 606—. L \nii.f, 6. •^ Reauni. vi. 393. L xxxvi./. 8, 9. 1. 1. ^ Jbid, 395. t xxxvi./. 8—9. c. c.

60 INTERNAL ANATOMY OF INSECTS.

dissecting a species of Nociua related to N. Pronvibaj but I do not recollect the particular species, — at the base of the abdomen of the male I discovered two bunches of long fkwnrcploured parallel hairs, planted each in an oval plate, plane above, but below convex and fleshy ; while the plates remained attached to the insect, they appeared to have a distinct pidsation. The hairs, which are about half an inch long, diverge a little, and form a tuft not very unlike a shaving-brush^. I have not since met with this species, but I have preserved the brush and scale. Somewhere in Bonnet's works, but I do not recollect where, I have since found mention of a similar fact in another moth*

II. Having considered the external respiratory organs of insects, by which the air is received^ we are next to consider the internal ones, by which it is distributed. These are gilU ; tracheae and bronchia ; and sac^ or pouches^.

i. Gills {Branchta^.) Having lately described what may be denominated fake gUls, or branchiform ap. pendages, I shall now call your attention to what may be denominated trtie ones, which are peculiar to the Arachnida Class : but what is remarkable, the animals that breathe by them are very rarely inhabitants, of the water, so that their functions cannot be perfectly analo- gous to those of fishes.

In the Scorpion^ on each side of the four first ventral segments a spiracle may be discovered, which has no

• Plate XXIX. Fig. 21. »» Marcel de Serres {Mem.

du Mtu, 1819. 137, &c.) calls the ttibular trachea that receive the air, arterial tracked;, and the vesicular ones, which act as retervoirs, pulntonart/ trachea, * P^iATE XXIX. Fig. 1. 2,

INTERNAL ANATOMY OF INSECTS. 61

lip as In other insects, but is merely a circular orifice* These orifices do not lead to tracheae or vesicles^ but to true giUsy which are situated belo<y a muscular web which clothes the internal surface of the crust Each gill con- sists of many semicircular very thin plates, of a dead milky white, which are connected together at the dorsal end like the leaves of a book. There aj^iear to be more than twenty of these leaves, which when- strongly mag^ nified look transparent and destitute of any vessels. Each g31 is fastened at the back to the spiracle*. In the spiders also, gills are discoverable, but differently cir- cumstaaced. On the under side of the abdomen, near the base, is a transverse depression, on each side of which is a longitudinal opening leading to a cavity, which is covered from above by a cartilaginous plate. In this cavily is situated a true g^ll, which is white, triangular^ and covered with a fine skin ; the leaves of this gill are far. more numerous and much finer and softer than those of the gills of the scorpion. On account of their softnessr they have ofi»n the appearance of a slimy skin ; but their lamisiAtedr structure shows itself very clearly in old spe* cinieoi^. and in such as have been immersed in boiling water**;'.

iL Trachea and Bronchia^, Parallel with each side of xhe body of most insects and extending its whole length, run two cylindrical tubes, which communicate wit^ the spiracles!^, and from which issue, at points opposite to those organs, other tubes which ramify ad infinitum^ ai^d

» Treviranus Ata^nid. 7 — . t» !• /. !• r. /. lO. Comp. iV. JXcL d^Hitt. Nat. XXX. 4 IGF. Latmlle calls these gills Pneumobranchei. * Treviranus IbitL ^. Plate XXIX. Pig. 1. « Plate XXI. Fio. 3. a 6. «» Ibid. «.

62 INTERNAL ANATOMY OF INSECTS.

are distributed to every part of the body *. The first of these tubes are called the trochees and the latter the bront^icBu This structure appears, however, not to be universal: it is to be found in caterpillars and many Dipterous \BXi.Bd\ but in that of the rhinoceros^beetle {Oryetes nasicomis) and other Lamellicoms, ihebronchiig branch directly from the spiracle, the bottom or interior mouth of which is lined by a m^oabrane from which they proceed^.; something similar has been observed to take place in many insects in other states^ as the common cockchafer^; the pupa of SmerintJms Poptdi^; in the CicadcB^ ; in the Locust taribe^; and many oth^s. In the Cossusy or larva of the great goat-moth, th^ trachea commences ynAk the first spiracle, and finishes a littlb beyond the laat, after which it diminishes considerably in diameter, and terminates in several branches or bron* cki^Pf which proceed to the anal extremity of the body'. The bronchia which originate fit)m the trachece in the vidmity of each spiracle, may be considered as consisting m general of three packets \-^^dormL ones, whicb are dis^ tributed to the back and sides of the anunal ;^ visceral ones, which ent^ the cavity of the body, and are lost amongst the viscera and the caul; and gastric oiies, which dipping from the trochees overrun the lower part of the sides and belly''.

Hie trachea and bronchia consist of three tunics * ; the

» Plate XXL Fig. 3* A. *» Sprengel Conmentar. U i./. 1*

« Ib%d.f. 10. <* Ibid, t. ii./. 15.

* Malpigh. Be JSombyc. U iiL/. 3. ^ Ibid. t. iv./. 1.

' Lycmn€t Attat. 101. ^ Ibid.

^ Sprengel (ubi supr, 16.) says that he never found more than /tew/ but as Lyonnet affirms thcb| he has very often separated them (102), his accuracy cannot be questioned.

INTBRNAL ANM'OMy OP INSECTS* 6S

Jirst or external one is a thickish membrane^ strengths ened by a vast number of fibres or vessels, which form round it a number of irreguliu* circles ; the ieeond is a membrane more thin and transparent, withiout a vascu^ lar covering^ ; the third is foimed of a cartilaginotfi^ thread running in a spiral direction, which may be easily unwound^. This structure gives a great elasticity to these organs, so that they are capable of considei'dtble tension, after which they return to their usual length ^« The BronchiiB are cylindrical or slightly conical, insen- sibly diminishing in size as they leave the trunk, in which diey originate. In larvse, after losing their spiral fibre^ they f^pear to terminate in membr^e, but in perfect insecte they pass into vesicles^. luvthe Cossus the truchea is flattened, and in every segment, exc^t the first and two last, is bound by a fleshy cord four or five times as Aide as its threads. Where this occurs, there is a slight constriction^-^probably here is a sphincter, by the con-^ tractimi of which Lyonnet supposes the tirackea may be shut wh^i it is necessary to stop the passage cf xht air, and direct it to any particular point ^. The structure here described is admirably adapted for the purpose it is intended to serve; for had these vei^els been composed of membranef they could not possibly have been prevented from collapsing; but by the intervention of a spiral cartila-^ ginous thread this accident is eftectually guarded against, and the necessary tension of the tubes provided far. However violent the contortions of the insect, however

* Lyonnet Anat, J 03.

^ IhicL Cuv. AnaL Comp, iv. 438. This author says that the m- termediate tunic is the spiral thread (437). ^^ Lyonnet 10^.

** Ibid. 104. Sprengel Comwentar. 17. * Lyonnet Ibtd*

64 INTERNAL ANATOMY OF INSECTS.

small the diameter of these vessels^, they are sure to re- main constantly open, and pervious to the air. And by this circumstiEuice. they may be always distinguished frcHR the other organs of. the animal, and likewise by their pearly or silv^y hue, for from being constaatly filled with air, these tubes, when viewed under a powerful mi- croscope in a recently dissected insect, present a most beautiful and brilliant appearance, resembling a branch- ing tree of highly polished silver or pearl : — though sometimes they are blue, or of a lead colour, and some- times assume a tiiit of gold. In the dead insect theiarger tubes soon turn brown, but the finer ones preserve, thdir lustre several weeks ^. The ramifications of the tracheal tree may be seen without dissection through the trans- parent skin of the common louse ^ and most of the thin- skipned laryae.

You will not expect to view in this way the minuter ramifications of the bronchice^ when I have mentioned their number and incredible smallness. Nothing butlhe scalpel of a Lyonnet and the most powerfiil leiises are adequate lo trace ^e extremities of these vessels; and even with every belfl, they at last become so incan'ceiTaf- biy. slender as tQ;elude the most piercing si^t. That ilhistrioiis anatomist f6und that the two irathea f£ the larva of the Cobsus gave, birth to 236 broi^hial tubesy and that these ramify intQ no less than 1336 smaller tubes, to which, if 232, the number of the detached branchiae, be added, the whole will amount to 1804? branches'". Surprising as this number may appear, it is not greater

» Lyonnet 102. Malpigh. DeBombt^c. 12, Reaum. i. 130. »> Swamra. Bibl. Nat. t. \uf, 7. *= Lyonnet 411,

INTERNAL ANATOMY OF insects; 65

than we may readily conceive to be necessary for 'com- municating with so many different parts. For, like the arterial and venous trees, which convey and return the blood to and from every part of the body in vertebriate animals, the bronchia are not only carried along the in- testines and spinal marrow, each ganglion of which they penetrate and fill, but they are distributed also to the skin and every organ of the body, entering and travers- ing the legs and wings, the eyes, antennae, and palpi, and accompanying the most minute nerves through their whole course. How essential to the existence of the animal must the element be that is thus anxiously con* veyed by a thousand channels, so exquisitely formed, to every minute part and portion of it! Upon considering this wonderful apparatus we may well exclaim, This hath God wrought. Of id this is the work of his hands.

Though in general there is only a^^a/r of tracheae^ yet in some Urvse a larger number have been discovered. In those of the Libelhdime there are six. According to M. Cuvier, Reaumur, who mentions only four^ overlooked the two lateral ones that are connected with the spira^ cles*. The reason of this and other parts of their in-» temal structure I shall explain under the next head* In the grub of the gad-flies of the horse {(Estri gastrin cola eiark), Mr. B. Clark discovered eight longitudinal trachea, — six arranged in a circle and tox) minute ones, which appeared to him to terminate in a pair of exter- nal nipf>Ies (spiracles) in the neck of the animaP. This

•

• N. Diet. d^Hitt. Nat, xvii. 641. Reaum. vi. 397. Plate XUll^ Fig. 8. shows three of them ait a.

*• Essay on the Bots, S^c. 23j t. If. 7, 32, &c.

vol.. IV. F

66 IHTERNAL ANATOMY OP INSECTS.

k a Mngnlar anomaly, as the other (E^rida have only a pair of trachea ■.

JiLBespiraton/ Sacs or Pouches. Besides thar ftvwAe* and bronchia, many insects are furnished with a kind of reservoir fot the «r, under the form of sacs, pouches, or vesicles. These are commonly formed by the bronchial tubes being dilated at intervds, especially in the abdo- men, into oblong inflated vesicles ; from which othta- bronchial tubes diverge, and agwn at intervals expuid into smaller vesicles, so as to raihibit ho um^t resem- bhmce— as Swammerdam has observed with respect t» those of the rhinoceros-beetle— to a spedmen of Fitaa vesiculostis. Cuvier compares them in tha Lamdhcon* beetles in general to a tree veiy thickly laden with leaves" t and Chabrier obsttves that they particularly occur in the intestinal canal'. This stracture of the pulmraiary or- gans may be seen also m the common hive-bee, and other Hymenoptera ,■ but the vesides are less numerous, and those at the base of the abdomen much larger than the rest''. These vesicles^ by a vay rough disseotioo, may be distinctly seen m the rfjdemen of the cockrfwfer, wYaOk appears to be ahaost filled wUh thran*. Not being oaet- posed of cartilapnous rings like the air-tubes, bnt of mere membrane, if a pin pierCes one, the air that inflates it esc^es, Mid it collapses. In the larva of a Uttle gnat ( Corethra adicifcrmis) the trachea ^pear to proceed fi?om

• Etiag on the Bolt, *o. 49. VaJianieri i. 101. (. vi./. 4. &c. *■ BUt. 2/at. i. 149. e. /. xxix. /. a. Cuv. Anat. Comp. iv. 439. Malpigh. De Bombyc. t. iii./ 2.

' SurU Voider Im. c. ii. 338. note I.

■" Swaraic. BOt. Wri. (- xvii./. 9. Curier Ibid. 440.

INTERNAL ANATOMY OF INSECTS. 67

apair of oblong vesicles of considerable size^ in^the trunk, and towards the anus they form two other smaller ones^, -^upon piercing the former, De Geer observed a consi- derable quantity of air to make its escape^. Another sper . cies, probably of the same genus, described by Reaumur, exhibits something similar^.

But one of the most remarkable structures, in this re- spect, is to be seen in the larva and pupa of the dragon- dies {LibeUtdina)» I have before noticed the number of their trachea^ but I shall here describe their whole in- temal respiratory apparatus. I must observe that Reann mur, Ciwier, and most modem writers on the physiolo^ gical department of Entomology, have affirmed that they respire the water^ and that they receive it for that pur- pose at their anal extremity: but M. Spretigel, from itaving obseirved in fhe larvae abdominal spiracles, is Un- unwilling to admit this as a fact<^; and De Geer also seems to hesitate upon it, especially as he discovered that tlie animal seemed to absorb the water to aid it in its motions^* But when we consider that it is by the aetionT of a pneumatk apparatus that the absorpticm and ex- pulsion^ of the water takes place, aiid that the animal when it has-been taken out of l£at eli^eht, upon being restored to it, imtHediately has eager re<6ourse to this ac- tibti^, we shall Sbel indtiied rather to i^opt the opinion 6f ^lose great physiolbgi^ Reaumur^- Lyomiet, and Cuvier, and admit that it absorbs water for the purpose 6f respirati'oni I shall how estplain how this takes place*

» Plate XXIX. Fig. 10. a. " Ibid. b.

* De Greer vi. 374. ^ Reaum. v. 40. t, \\,f» 4, 7-

• Sprengel Comment, 4. ^ De Geer ii. 667, 675:

f Reaum. vi. 394-:-,

F 2

6$ INTERNAL ANATOMY OF IN&ECTS.

The pieces both internal and external that close the anal orifice have been before described ; the others employed in ih.e admission and expulsion of the water are evidently respiratory organs. When this orifice is opened^ the parts that are above it are drawn back in an opposite di- rection, so that the five last segments of the abdomen be- come entirely empty, and form a chamber to receive the water that enters by it When the water is to be ex- pelled, the whole mass of air-vessels which had receded towards the trunk, is pushed forwards, and forms a pis- ton that again expels the water in a jet It consists of an infinite number oibronchiaj entangled with each other, which proceed from the middle and posterior end of the trachedB. M. Cuvier in the interior of the rectum of the larva discovered twelve longitudinal rows of little black spots, in pairs, which exhibited the resemblance of six pinnated leaves. These are minute conical tubes, of the spiral structure of trachea^ which decompose the wa- ter, and absorb the air contained in it. He also disco^ vered that each of these tubes gave birth to anotber out- side the rectum^ which connected itself with one of the six great longitudinal trachea ; two of which are of enorr mous size, and appear to serve as reservoirs, since they furnish air by transverse branches to two other tubes ; they have each a recurrent brandij which follows the course of the intestinal canal, and fiunishes it with an infinity oi bronchia^. These trachea are found in the perfect insect The principal ones in some send forth many branches, terminating in vesicles, which in shape

» Reaum. vi. 394—. Cuv. Anat. Comp,iv, 440—. JV; Diet, d'WsL h^at. xvii. 540—.

INT£ltNAL ANATOMY OF INSECTS.: 69^

resemble the seed-vessels of some species of Thlaspi^\ while others appear to form a file of oblong ones*. Near each of their spiracles also is a vesicle which ap- pears to be a reservoir ^

' Bat this kind of structure is not confined to insects strictly aquatic* Even such species of terrestrial ones a& live upon aquatic plants, and are, consequently, necessa- rily or accidentally often a considerable time under wa- ter, are furnished with some apparatus by means of which diey can exist in this element for a considerable period. For example, most of the Weevils {Curctdio L.) die in a short time if immersed in water; yet the species of the genera Tanysphyrus Germ. Bagous Germ., and that to which C. pericarpius L. belongs, and which feed on aquatic plants, can exist for days under water^ as I have ascertained by experiment C. leucogaster Marsh, and another of the same tribe, swims like a HydrophiluSf and will live a long tim6 in a bottle filled with water and corked tight Other insects also, that are not at all aquatic, have pneumatic pouches. A striated or channeled ve- sicle I have found under the lateral angles of the collar in the humble-bee, where Chabrier supposes the vocal spiracles are situate ; and also at the mouth of the spira- cles of the metathorax in Vespa^ &c.*^ In Sphinx Li- gustri the bronchice terminate in oblong vesiculoso-cel- lular bodies, almost like lungs** ; in Smerinthus Tilia: these are preceded by a simple vesicle bound with spiral fibres^. M. Chabrier thinks that these air-bladders of

» Plate XXIX. Fig. 9. a, 5. Reaum. vi. 418— . 450.

*» Cuv. Anat Comp, iv. 441. '^ Vol. HI. p. 585.

** Sprengel Comment, 17, t, iii./. 24. * Ibid, L '\.J\\\,

70 INTERNAL ANATOMY OF INS£CTS»

insects, amongst oth^r flinctions, give more fixity and force to tlie muscles for flight'.

Many phjrsiologists have seen an analogy between the spiral vessels of plants and the trachea of insects; and some of great name, as Comparetti, Decandolle^ and Kieser, have thought that in some instances they termi^ nated in the oscida or cortical pores : but Sprengd coa-* tends that they are not accurate in this opinicm^. In &ct, the principal analogy seems to be in the ^iral structure of both these vessels*

«

Having considered the different organs o>f respiration both external and internal, I shall make a few further observations upon this function. We know litde more respecting the mode in which insects respire^ except that they breathe out the air by the same kind of organs by which they receive it,— rnamely, the spiracles^ or their re- presentatives. This has been satisfactorily proved by Bonnet, who showed that the experiments by which Reaumur thought it established that insects inspire by their spiracles, but exspire through the mouth, anus, or pores of the skin, are founded on an erroneous assump- tion. This physiologist, having observed on the surface of submerged insects numerous bubbles of aii*, concluded that they had passed through the above orifices^: but Bonnet found by various experiments carefully conduct- ed, that this appearance was caused by air which ad- hered to the skin and its hairs, and that when the access

* Sur le Vol det Ins. c. ii. 336. note 1.

*» Sprengel Comment, 13—. These oscula or pores in the straw of Tritieum hifbemum, as figured bj Mr. Bauer's admirable pencil, (Sur J. Banks On the BUgkt, ^c. t. ii./. 3.) exactly resemble the spi- racles of insects. « Reaum. i. 130.

INTEBKAL ANATOMY OF IN8BGTS. ?1

of this was precluded by carefiiUy moistening the skin widi water previously to immersion, this accumulation of atP-bttbbles on its sur&ce did not take place ^« And in a variety of instances he observed large ones issue firom all the spiracles, especially the anterior ones. These bubbles sometimes wer^ alternately emitted and absorbed without quitting the spiracle^, and at others were darted with force to the surface <^ the water, where they ap- peared to burst with noise *^, This author is of opmion that the Jirst and last pair of these organs are of most im« portance to respiration^. Reaumur subsequently owned that B(HM»et's arguments had shaken his (pinion ^; and some observations of his own, with respect to the respi- ration of the bot of the oj7, go to prove that expiration and inspiration are not by Hiesajne spiracles ; for he found that the air in this animul was expired by the eight little lower orifices before mentioned^, from which he clearly saw the air-bubbles issue— the upper one he conjectures receives the air^. As the only comnranication that this grub has with the atmosphere is by its posterior extre- Hiity, it follows, reasoning from analogy, that the ante- rior respiratory plates of Dipterous larvss, which may be r^arded as repres^iting the spiracles of the trunk in in- sects in genial, are destined for the escape of the air, after it has parted with its oxygen, received by the anal ones^ So that there seems very good ground for

* Bonnet CEuvr. ui. 39—. ^ Ibid. 43. ^ Ibid. 50.

d md, 69. • De Geer ii. 117.

f See above, p. 50. ■ Reainn. iv. 5^.

^ Mr. B. aiA- thinks that he has discovered spiracles in this larva in the usual situation, {Euay on the BoU^ ^c, 48. /. ii./. 3.) but they are probably analogous to the spiracuHform tubercles of CE. Ovii. Reaum. iv. 566. /. xxxv. 17—- 10. t. Valiisnieri (Eaperienz. Jjrc. 136) notices them.

72 INTERNAL ANATOiMT OF iKSiECtS.

M. Chabrier's opinion that inspiration is ordinarily by the abdominal spiracles, and expiration by those of the trunk of insects ^. He seems to have been led to the adop- tion of this opitiion, not so much by experiments similar to that of Reamnur just stated, but by observing that in many instances these, two sets of spiracles differ from ea^h other, the latter having a convex and the former a con^ cave mouth or bed**. In some cases, however, — for in- stance duringflight,— he supposes the spiracles of the trunk may receive as well as emit the air ^ : he likewise is of opi- nion, and it seems not improbable, that by me^ns of these openings in the trunk, from the rifsh of the superfluous air through them, insects produce those sounds for which they are remarkable, — as the humming of bees and flies. In the former he thinks the sound is produced by the pneumatic apparatus covered by the ends of the collars while in the latter he attributes it to the spiracles in the metathorax behind the wings attended by a poiser^. I incline, however, to M.Dufour's opinion % — that the vocal spiracles in the Hymenopteray as well as in the Dipteral are those behind the wings. Perhaps both theories may be right; for if you take any common humble-bee, you will And that, in the hand, it produces one kind of sound when its wings are motionless, and another more com- plex and intense when they vibrate* In numerous in- stances, however, there is no very striking external dit ference between the spiracles of the trunk and those of the ahdomen : this observation applies more particularly to the caterpillars of Lepidoptera ; but whether these re- ceive the air by those of die abdomen, and return it by

^ Sur le Vol des Ins. c. i. 423.

^ Ibid. 454. and c. iv. 66. note I. "^ Ihid. c. i. 453.

- Ibid. 459, 456. ^ Ibid. 459.

INTERNAL ANATOMY OF INSECTS. 7S

those of the trunk, has not yet been ascertained; and indeed, too little is at present known upon the subject, and too few &cts have been collected, to admit of dog- madzmg.

The external signs of respiration in insects are not uni-- versally to be discovered. The alternate contraction and expansion of the abdomen is, however, very visible, in some beetles, bees, the larger dragon*fiies, and grass- hoppers. In one of the latter, Acrida viridissima K., Vauquelin observed that the inspirations were from fifty to fifiy«-five times in a minute in atmospheric air, and from sixly to sixty-five when in oxygen gas *. But M. Chabrier has> given the most satis&ctory account of these signs : The abdomen, says he, is the principal organ of inspi- ration ; it can dilate and contract, lengthen and shorten, elevate and depress itself. In flight, in elevating its ex- tr^ooily at the same time with the wings, it contracits it- self pushes the air into the trunk, and diminishes the weight of the body by the centrifugal ascending force ^. Id the ma^iity of insects perhaps the dilatation of the abdomen takes place by the recession of the segments from each other by means of the elastic ligaments that connect them ; in others, as ih&Djfnastid^e^ Solpuga^ &c» by the longitudinal folded membrane that unites the dor- sal and ventral segments— in the LibeUidime by similai* venial folds; iand mCimbex by membranous pieces in the first dorsal s^ment, which. De Geer observed was elevated and depressed at the will of the animal ^« . Air is as essaatial to insects in their pupa as in their

* AfMol, de Ckim, xii.

^ Sur le Vol des Im. c. i. 423, 4d4. c. iii. 344. c. iv. 66.

« De Geer ii. 946—.

7t IHTSRKAL ANATOUY OF IH8ECTS.

larva or pafeet states. •■'Lyaaaei, bowerer, Musschen- broeki Martbet, uid some other pbysiolt^ts, Lave doubted whetber ^itxceat pnpK broathed ■ ; but Reau- mur and De Oeer seem to have proved that tbey do^ i and M" tbromn into wate^; ihe same proof of rsq>iration, by the ewwion and rBtractioii.i^ a bubble of air takes [^ace, as in tbe larvn; and De Geo- found tbat if one be trau^rred upder wfUer &om mie spiracle to uK^hsr, it Till be absorbed by it^. Indeed, unless these papee had breathed) vbere would have been the necessity Sat the piracies ^th vhidj all are iumished? It is remark- ably however, that all Ihew sfHracles do not seem of equal iiii|)CHtaoce in this respect. Beanmur found tbat if the posterior spiracles only were doaed with oil, the insect Bu£^red no iiyuiy ; but that jf the anterior ones were similarly treated, it in&Uibly died'*. The ro^irm- ti^n bpwfiver of pupse seems more perfect in those that hare rec^pdy assumed that stot^ than in those that aie more advanced towards tbe imago ; in which at first, from If^immr's experimfots^ it aiq»ara that the posterior piracies were stopped ; and in olbera still older, from Mosscbenbroek's ', ereo the anterior ones. Those qnie». cent pupw that during that sta^ remain fubmergedt re- spire air- Be Geer has given an interesting record of ibis, in the case of Bo^ ttratiolans. This insect spins a double cocoon, the outer one thin, and the inner one of a dose texture. In the pupa there ore three pair of ctmspicuous tfurades on the second, third, and iburdi segments of the abdcHuen, which are placed cHt cylindri-

' Lesser, L. i. 124. uote *. Ljomiet ..Jiiatom. pref. xu. DeGeerii.

H. " Resum. i. 389—. Do Oeeri. 37—.

■ Ibid. 40. ■■ Reaum. i. 400. ■ /AW. ' De Geer ii. 129.

IHTBRNAL ANATOMY OF INSECTS. 70

eal tubesy and they appear to have no other air-vessdis. Hie jrespiiiUory gills of the larva having vanished, like soQie others of the same genus, they know how to snr* round themselves widi an atmosphere of air in the midst of the water, so that the interior. of their inner coeoon is imparviotts to the latter element-«-rhow lihey renew the air has not been ascertained. Though they respire air, water is eqwdly necessary, for the animal died when kept outofwatar^.

The great majority of insects respire in much the same manner in aH their states, particularly as to their ej^/ernoi organsi for when the larva breathes by the lateral spira- des, the pupa and imago usuaUy do the same. The con* verse of this, faoweveiv by no mefms holds ; for it not un* fireqnently happ^is Aat the two latter breathe by means of lateral spiracles, though lliey received the air in their larva state by an apparatus altogether different. Tlius die iarvffi of many Dipiera breathe by an anal tube, while the pupa and imago fijillow the general system. Some^ times ^ tdbe of insects bseathe by an apparatus quite difl^sDent in' all their 8tates,i» we have seen to be the case with the oommifin gniit'*, which has an anal res{Mratory tube iait^ first state, thorifeic reiqnratory horns in its se^ cond^vtidi die ordinary lateral grades in its third.

Oiange&ilso take place in their internal organs. In the larvae the respiratory ajqiaratus, especially the tra^ cheal tubes, is often much larger and more ramified than in the imago ; and ast the former is the prindpnl/eeding state, there seems good ground for Mr. B. Clark's opi-

* De Geer i. 531—. ^. zxxviL/. 13. s. Compare Reaum. ii. 386—. ** See above, p. 51— •

76 INTERNAL ANATOMY OF INSECTS.

nion — that the respiration is intimately connected with the conversion of the food'* In the imago, there ap- pears to be more provision for storing up the air in vesi- cular reservoirs, than in the larvcu /Wonderful is the mode in which some of the changes in the internal struc- ture,, which these variations indicate, must necessarily take place. They are, however, probably not more sin- gular than those which less obviously occur in the air- vessels of all insects in their great change out of the larva into. the piipa state. But having before enlarged on this subject, I' need not repeat my observations^.

The access of air is as necessary to insects even m their egg state ^, and in many cases its presence seems provided for with equal care, by means as beautiful as those Messrs. Home and Davy have shown to occur in the oxygenation of the eggs and foetuses of vertebrate animals^. It is only necessary to view the admirable net-work of air-vessels which Swammerdam discovered spread over the surface of the eggs of the.hive-bee whil^ in the ovaries % — ^a provision which, from analogy, we may conclude obtains generally ; from the importance which nature has attached to the oxygenation of the germ while in ;the matrix. And juicing from anialogy, we may infer that the access of this element is as carefully secured after the egg is laid, as before. The eggs of most insects being of a porous texture, often attached to the leaves of

* In Lmn.Tr0ns. Ill 30^. »• Vol.111, p. 196—.

*' Spallanzani found that the eggs of insects placed under the ex- hausted receiver of an air-pump, or in any small closed vessels, did not hatch, though every other condition for their developement wasi present. Opusc, dePhysJ 1 141. «« PhUos. Trans. 1820. 213.

« BiU. Nat. i. 204. b. t. xix.f. 5.

INTERNAL ANATOMY QF INSECTS. 77

plants, and some of them embedded in the very substance of a leaf or twig^, are in a situation for the abundant absorption of oxygen: and the pouch of silk in which the eggs of spiders and Hydrophili are deposited, may probably, from Count Rumford's experiments, be of uti- lity in the same point of view. In the case of the 7W- chapter a and other insects^ whose eggs are dropped into the water enveloped in a mass of jelly, this substance per- haps serves for aerating the induded embryo, in the same way with the jelly surrounding the eggs of the frog, dog« fish, &c« It would be desirable to ascertain whether the former jelly be of the same nature as the experiments of Mr. Brande have shown the latter to be ^. It is not im- probable that the singular rays that terminate the q;gs oiNepa^ may in some way be connected with the aera«« tion of the egg.

To what I have before remarked with regard to the xnkd heat of insects % I may under this head very pro- perly add a few further observations. I there stated, that the temperature of these animals is usually that of the me- dium they inhabit, but that bees, and perhaps oth^r gre- garious ones, fiurnisb an exception to this rule ^ A con- firmation of this remark is afforded by Inch, a German writer, who, upon putting a thermometer into a bee-hive in winter, fopnd it stand 27^ higher than in the open air; in an anthill, he found it 6^ or 7^ higher ; in a vessel containing many blister-beetles, {Caniharis vesicataria Latr.) 4^ or 5® higher. A thermometer, standing in the air at 14^ R., put into a glass vessel with Acrida vitidis^

» Vol. I. p. 449—. IIF. p. 76. ^ IMd. 68—.

e PhUos. Trans. 1820. 218. ^ Vol. III. p. W.

« Vol. II. p. 220—. ^ Ilnd. p. 214.

?8 INTERNAL ANATOMY OF INSBCTS*

sima^ in nine minutes rose to 17% and a similar re^lt was observed with respect ta other insects*. Dr. Mar- tine says that caterpillars have but two degrees of heat above that of the air they live in ^, Cote<^iterdus insects are said to move dlowly md with difficulty when the thermometer sinks to 36% to become torpid at S4% and to lose muscular irritidnlity at a. lower d^ree^^ I have befinre observed that some insects wiU bear t(> be ftoixs& into an icicle, and yet survive^ : they share this power yf^ reptiles^ fishes, and amphibia. But, however small: the excess of it in some insects above that of the medium- they inhabit^ it proves thatthey possess the power of gene- rating heat Whethlsr, like the warm-blooded animals, they generally possess that of residing heat by perspira- tion, &C. is not so dear. Yet the heat to which some can bear to be exposed, basking at noon, as I>p. Clai*ke iolbrms ur% on rodcy abd sandy places, exposed to the fill! action of die sun, appears suffieiexil, if not resisted by some principle of counteraction, to roaiit them to a cinder. That bees persnpuf^; i» well knowu, but probably

When die respiration Of insects iJ» suspe«d<^ \^ ittii«^ mersion in any fluid, it is often resumed^ «ven when it has beeil ks^ and l^ey^ are apparently dead, if they be brought into dontact widi the atmosphiar^. ReaiiniUI* found this to be the case with bees ^ ; and Swamnaetdam^ tdls us that die maggot of the cheese-fly {Tyr&p%itgaCdim K.) lived six or seven days in rain-water^ : he found it

* Inchy c. iv. Ideen zu Einer Zoocheniie, 68-—.

• »» On Themum. 141. « Carlide in PkSos, TfaHs. 1805. 25.

* Vot, 11. p. 231. * Traneh ii, 48^.

' Reaum. v. 540. ' Swamm. JBi6l» Nat. ii. 65; a.

INTERNAL ANATOMY OF INSECTS* 79

r

SO difficult to kill the larva of Siratyamis Charmdeon^ which he first immersed twenty-four hours in spirits of wine^ and then put them several days in water, without killing them, — ^that he lost his patience, and dissected them alive. He tried to drown them also in vinegar, in which they held out more than two days^.

That the suspended animation and subsequent death of ttiost terrestrial insects when thrown into water is caused by the want of aitj is evident from this, — ^that the same dOTect ensues if the spiracles be covered with any oily or fittty matter. In this ease too, their vital powers soon become suspended : they revive^ if the suffocat- ing m^itt^ be soon removed ; and if du^ be not done, in- fidlibly perish* This fact was known to the ancients^ for Pliny observes that bees die if dipped in oil or ho- ney **• On^ exertion to this law has been be£:»re mm^ tioned^ t a siifiilar cimtrivauce secafes the cheeseifaaggot firotti having its respilrati^n tmentipled by its moist and greasyfiodd; the grab also of Mwa«iyrnarta,aad of other JlAismte pcobably, has its posterior qdr^es placed in sr j^te at the bottom of a kind <tf fleshy poudi', miiieh hoa like shape of a hdkmv tnuicaited, and reversied cme. This poueb the. gfub can dose* whenever it pletfses^ stt as to oov«r its spivades^. And numerdus oth^ iarva^ both of i){>tef^aiidCt»toip^^m thftt devour laid^daH and oUy fiyod, have doiibdess some proceietioii dT dki^ kind ftr tbeir spiraclies and r^iratory plata&

• Swamm. BUd. Nat. ii. 48. a. ^ Hut. Nat^ I. xi. c. 19.

" Swamm. Bibl, Nai.iL 641 a.

^ Reaum. iv. 4^8. t. xxix./. 2. c> $,

LETTER XXXIX.

INTERNAL ANATOMY AND PHYSIOLOGY OF INSECTS, CONTINUED.

CIRCULATION.

We learn from the highest authority^ that the blood is the life of the ammal^ : every object of creation, there- lore, that is gifted with animal life, we may conclude, in some sense, has blood, which in this large sense may be defined — Thejkdd that visits and nourishes every, part of a living body^. But the Great Author of nature hs^ varied the Tnachinery by which this nutritive fluid is formed and distributed, gradually proceeding firom the most simple to the most complex structure ; in which he seems to have seen it fit to invert the process observable in the systems of sensation and respiration, where the ascent is itovcL the most complex, to the most simple structure.. In the lowest members of the animal citation, the blood seems the portion they imbibe of the fluid medium in which they reside, which when chylified, distributes new mole^ cules to all parts of their frame <^. In others, as in insects, it is formed by the chyle that transpires through the intes-

F Genet, ix. 4. »• N, met. tTHist Kat. xxx. 130.

* Cuv, Anat, Cofiip, iv. 167.

INTERNAL ANATOMY OF INSECTS. S^l

tinal canal into the general cavity of the body, where it receives oxygen from the air-vessels, and is fitted for nutrition ^. In these animals it is accompanied by a long dorsal vessel, the first step towards a hearty which alter- nately contracts and dilates with an irregular systole and diastole, but appears to have no vascular system con- nected with it. Again : in others, as the Tubicoles^ An^ nelida^ &c., a real circulation has been discovered; that is to say, a system of veins and arteries, but unaccom- panied by a muscular heart ^. In the Arachnida and Branchiopod- Crustacea the long dorsal vessel is also found ; but in these it is connected with an arterial and venous system, which receives, distributes, and returns the blood <^. It has therefore now become a true heart, and there is a regular circulation ; and in the Decapod Crustacea the dorsal vessel is contracted into an oval form, and placed nearly in the centre of this trunk**. In the great majority of invertebrate animals the blood is "white, but in the Annelida, to which Class the common dew-worm belongs, a curious anomaly takes place— *for it is red^. Thus a gradual ascent is made to the circu- lating system of the vertebrate and red-blooded animals. In ail, however, the blood is the principal instrument of nutrition an3 accretion ; and is on that account properly so denominated, though not connected with a circulating system.

Having given you this general outline of the means by which the blood is distributed in the different Classes of animals, I shall now confine myself to the case of insects

• Herold Schmetterl, 25. note *. Vol. III. p. 53.

«» N. Diet. (PHitt. Nat, vii. 313. Cuv. Anat. Conip. iv. 411.

«= Ibid. 419, 407. •* Itnd. * Ibid. 410.

VOL. IV. G

82 INTERNAL ANATOMY OF INSECTS.

inul Aracknida, beginning with ikejbrmer. As their nu- tritive fluid and their dorsal vessel have not been disco* vered to be connected, I shall consider them separately : but I must first observe, — that the term Circutation^ with which this letter is headed, though not strictly applicable to insects^ is perfectly proper when used with respect to Arachnida; you will not therefore stumble at the thresh* old, and object to my employing it*

L If you examine attentively the back of any smooth caterpillar with a transparent skin, you will perceive in that part an evident pulsation, as though a fluid were pushed at regular intervals towards the head, along a narrow tube which seems to run the whole length of the body. Accurate dissections have proved that this ap* pearance is real, that there is actually present in the back of most insects, placed immediately under the skin and furnished with numerous aur-vessek, a longitudinal vessel^ originating in the head near the mouth ^, running parallel with the alimentary canal nearly to the anus, 'containing a fluid which is propelled in regular pulsa* tions of from 20 to 100 p^r minute^ more or less as the weather is colder or warmer^, causing a sensible alter* nate systole and diastole from the anal extremity to- wards the head. In the Cossus these pulles were ob* served by Lyonnet to be^n in the eleventh segment, from which they passed from segment to segment, till they arrived at thefourthy where they terminated"^. This ves- sel is what Malpighi, who first discovered it, termed a hearty or rather series of hearts * ; but which Reaumur,

•

^ Plate XXII. Fig. 15. «» Lyonnet AruU. 105. *= Ibid. 426. *• Ibid, 105—. " BeBwnbyc. 15—,

INTERNAL ANATOMY OF INSECTS. 83

who injected it, regarded as a simple artery witho,ut strik- ing contractions^ : but to steer clear of any hypothesis, I shall merely call it the dorsal vessel {Pseudocardia). When carefully taken out of the body it is found to be a membranous tube closed at each end^, in many larvaB of equal diameter every where, but in perfect insects usually widest at the anal extremity ^, and attenuated into a very slender filament towards the head. In some in- sects, however, as in the larva of the chamaBleon-fly {Stratyomis Chamale(m\ it is attenuated at both ends, and in the Ephemera is alternately constricted and dilated as Malpighi describes that of the silk-worm^, a dilated por- ticHi belonging to each segment^. In the Cossusj and probably others, after the third s^ment, it is furnished with nine pair, the three posterior pair being the largest, of triangular trans vei'se bundles of muscular fibres, which Lyonnet denominates its wings ^, the action of which pro- duces its systole and diastole, and their propagation from tlie Ual towards the head ^. Under the last pair of these wings it is strengthened by a large number of circular muscular fibres^. I have stated it, with most writers, to be closed attach extremity; but from Lyonnef s words it should seem that, in the Cossusj he considered it as open and expanded at its anterior end^ He seems also to* suspect, that, by means of what he calls the fi*ontal ganglions, a fluid is derived from the dorsal vessel to the

■ Reaum. i. 160—. ^ Cuy. Anat. Cornp* iv. 418.

c Mai'cel de Serres Menu du Mm, 1819. 69.

** Swamm. Bibl. Nat. t, xl./. 4. t. xv.f, 4.

« Be B<md>yc, t in./. 4. ^ UU stipr. 414. ^ Ibid, 425 —

*> Ilfid, 419. » I6uL 412.

o 2

S4 INTERNAL ANATOMY OF INSECTS.

spinal marrow. He likewise describes a large nerve as passing through it and becoming recurrent'.

The^uid which this vessel contains is very abundant ; in the animal it appears colourless and transparent like water, but when collected in drops it becomes more or less yellow, and even orange**. Examined under the microscope it appears filled with a prodigious number of transparent globules, of incredible minuteness ^. When mixed with water, which it does readily, its globules lose all their transparency, and coagulate into small clammy, masses. After evaporation it becomes hard, and cracks like gum, as blood does also. This gummy substance is so abundant, that the fluid contained in the dorsal vessel of the caterpillar of the Cossus yields a mass of it of the size of a grey pea**.

From the situation of this dorsal vessel, which is pre- cisely the same with that of the heart in Arachnida and the Branchiopod Crustacea^ and from the systole and diastole which keep its fluid contents in constant motion, who can wonder that the physiologists who first disco- vered it, maintained that it was a true heart ? And even now, our knowledge of this organ is so very circumscribed that, till insects have been more widely examined with this view, and its real functions are ascertained, it seems to savour of temerity to assert, that in no respect.it can answer the purpose of a heart. Before I advert to those arguments that at present may be regarded as proving that it is not a heart, I will notice those upon which the upholders of the original opinion have founded their

* Lyonnet Annt. 413. •» Ibid. 426. Cuv. Anat. Comp. iv. 419.

"^ Lyonnet says (4^6), ** au-dela de trois millions de fois plus petits qii*un grain de sable " ! ! ** Ibid.

INTERNAL ANATOMY OF INSECTS. S&

jddgemenL No one will deny that the argument from analogy is strongly in favour of the old theory : I shall not therefore dwell upon it, but proceed to others. Swam- merdam, to whose exactness in observing, and scrupulous accuracy, every reader of his immortal work will bear .testimony, expressly asserts that he has seen vessels is- suing from the dorsal vessel in the silk-worm, and even succeeded in injecting them with a coloured fluid ^. Now it seems extremely improbable that so practised and ex- pert an anatomist should have been deceived, especially upon a point which would naturally excite his most earn- est and undivided attention. Without this recorded ex- periment, perhaps, it might be thought, though this was very unlikely, that he had mistaken bronchia for veins and arteries : but how could they have been injected from die supposed heart? Another great physiologist, Reau- mur, in the cate^illar of the saw-fly of the rose {Hi/lo* toma Mosie, Lat.) observed, besides the dorsal vessel, a ventral one of similar form, in which also was a pulsa- tion, but slower than that of the other. This he sup- poses may be the principal trunk of the veins''. Bonnet thought he discovered a similar vessel in a large cater- pillar, but with all his attention could perceive no mo- tion in it^. Reaumur also, thought he perceived in the

* His words are— '* In silk-worms I have clearly seen various small vessels spring from and approaching to the heart, which I have even filled with a coloured liquid. But whether they were veins or ar- teries I cannot yet affirm ." i. 112. a. 176. a. According to Cuvier (Anat, Comp, iv. 418), but I cannot find the passage, Swaramerdam alsomentions having seen a red fluid issue from small vessels in grass- hoppers. ^ ^ Reaum. v. 103.

<: Bonnet ii. 309. Perhaps in both cases the alimentary canal was the organ seen.

86 INTERNAL ANATOMY OF INSECTS.

grub of Mtisca vomitaria^ in which he in vain looked for the dorsal vessel, a fleshy part which exhibited alternate pulsations ; and when with a pair of scissors he made a lateral incision in the insect, amongst other parts that came out, there was one that had movements of contrao- '* tion and dilatation for several minutes, — this experiment

was repeated with the same result upon several grubs*. De Geer, whose love of truth and accuracy no one will call in question, saw the appearance of blood-vessels in the leg of the larva of a Phryganea L. (as Lyonnet did in those of a flea**) ; and in the transparent thigh of Orni-^ thomia avicularia he discovered a pulse like that of an artery*^. Baker, whose only object was to record what he sww^ speaks of the current of the blood being remark- ably visible in the legs of some small bugs^ : what he meant by that term is uncertain, but they could not be spiders^ which he had just distinguished. This author has likewise seen a green fluid passing through the ves-^ sels of the wings of grass-hoppers ^ ; and M. Chabrier is of opinion that insects possess the power of propelling a fluid into the nervures of their wings and withdrawing it at pleasure, as they are elevated or depressed ^ : but these two last ikcts must be accounted for on other principleS| as there is clearly no circulation.

But though these arguments, which I have stated in their full force, appear strong, and at first sight conclu- sive, those which may be urged for the more modem opi- nion— that no circulation exists in insects, properly so called, — appear to me to have by far the greatest weight.

* Reaum. iv. 171 — . ^ Ijesser L. ii. 84. note.

** De Gcer ii. 606-. vi. 287. '^ On the Microtcope. i. 130.

« l6id. f Sur le Vol det Ins. 326—.

INTERNAL ANATOMY OF INSECTS. 87

Lyonnet, whose piercing eye and skilful hand traced the course of so many hundred nerves and bronchtie long after they became invisible to the unassisted eye,* and which were a thousand times smaller than the princi- pal blood-vessels, opening into so large an organ as the supposed heart of insects, might be expected to be, could never discover any thing like them. His most painful researches, and repeated attempts to inject them with coloured liquors, were unable to detect the most minute opening in the dorsal vessel, or the slightest trace of any artery or vein proceeding from or commu- nicating with it^. And Cuvier, whose imrivalled skill in Comparative Anatomy peculiarly qualified him for the investigation, repeated these inquiries, and tried all the known modes of injection, with equal want of success ; and is thus led to the conclusion, that insects have no circulation, that their dorsal vessel is no heart, and there- fore ought not to be called by that name : that it is ra- ther a secretory vessel, like many others of that kind in those animals. As to the nature of the fluid that it se- cretes, and its use, he thinks it impossible, from our present information on the subject, to form any satisfactory cgn-^ elusion^. Marcel de Serres informs us — ^which further proves that it can be no real heart — that this vessel may be. totally removed without causing the immediate death of the insect^. This opinion receives further confirma^ tion from the mode in which respiration is performed in insects. In those animals that have a circulation, this takes place by means o£ lungs or gills ; — thus we find, even

* Lyonnet Anai, 427 — • ^ Cuv. AnaL Comp, iv. 418-

^^ Mem. du Mut. 1819. 71.

88 INTERNAL i&NATOMY OF INSECTS.

in the Crusiaeea and Arachnida so nearly related to in- sects, that the organs of this function are true gills ^ whereas in insects, though in some of their states their respiratory tubes are branchiform, yet they are not jgills, and the respiration is by tubes and spiracles. And these tubes, as you have seen, are so numerous and so infinitely ramified and dispersed, as to occupy the place of arteries and veins, and to imitate their distribution,— and thus to oxygenate what may be deemed the real analogue of the blood, which bathes every internal part of the body of an insect. Those animals likewise that have a circulation $.re furnished with a livery as is the case with the Amchr- nida and even many aggregate animals that have a heart ; but in insects there are only hepatic ducts. M. Cuvicr has also proved that the conglomerate glands^ which ex- ist in all animals that have a heart and blood-vessels, do not exist in insects, in which they are replaced by long slender secretory tubes, which without being united floait in the interior of the body : from this circumstance, he is led to conclude that their nutrition is by imbibition or immediate absorption, as in the Polypi and other zoo- {^ytes, the chyle transpiring through the alimentary ca- nal, and running uniformly to all parts of the body^.

But although it be granted that no circulation of the blood takes place in insects, yet, reasoning firom analogy, the dorsal vessel should in some degree and in some re- spects represent the heart, and its pulsations be in some measure for a similar purpose ; but what that purpose is, has not yet been ascertained : and on the whole, in the present state of our knowledge, it seems the most prudent

* N, Dici, d'Hist. XaL xvi. 208.

INTERNAL ANATOMY OF INSECTS. 89

course to leave this matter for the investigation of future physiologists*.

* Since writing the above, I have been favoured with a sight of Marcel de Serres' Observations on the Dorsal Vessel of Insects ^, in which his object is to prove that the principal use of that vessel is the more perfect animalization of the chyle that, transuding through the pores of the intestinal canal, is imbibed by it. In insects, he ob- serves, that undergo metamorphoses, in which the growth or develop- ment of parts is often very rapid, it is requisite that a considerable portion of the chyle should be in reserve for this purpose. On this account it is that the Epiploon or adipose tissue is so abundant in larvae to what it is in the perfect insect. That the importance also of this part to insects is proved by the circumstance, that all their in- terior parts communicate by fibrils with this tissue, and that proba- bly their various organs derive the nutriment from it by their means. He then asks by which of the viscera is the fat elaborated. Or by what means does tiie chyle which transudes from the intestinal canal pass to the state of fat? Facts seem to indicate, says he, that the func- tion of the dorsal yessel is to pump up the chyle, and to cause it then to transude through the meshes of the adipose tissue, where it finishes by elaborating that mass of fat so abundant in larvas and certain perfect insects, which are thus enabled to sustain the effbcts of a long fast. So that this vessel is only a secretory organ, analo* gous to so many others that exist in insects ; but the secretion which it has to produce is the most important of all, since the support of the vital powers depends upon it : it is, in effect, that vessel which completes the function of animalization, and which itself prepares the nutritive fluid*. He observes, amongst other reasons he brings to support his theory, that the colour of the fluid which it contains is always analogous to that of the adipose tissue that surrounds it, and that the colour of that tissue never changes without that of the fluid undergoing a corresponding alteration, — that when, as in many per- fect insects, the quantity of fat diminishes, the dorsal vessel also di- minishes in size, and that the same reagents which coagulate the fat, coagulate equally the fluid in the dorsal vessel, which seems to indi- cate an identity between them^.

The only circumstance that strikes me as militating against this hypothesis, is the analysis which Lyonnet has given of the fluid con- tuned in the dorsal vessel of the Cossus^, which seems to prove that

» Mein. du Mm. 1819. ^ /^. 68-^.

3 Ibid, 69~. * See above, p. 84,

90 INTERNAL ANATOMY OF INSECTS.

. Whatever be the functions of the dorsal vessel, this seems the most proper place to state to you what further is known respecting it* Its construction is nearly alike in insects in all their states, except that ki the imago it is shorter and narrower. Reaumur has affirmed, and before him Malpighi made a similar observation, that in chrysalises newly disclosed from the larva, and yet trans- parent, the motion of the included fluid is the reverse of what it has been in that state, it being propelled from the head to the tail, which he found to be tl\p case also in the imago ^. If this be true, and there is no reason to doubt his accuracy, when they are more advanced, it re- sumes its old course, as Lyonnet observed, from the tail to the head^. But probably it is not always uniformly in the same direction, since Malpighi states that a very slight cause will change its course, and that the pulsa- tions differ in quickness in difierent portions of the heart^. If its course were really always the same, and in one di-

it is more analogous to gum or varnish. He saw indeed a few globules, which appeared ten times as big as the others, which swam upon the water, but which he did not regard as component parts of the fluid, but as little drops of grease extravasated by dissection. The fluid of the vessel itself easily mixed with water, and appeared to sink in it to the bottom ^ These circumstances seem to indicate that it is not of a fatty or olea^nous nature. Further experiments however seem necessary to ascertain the nature of the fluid and its object : but I think it is a fair and reasonable conjecture, that as the vessel in question is in many respects analogous to the real heart in Arack- mda and some Crtutacea, it so far performs the functions of a heart as to produce an important effect in the nutrition of the animal. A more satis&ctory elucidation of the uses of this vessel may be ex- pected from the able pen of Mr. W. S. MacLeay.

1 Lyonnet Anat* 426—.

^ Reaum. i. 409, 64a—. Malpigh. De Bombyc, 38. ^ Lesser L. ii. 87 note *. * Uin supra.

INTERNAL ANATOMY OF INSECTS. 91

recticn, without any reflux, it would seem to follow that the fluid must be absorbed at one end, and, if there was no outlet, transpire at the other, which would be a kind of circulation. In Syrphm Pyrastri and other aphidi- vorous flies, this dorsal vessel, instead of the usual form which it had in the larva, assumes a very peculiar ap- pearance. If, taking one of these flies by the head and wings and holding it up to the light, you survey under a lens the base of the lower part of its abdomen, you will see through its transparent skin, which exactly forms such a window as physicians have sometimes wished for in order to view the interior of their patients, a flask- shaped vessel having its long end directed towards the trunk, in which there is a manifest pulsation and trans- mission of some fluid. This vessel extends in length from the junction of the trunk with the abdomen to about the termination of the second segment The in- cluded fluid does not run in the dorsal vessel in a regu- lar course, but is propelled at intervals by drops, as if fn»n a sjnringe, first from the wide end towards the trunk, and then in the contrary direction, forming a very in- teresting and agreeable spectacle. One circumstance led Reaumur to conjecture that the neck of this vessel, which he at first regarded as simple, is in fact composed of two or more approximated tubes, and that the blood is con- veyed forward by the outward ones, and backward by the intermediate one^ : he even thinks that he saw a kind of secondary heart, at the extremity next the trunk, for the purpose of causing the reflux. This illustrious au- thor observed the above remarkable structure not only

■ Reaumur iv, 264.

<

92 INTERNAL ANATOMY OF INSECTS.

in the Syrphi, but in many of their afSnities, and thinks that it is also widely diffused amongst the Muscida^.

I must now say something upon what I conceive to be the real blood of insects ; for I think no one will object to that name being given to their nutritive fluid, though it does not circulate by means of a vascular system. The chyle that is produced in the intestines of animals from the food, is that fluid substance from which their blood is formed : in insects it is not absorbed by the lacteals, but transpires through the pores of the intestinal canal into the general cavity of the body, where, being exposed to the influence of the oxygen in the air-vessels, it becomes, though retaining its colour, a different fluid from what it was before, and analogous to blood in its use andofiice^ ; only that in these animals, as Cuvier has ol^served, the blood, for want of a circulating system, not being able to seek the air, the air goes to seek the blood ^. The dispersion of this fluid appears to be universal, so that all the parts and organs contain it in a greater or less degree **. In many insects, if you break only an antenna or a leg, a drop of fluid flows out at the wound. In larvae, the fluid which bathes all the internal parts and organs is not only sufficient for their nutriment, but a large

» Reaumur iv. 260—. »» Herold SchmetterL 24.

' Anat Comp. iv. 165. <* Marcel de Serres (p. 67).

speaks of this fluid as being, after it has transuded through the in- testinal canal, a fluid in repose, which seems to indicate that it b per* fectly stagnant ; but when we consider that it is not only incessantly entering the body and making its way to every part, but is also, by means of the various secretory organs, constantly converted into new products, and so going out again in many cases, it will appear evi- dent that it cannot be considered as a stagnant fluid, since there must be a constant though probably slow motion towards the points of absorption or imbibition.

INTERNAL ANATOMY OF INSERTS. 95

quantity of seemingly superfluous blood remains that is not wanted for this purpose. This is expended in the production of th^ caul or epiploon {Carps graisseux Reaum.), which laps over and defends all the viscera of the animal, and goes principally to the formation of the imago*. I have said that Cuvier conceives nutrition in insects to take place by imbibition or immediate absorp- tion ; that is, I suppose, the different parts and organs thus constantly bathed in the blood, imbibe from it the particles necessary for their consjtant accretion. M. Cha- brier seems to think that it is the compression and dila- tation of the trunk that duly distributes the nutritive fluid**; Lyonnet compares the nutrition of insects by their fibres from this fluid, when formed into the corps graisseux^ to that of plants that draw their support by their roots from the earth *^. Much obscurity, however, at present rests upon this subject — much for future inves- tigation to explore ; but in all the works of the Most High there is always something inscrutable, something beyond the reach of our senses and faculties, which teaches us humbly to adore his infinite perfections.

II. The circulation of the Arachnida is next to be considered; and the term applied to these becomes strictly proper. Two great tribes, in our view of the subject, constitute this Class,— the spiders {Araneidai) and scorpions {Scorpionidai) : I shall give you some ac- count of the circulating vessels of each. — In spiders^ the heart in general is a long dorsal vessel as in insects, but supposed to be confined to the abdomen^ growing slen-

r

* Cuv. AttffL Comp. iv, 158. Herold SchmeitcrL 28.

*» Siu' le Vol dei Lis, c. iv. 88. note 1. '^ AnaL 4"28.

94f INTERNAL ANATOMY OF IN6£CTS.

derer towards each extremity, particularly the anal. In some also, as in Aranea domestical like that of insects, it has lateral muscular appendages ; but in others, as in Clubiana atrox^ it is without tliem ^. It exhibits a pair of vesseb that appear to connect with the gills, by which the oxygenation of the blood takes place, and a number of others that ramify minutely and are lost in the ana- Xogne of the epiploon, supposed to be their liver\ Whe- ther these last are to be regarded merely as veins, has not been ascertained ; they seem rather to convey the blood outwards, than to return it back to the heart : but this question must be left for future investigation. I may observe, however, that though the heart of the spider has been traced only in the abdomen, it may probably ex- tend into the trunk.

The heart of the scotpion has been examined both by Treviranus and Marcel de Serres ; but as the descrip- tion of the latter is most clear and intelligible, I shall princ^Htlly confine myself to that The heart, then, of these animals is elongated, almost cylindrical, but atte- nuated at each end ; it is extended firom the head to the extremity of the tail, and appears to have four pairs of lateral muscles. On each side are four pairs of principal vessels, which go to the pulmonary pouches, and there ramify. These may be assimilated to veins. Besides these, there are four other vessels that cross them, form- ing with them an acute angle, and which, with four branches of smaller size, receive the blood from the pul- monary pouches, and distribute it to the different parts

* Treviranus Arachnid. 2S. t, iii./ 28, 29. " Ibid 29. t. iii./. 30, 31.

INTERNAL ANATOMY OF INSECTS. 95

of the body, — these are the arteries. Before it enters the taO, the heart throws out two vascular branches which do not go to the gills, but distributing the blood to different parts, ought to be considered as arteries'. Treviranus mentions bunches of reticulated vessels, con- cerning the use and origin of which he seems uncertain^ ; but as they approach the gills, they are probably the branching extremities of what M. de Serres considers as the veins.

• N. Did. (THist. Nat. ixx. 420. Comp. Treviran. Arachnid. 10—. »» Ibid. 9-.

LETTER XL.

INTERNAL ANATOMY AND PHYSIOLOGY OF INSECTS, CONTINUED.

DIGESTION.

** XHE immense Class of insects," says the immortal Cuvier, " in the structure of its alimentary canal exhibits as many variations as those of all the vertebrate animals together : there are not only the differences that strike us in going from family to family and from species to species ; but one and the same individual has oflen^a Go- nial quite different, according as we examine it in its larva or imago state ; and all these variations have rela- tions very exact, often easily estimable, with the tempo- rary or constant mode of life of the animals in which it is observable. Thus the voracious larvae of the Scara-^ beet and butterflies have intestines ten times as large as the winged and sober insects — if I may use such an ex- pression—to which they give birth*."

In the natural families of these creatures, the same analogy takes place with respect to this part that is ob- servable in the rest of the Animal Kingdom ; the length and complication of the intestines are here, as in the other Classes, often an index of a less substantial kind

* Anat, Comp. iv. 129.

INTERNAL ANATOMY OF IN$S£CTS. 97

of nutriment; while their shortness and slendemess in- dicate that the insect lives by prey".

In oMiaidering therefore the parts connected with the digestive functions of the insect world) it will not be amiss to have reference to Xhevtfood^ and their mode of taking it; but first it will be proper to state and define the parts of this important organ.

In general the alimentary canal ^ is composed of the same essential iunicks as that of the vertebrate animals, consisting of an interior epidermis, a papillary and cellu- lar tunick, and an exterior muscular one^. The first is nsuall}^ tender, smooth, and transparent ; but not always discoverable, probably on account of its tender sub- stance^. Ramdohr does not notice the papillary and cellular tunicks; they are probably synonymous with what he demmiinates — the^fiociy layer {Diejhckige lage)^ and which he describes, when highly magnified^ as appear- ing to c<msist of very minute globules or dark points, and as being of a cellular structure*. The exterior tunick is thicker and stronger than the interior^ and composed ci muscuW fibres, running either longitudinally or trans- versely, so as to form rings round the canal. This tu- nick mostly begins at the mouth, and goes to the anus« changing its conformation in different parts of the above intestine. Sometimes however it originates only at the beginning of the stomach ^. With respect to its general disposition, that canal — ^in its relative length, in the size of its different parts, in the number and form of it^ dilatations, and particularly of its stomachs and its

• Cuv. Anat. Comp, iv. 129. •» Plate XXI. Fig. c, d, e, is the intestinal canal of the larva of the Cossus, '^ Cuv. IM, 112.

•* Ramdohr Anat. der Im, 6. ' Ibid. 25. f Ibid. 6.,

VOL. IV. H

98' INTERNAL ANATOMY OF INSECTS.

coecumS) and In the folds of its interior — exhiHts varia- tions altogether analogous to those of vertebrate animals, and which produce similar effects^. As to itsparts^ it may be considered as consisting of two larger portions, between which the biliary or hepatic vessels form the point of separation. In the first, the most universal parts are the gullet and the stomach ; and in the second, the small intestine and the large intestine^.

1« The gtdlet {CEsophagtis^) is that portion of the in- testinal canal which, receiving the food from the pharynx, or immediately from the mouth, conveys it to the sto- mach. Though it often ends just behind the head\ it is usually continued through the trunkj and sometimes even extends into the middle of the abdomen^ ; it there- fore seldom much exceeds in length half the body. It is constantly long when the head is connected with the trunk by a narrow canal — as in the Hymenopiera, Neuroptera^ Lepidoptera^ &c. ; but is frequently short when these parts are more intimately united ^ It oflen ends in a kind of sac analogous to the crop of bnrds. Under this head I must mention a part discovered by Ramdohr, which he calls the food-hag (Speisesack), pe- culiar to, as he thinks. Dipteral. From the mouth in these proceeds a narrow tube into the abdomen, where it expands into a blind sac having no connexion with the stomach ; so that the fluid food, as blood, &c stored in it, must be regurgitated into the mouth before it can

" Cuv. ubiiupr, 113. •» Com p. Ramdohr Aiutt, 7.

' Plate XXI. Fto. 3. c. ^ TVn^rto Ramdohr, tibisupr. 9.

/. iv./. 1. * Agrion, Ibid, t, xv./. 4, a, b. ^ Ibid.

' Many other insects that live by suction have something simflar, as the honey-bag of butterflies, Plate XXX. Fig. 10, 11. a. Ram- dohr L xviii./. 2, with t. xix./. 1—3. and xxi. 1. 3, &c.

INTERNAL ANATOMY OF INSECTS. 99

i

pass into that organ '. Thus these animals, besides their I stomach, have a reservoir in which to store up their food ;

the product therefore of a single meal will require seve- ral days to digest it

. 2. The stomach {Ventricultis^) is that part of the in- testinal canal immediately above the bile*vessels, which receives the food from the gullet for digestion, and traps- mits it when digested to the lower intestines^. By its admixture with the gastric juice, the food acquires in the stomaeh a quite different colour from what it had in the gullet. In herbivorous insects it contains no acid, but, like the gastric juice of herbivorous quadrupeds^ is of an alkaline nature^. The chyle is forced through this or- gan, probably in part by the pressure of the muscular fibres during the peristaltic motion ^ and being pressed through the inner skin, is first collected in the interme- diate cellular part, and ultimately forced through the outer skin^. At its posterior end it terminates in the pylorus^ a fleshy ring or sphincter formed of annular mus- cular fibres ^. The stomach often consists of two or more successive divisions, which are separated from each other, and are often of an entirely difierent conformation and shaped. In the OrthopterOf Predaceous CokopterOf and several other insects, an organ of this kind precedes the ordinary stomach, which from its structure Cuvier deno- minates a second stomach or gizzdtrd^ ; Possdt impro- perly calls it Cardial ; and by Ramdohr it is named the

• Ramdohr Anat. 11—. ^ Plate XXI. Fig. 3. d.

<" Ramdohr Ibid. 28—. ^ Herold (Schmetterl 24)

says that Ramdohr is mistaken here, and denies the existence of this juice in insects; but as Ramdohr's researches were so widely extended, he is most likely to be right. * Ramdohr Ibid. 99.

^ Ibid. 31. J? lind. 28.

^ Anat. Comp. iv. 135. ' Ramdohr, M tupr, 15.

H 2

r^

i ] '^. -*'

t

100 INTERNAL ANATOMY OF INSECTS.

plaited'Stamach [Falten-magen^). It is a short fleshy part, consiisting of two skins, placed above the opening of the stomach, and perhaps rather belongs to the gullet. The inner skin is formed into longitudinal folds, and sometimes armed with horns, teeth, or bristles. Its ca* vity is very small and compressed, so as to admit only small masses of food, and yet present them to a wide sur* lace for the action of the teeth or bristles ; — in this sto- mach therefore, as in the gizzard of birds, to which it seems clearly analogous^, the food is more effectually Comminuted and rendered fit for digestion. The mus-> cles, by which its action upon~ the food is supported, in some species amount to many thousands'. Rudiments of a gizzard are sometimes found concealed in the gullet of many insects **. The idea of Swammerdam, Cuvier, &c. that grasshoppers and other insects that have this kind of stomach, chew the cud% Ramdohr affirms is entirely erroneous ^. Besides its divisions, the stomach has other appendages that require notice. In most Ortkoptera^ a pair or more of blind intestines or cceca may be found at the point of union of the gizzard with the stomach*^, which have been regarded as forming a third stomach : they also begin the stomach in the louse** ; they form a coro- net round the apex of that organ, in the grub of the cock- chafer * ; and in that of the rose-beetle, there is one at the apex, one in the middle, and a third at the base*. Be^ sides these appendages, which are formed of the «kin of

' Ramdohr Ami. 16. »» Ibid. IS. « IMd. «» Ibid.

• Swamto. £ibl. Nat. I 94. b. Cuv. Anai. Con^. iv. J 34.

' Ubi iupt. 18. » Ibid, t If. I.e. 5. c. 9. g, h,

^ Ibid, t XXV. /. 4. bb. i Ibid. t. viii./. 3. cc.

^ Ibid. t. vii./. 2.

■ ' .' • • •.• ••

INTERNAL ANATOMY OF INSECTS. 101

the Stomach, there are others that are not so. In the Pre- daceous and some other beetles, the whole external sur- face cf this organ is covered with small blind appendages opening into the space between its two skins, which cause it to resemble a shaggy cloth ; these Ramdohr calls shags {zoite^\ and Cuvier, hairs^ {villi). These appendages the latter author seems to regard as organs that secrete the gastric juice and render it to the stomach^ ; but the former thinks their use uncertain^.

5. The small intestines {Intestifia parva) are the .por- tion of intestines next the stomach, and consist often of three distinct canals ; — the first is supposed to be analo- gous to the duodenum ; it is found only in the Coleopterous genera Silpha L. and Lampyris L., and is distinguished from the succeeding intesdne by being perfectly smooth ^. Next follows the ikin intestine {Dunndarm Ram.), which in the above insects is wrinkled ; it most commonly imme- diately follows the stomach. Sometimes it is wholly want- ing, as In Agriony the Hemiptera % &c. Ramdohr conjec- tures that it is not solely destined for conveying the ex- crement, but that probably some juices are separated in it from the food especially for the nutritioa of the gall- vessels, as their principal convolutions are mostly near this intestine <^ ; which perhaps may in some cases be re- garded as analogous to the jejunum in vertebrate ani- mals. The third pair of the small intestines, which per- haps represents the iteum^ Ramdohr distinguishes by the name of chdh-shaped (Keulformigen Darm^). It may ge-

* Tbid. 20. * Anat Comp, iv. IS±

' Ibid, and 130. ^ Ubi tupr. 30.

« Ibid. 31. t, Vf.f, 2. e. i. v./. 1. d,f. 4. D. ' Ibid, 32.

« Ibid. 34. " Ibid. 36.

102 INTERNAL ANATOMY OF INSECTS.

nerally be regarded as only a continuation of the former thickened at the end so as to resemble a club reversed. It is however sometimes separated from the thin intes- tine, as in Callichroma moschatum*.

4. The large intestines {Intestina magna) consist some- times of two portions. The tkick intestine {Dicken^darm^ which may be regarded as a kind of cacum, is found only in the larvae of the Lamellicom beetles, but never in the perfect insect In shape it is oval and folded ; whence it is thicker than the rest of the intestinal canal, and is constantly filled with excrement^. The second portion of these intestines is the rectum {Mastdarm\ which ter- minates in the anal passage* This part is scarcely ever wanting, except when the insect evacuates no excrement, which is the case with the grubs of bees, wasps, and the antlioh {Myrmeleon). In the imago of Telephorus^ at least in T.fmcus^ it is also obsolete ^ : in most cases, how- ever, it is very distinct from the preceding intesdne. ^ Sometimes it consists of only one tunick composed of muscular fibres**. When the gullet is wide, the rectum is usually so likewise ; but when it follows a club-shaped or thick intestine, it is narrow •. It generally may be termed short ^. When wide, it often contains a great quantity of excrement, as the gullet does of undigested food ; but when narrow, the excrement seldom remains long in it. This intestine also in a few cases has a lateral enlargement or ccecum {Blind'darm\ being a continuation of the same skin ; but perhaps this enlargement is really

■ Ramdohr Anal, t, xxiv./. 1. F.

• Ibid, 36. /. vii./ 2. kk. t viil./. 3. g, hh.

• JbitL U xii./. 1. /. xvii./. 1. /. vii./. 6. * Ibid. 37.

• Ibid, 38. f Ibid,

« « '

INTERNAL ANATOMY OF INSECTS. 103

analogous to what Ramdohr calls the thick intestine^ though in these cases he regards it as an appendage of the rectum*.

I must npw call your attention to the bile^vessels of in- sects. These^ by M alpighi ^ and the earlier physiologists, who regarded them as a kind of lacteals, were denomi- nated varicose vessels : but Cuvier — and his opinion after some hesitation has been adopted by Ramdohr — consi- ders them as vessels for the secretion of bilcj and as ana- logous to the liver of animals that have a circulation ^. As the want of blood-vessels prevents insects from hav- ing any gland, the bile is produced with them, as all their other secretions, by slender vessels that float in their nutritive fluid, and from thence secrete the elements proper to form that important product, which usually tinges them with its own yellow hue ; though in the La^- mdlicoms and Capricorns they are of an opaque white, and in the Dytisci of a deep brown colour^. Their bitter taste further proves that they contain the bile ^. They are long, slender, filiform, tortuous or convoluted, and mostly simple vessels ; sometimes gradually smaller toward the base^, at others, towards the apex^. In some, screw- shaped^: in one larva, with hemispherical elevations^: in the cockchafer, part of them are fringed on each side with an infinity of short, blind, minute, setiform tubes, while the rest are naked ^ ; they are composed of a single, thin, transparent membrane, according to Ramdohr ' ; but

• IHd, 40. •» De Bombyc. 18—. « AnaJt. Comp. iv. 153,

* Ibid. • Ibid,

f Ramdohr 43. Cicindela campettris, t in./. 1. K. ' Phiyganea grandity Ibid, t xvi./. 2. ** Notonecta glauca.

Ibid, i, xxiii./. 5. * 0£ Mtuca vomitoria. Ibid, t, xix./. 5,

^ Ibid. t. Viii./. 1. H. and G./. 2. ' Ibid, 50,

104 INTERNAL ANATOMY OF INSECTS.

Cuvi^r thinks their texture is spongy^. They appear to contain a number of small, irregular, dark granules, which float in a peculiar fluid, with which, however, they are not always filled throughout, nor are thev constantly permeable from one end to the other. Thus, in the meal- worm beetle ( Tenebrio Molitat^), the common trunk by which they are attached to the intestinal canal is com- posed of gelatinous granules^. The place of their in- sertion is generally a little below the pylorus^ but in the common cockroach they are inserted into the stomach just above that part^. Usually each vessel opens singly into the intestinal canal, which the whole number sur- round at an equal distance from each other ^. Some- times, however, they are connected witii it by a common tube in which they all unite, as in the asparagus-beetle {Lema Asparagi *) ; in the house-fly ( Musca domestical and pther Muscidcey each pair unites so as to form a single branch on each side of the canal previously to their in« sertion^ ; in the field-cricket {Gryllus campestris) they are all inserted in one spot ^: and when numerous, they are ge- nerally attached singly though irregularly**. These ves- sels at their base do not open into the cavity of the in-> testinal canal, but merely into the space between its outer and inner tunicks, the last being constantly imperforate*. With regard to their apex^ the bile-vessels are some- times^<r^rf singly or connectedly to the intestine merely by a few muscular fibres ; for they do not enter it, their ends having no orifice. This structure is mostly to be met

■ Ubitupr, •* Ramdohr, ubi supr, "^ Ibid, 44. t. \,f, 9.

o Ilfid, . • Ibid, t, vi./. 5. H.

f Ibid. t. xix./. \.N,N, O,/. 2. P, P, O. « Ibid. 1. 1./. 1. kkk.

^ Ibid. L xiii./. 1—3. » Ibid. 44.

•

•

■••• ••• • • »

INTERNAL ANATOMY OF INSECTS. lOJ

with in the Caleoptera *. in caterpillars, the tops of these vessels perforate the outer skin of the rectum, and pro* ceeding in dense convolutions to the anus, become at last so fine thft their terminations cannot be discovered **. In other cases, the extremities of a pair of these vessels unite so as to form a double one : this may be seen in those of Staphylinns politus^j and probably other rove* beetles : and lastly, in others the bile-vessels arejieet hanging down by the intestinal canal, without being at- tached to it or to each other. This structure is con- stantly found in the Orthoptera and Hymenopta'a Or- ders, fee.**.

With regard to their numbe?^ the bile-vessels vary from two to upwards of one hundred and fifty, yet so that their whole amount is constantly the product of the num- ber two, — at least as far as they have been counted : and even when those on one side are not alike, a similar va*- riation takes place in the other, as may be seen in Gal- leruca Vitellince^ where on each side are two long ones and one shorter • ; the most usual numbers dLTe^four — six — or manyj that is, more than twenty —

Two bile-vessels are found in the larva of Cetonia aurata ^. JFour • • most Coleqptera^ Diptera^

and Hemiptera^. Six •••••••.•••• LepidopterOy some Coleth-

pt€ra\ &c.

» IbuL 45. b Ibid. 45. Plate XXI. Fig. 3././

«= Ramdohr, Ibid, /. iii./. 6. E.

•» Ibid, t. i./. 1. 5 9. /. xiv./. 1—3. * Ibid. 46. <. vi. /. 3.

' Ibid, t vii./. 2, ' Ibid. t. ii. ui. &c. /. xx./. 1,^. 6.

/. xxii./. 1—5. &c. * Ibid.t. xviii./. 1. 5. t. vr.f. 1. Sec

also t vi./. 1. 3.

106 INTERNAL ANATOMY OF INSECTS.

£igfA^ bile-vessels areToimd in Myrmeleonj Hemerobim^.

Fourteen Formica rufa^.

Twenty larva of Tenthredo Ame^

rime^. »

Many Libellulinoy Orthopfera^

and Hymenoptera^.

The bile-vessels vary considerably in length : in many cases where they av^Jree they are short * ; they are often very long, and perhaps those that ^x^Jixed may be gene- rally stated as the longest. In the Lamellicom beetles they are remarkable for their great length ^.

Having, given you this general account of the intesti- nal canal and its parts and appendages, I shall now state some of the peculiarities that in this respect distingubh particular tribes and famiUes.

The Coleoptera alone, exhibit as many variations in the structure of the alimentary tube as all the other Or- ders of insects together : — to particularize these would occupy too large a portion of this letter, I shall therefore only notice a few of the most remarkable. In general they may be stated as having universally a stomach, a small intestine and rectum, and not more than th^ee pairs oi^ed or united bile-vessels. In the Predaceous beetles, the gullet mostly widens at the base into a considerable cropi followed by a gizzard^ a shaggy stomachy and two pairs of united bile-vessels. The whole alimentary canal in these, is never less than dovble^ and sometimes treble the length of the body*. In the carnivorous beetles, at

• Ramdohr Anaf. t. xvii./. 1, 2. 6. «> IM. i. xiv./ 3.

« /«</. /. xiii./. 4. ^ Ibid. t. xv./. 3, 4. 1. 1./. 1. 5. 9. /. xii.

/. 4, 5, 6, &c. « Ilnd, t, x\.f. 4. t xii./ 4—6. t. xiii./. 2-4, &c. ' Ibid, t, vii./ 1. /. viu./ I, &c. « Ibid, t ii. iii. XXV.

INTEENAL ANATOMY OF INSECTS. 107

least the Staphylinida and Silphidce^ there is little or no crop — the gizzard is hidden : in the former, the whole length of the intestinal canal is not tv^iice^ while in the latter it is tflore than four times that of the body ^, In these also the ihtermediate portion of the large intestine is singularly annulated'^. In the Lamellicoms the sto^ mach is usually longer than all the rest of the intestines together, and often convoluted : in the cockchafer the whole intestinal canal is neBilyJive times the length of the hody,,/&ttr parts of which is occupied by the stomach^. In the grub the canal scarcely exceeds the length of the animal^. In Lampyris the stomach exhibits a remark- able appearance, having on each side a series of spheri- cal^^ or vesicles^. Have these any thing to do with the secretion of its phosphoric matter? Tenebrio has a gizzard armed internally with calluses, and a shaggy sto- mach, and Blaps does not difier materially ; their entire canal is more than twice the length of the body ^. In the vestcatoryheeHes {Cantkarisj Meloe^ &c.) there is no giz^ zardj and the canal is less than twice the length of the body ^. Little Is known with regard to the alimentary canal of the beetles distinguished by a rostrum {Curcidio L.)» In the only two that appear to have been examined, Atte^ labus Bettdeti and Cryptorhynchus Lapathiy that canal is moderately long, the stomach partially shaggy, and the small intesdne inversely claviform ; but in other respects they differ materially^. In the former there is no crop or gizzard, the stomach is fringed on each side, except at its upper extremity, with a series of small cceca or shags, and there are three pairs of bile-vessels* ; while

• Ihid. U iii./. 6. /. iv./ «. L v./. I, •• IhidJ. 1. e.f, 3.

« Jhid, \n. ^ Ibid. 1«3. « Ibid, L v./ 4. B. f lUi, 94. « lh\d. 96-. "" Ibid, L x./. 1. 8. * Ibid./. 8. b c.

108 INTERNAL ANATOMY OF INSECTS.

in the latter the gullet is dilated into a crop which in- cludes a gizzard in which the skill of a Divine artist is singularly conspicuous : — ^though so minute as scarcely to exceed a large pin's head in size^ it is stated to be armed internally with more than 400 pairs of teeth, moved by an infinitely greater number of muscles*. A transverse section of this ^zzard represents two concen- tric stars, with nine rays each^ : the object of this struc- ture is, the comminution of the timber which this beetle has to perforate and probably devour^. The stomach is very slender, but dilates in the middle into a spherical vesicle^, and there are only two pairs of bile-vessels*. In the Capricorn beetles, the part we are considering varies much : in general we may observe that it is moi'e than double the length of the body, that the stomach is long and slender, and usually naked, that the gullet ter- minates in a crop without a distinct gizzard, and that there are three pairs of bile-vessels^. In the Herbivo- rous beetles {Ckrysomela L. Cassida L.) the canal is more tlian double the length of the body, and in some much longer^, the stomach is long, and commonly naked ; but m CArysomela violacea it is covered with hemispherical prominences \ and in Ckrysomela Poptdi it is shaggy' ; in the insect last named and Gall^ruca Vitellirue the rec- tum consists of ttsx) pieces K In this tribe the intestines of the larva resemble those of the perfect insect ^

In the Orthoptera the alimentary canal, which conti-

• Ramdohr 98. t. x./. 2 — 4. From Rarodohr's figure^ compared with the sire of the insect, it appears that the ^zard could scarcely have been of greater diameter. ^ Ibid./, 2.

« See Curtis in Linn. Trans, i. 88. ^ Ramdohr /. x./. 1. rf.

« /*irf. //. f Ibid. L ix./ 1, 2. t. xi./. 3, t xxiv./. 1. 2.

'« Ibid. 103. h /^-^^ 104. t. vi./. 4. D. » Ibid.f, % B.

k Ibid. L vi./ 3. E. » Ibid. 101.

INTEENAL ANATOMY OF INSECTS. 109

Hues the same in every state, is short, or only moderately long ; die gullet has one or two lateral pouches or cr<^s*, and terminates in a gizzard of curious construction, with singular folds and teeth ^ ; then follows a short stomachy usually with a pair or more of ccBca at its upper extre- mity^ ; the lower intestines are not distinct, and the bile* vessels numerous, short and free^.

In the Neuraptera^ maHiy of the genera are distin-^ guished by the remarkable length of the gullet, and by the lower intestines forming one short piece ^ In the Li* belUdina the bile^ vessels are numerous, short, and free, as in the Orthoptera ^^ In Hemerobius and Myrmeleon there is a gizzard <^, and just above it a coecum^ in the for-* mer very remarkable, is connected with the gullet^.

The Hymenofptera appear all to be distinguished by a long slender gullet, terminating in a dilated crop form* ing the honey-bag ; their stomach is variable, their small intestine slender, and the rectum dilated; — their bile-ves* sels, like those of the two preceding Orders, are nume- rous, short, and free*. In the ants and ichneumons there is an approach to a gizzard ^, In the wasp and humble* bee the stomach is very long, with muscular rings sur<- rounding it'. In this Order the larvee at first have no lower intestines and void no excrement", but as they ap- proach to the pupa state one begins to appear **.

» Ramdohr U If. 1. 5. 9. »» Ibid./. 2, 3, 4. 7, 8. 12.

« Ihid.f. I. <?,/. 5. c,f. 9. g A. « Ibid.f. 1. 9. *.

• Ibid. t. xv./. 3, 4. U xvii./. 2. 6. ' Ihid. t. xv./. 3, 4./. « Ibid, t, xvii./ 2. c./. 6. d. »» Ibid./. 2. b.f. 6. c.

i Ihid. t. xii./ 6. H. t. xiii./. 1./. ^ Ibid. t. xiv./. 2, 3, C. » /Airf. t xii./ 6. 1). t xiii./. 1. b. "» Ibid. 133. U xU./ 1-3.

• Ibid./. 4.

no IKTERtTAL ANATOMV OF INSECTS.

The next insects whose alimentary canal we are to consider, are those whicb, taking their food by suction, have no occa^on for masticating organs : tlus may in part be predicated of the preceding Order, in which most of the tribes in their perfect state imbibe fluid food* and use the ordinary organs of mastication principally in operations connected with their economy; and their crop, in wbiclt the honey in many is stored up tor regurgita- tion, may be regarded in some degree as analogouB to the food-bag of the Diptera and other suctorious insects.

TThe two sections of the Hemiptera Order differ widely in the canal we are considering, and I shall therefore give a separate account of each. In the Hetercqtterous section, upended to the gullet by a long convoluted ca- pillary tube, besides the usual saliva-reservoirs there is often a double vessel, which Ramdohr regards as dis- charging the same fimction, but which in many respects seems rather analogous to the food 'reservoir of the Di- ptera *. As I have had no opportunity of examining this vessel, I shall content myself with stating this idea, and describe the vessel more fully hereafter. The gullet, in these, usually terminates in an ample crop consisting of many folds'', followed by a long, slender, cylindrical tube, dilated at its base into a ^herical tiunour ; these two may be said to form the first stomach : to this suc- ceeds a second', which Ramdohr denominates' the bug- stomach ( Wanzen-magen), which varies in its figure, and in Pentatoma consists of four demi-tubes, so as to form a

' Comp. Ramdohr t.xxiL/. Z.M. Fig. 4.3. with txiL/. 1,7. IHd. t. xxii./. I. c./. 3, 4. B—. ' liid./. 1. D E.f. 3. CD.

JHTERNAL ANAl'OMV OF INS£GTS« 111

quadrangular canal*. In the Homopterom section of this Order Ramdohr seems to have examined but few; Chermes however «id Aphis exhibit one remarkable fea- ture ; they have no bUe^vesselSy at least he could discover no trace of these organs*'. Their intestinal canal is very simple, their stomach very long, widest above, and some- what convoluted, with a very slender gullet^. In Ce* reopis spumaria the structure is more complex, and ex- tremely singular. It has two or rather three stomachs^ the two first of a homy substance, and the last a slen- der somewhat convoluted membranous tube, which be- coming reversed, is attached by what should be deemed its lower extremity to the first stomach, from the other side of whicK emerge the lower intestines, terminating in a thick pear-shaped rectum. At the same point of the first stomach the four bile-vessels are attached, they groWs gradually thicker for about a third of their length, when they become twisted like a cord, and taper towards the rectum, to which also they are attached^. From this structure it should seem that the food has to pass twice through the first stomach, before the process of digestion is complete, and it is rejected at the anus.

The next suctorious Order is the Lepidoptera : in these the gullet is long and slender, surrounded at the banning with a loose transparent skin, and at the base fomished with a pair of lateral sacs, forming the honey- stomach, and probably analogous to the food-reservoirs of the Diptera^ which when blown up are of an oval form ; the stomach, as in the bugs, consists of tvoo por-

• Ramdohr t. xxii./. 1. D, E.fi 3. C, D./ 4. C. ^ Ibid. 198. ' Ibid, t xxvi./. 2. 4. * Ibid. t. xxxiii./. 3.

112 INTERNAL ANATOMY OF INSECTS*

lions, the first being the longest '. There are three^^^ bile-vessels on each side, proceeding firom a single branch^. It will not be uninteresting here to abstract from Herold the progressive changes which take place in the intestinal canal in this Order, during the transition of the animal from the larva to the imago state. In the larva, the gullet, the small intestine, and the rectum, are short and thick ^, there are a pair of silk reservoirs {se» ricteria\ as well as vessels for the secretion of saliva {sialisteria) : if you examine it two days after its first change, you will find the gullet and the small intestine much lengthened and become very slender ; the stomach contracted both in length and size ; the rectum also changed, and the silk vessels contracted^. These in a pupa eight days old have wholly disappeared ; the guUet is become still longer, its base is dilated into a crop or food*reservoir ; the stomach is still more contracted, and instead c^ a cylinder represents a spindle ; the small in- testine also is lengthened ^ : at a still more advanced pe* nod, when it is near appearing under its last form, the gullet and small intestine are still more drawn out; and the honey-bag, though very minute, has become a latersd appendage of the gullet '^; and lastiy, in the butterfly it appears as a large vesicle^ ; the small intestine is grown very long^ ; and the rectum has changed its form and ac* quired a coecum ■• When we consider the adaptation of all these changes of form, the loss of old organs and the acquisition of new ones, to the new functions and mode

• Ramdohr L xvin./. 1. F, G. ^ Ibid, L, K,

• Plate XXX. Fig. 7. * Ibid. Fig. 8. • Ibid. Fig. 9. ' Ibid. Fig. 10. « Ibid. Fig. U.a. ^ Ibid. p.

> Ibid. d.

INTERNAL ANATOMY OF INSECTS. 113

of life of the animal, we see evidently tfee all-powerful hand of that Almighty Being who created the universe, upholding by his providence, and the law that he has given to every creature, the system that he at first brought into existence.

We now come to the Diptera. These have a very sleit- der gullet, to which is attached on one side a long fili- form tube, terminating in the food-reservoir, which in some instances is simple^, but most generally coi^sists of two or more vessels **, collapsing when empty, but vary- ing in shape and size when inflated with food : the mouth of the stomach in many cases is dilated into a kind of ring^; sometimes there is on each side a blind appendage or caecum opening into it, in Bombylius covered with shags, which though not connected with the mouth by a tube, Ramdohr regards as saliva-reservoirs**; in Musca vomitoria the beginning of this organ below th^ mouth is covered with hemispherical prominences, and in T/- pula it is dilated and marked widi transverse folds. There are usually two pairs of bile-vessels; in the Muscida pedunculate ondjiee ^ ; in Ttjmla, Bombylitis^ and RhagiOy sessile and united^ ; and in Tabanus sessile nnAJixed^^ It is remarkable that in some of this Order — the reverse of what usually happens — ^the alimentary canal appears to be much longer in the larva than it is in the iinago ; in^ Musca vamiloriay its length in thejbrmer is two inches and a quarter, while in the latter it is only one inch and

* Ramdohr, Ibid, t xx./. 1. E,f. 6. C.

»» Ibid, L xix./. 2. C,f. 3. CCD, t. xx./. 2. E.

^ Ibid, t. xix./. 2. Z>.

^ Ibid. t. xx./ 2, FF.f, ©. DD. 184. 180.—

* Ibid. t. xix./ 1. ON,f. 2. OP.f. 3. F. t. xxviii./ 1, 2. p. q,

* Ibid. U XX. f. 1. G.f. 2, 3. L. ' Ibid, t xxi./ I D,

VOL. IV. I

114 INTERNAL ANATOMY OF INSECTS.

one third*. A singular organ distinguishes the imago of this species, the use of which appears not to be disco- vered. It succeeds the rectum, and has on each »de two short club-shaped appendages, op w at die end, which receive trachea^ and terminate in a short piece that opens into the anus^

In Hippobosca and its affinities the canal in question differs from that of other Diptera, in having no food-re- servoir; in other respects it resembles it^.

From the above statement it appears that the princi- pal character which distinguishes those that take their food by suction^ from those that masticate it, is the faculty with which they are iurnished by means of an ample crop, honey-'Stomach, or food^-reservoir, of regurgitating the food they may have stored up. Another distinction still more striking, which will appear more evidently here- after, is to be seen in the saliva-secretoi^s with which the siictorious tribes are famished, to be found in very few masticators, by which they are enabled to render the juices more fluid and fit for suction.

The only insect amongst the Aptet^a whose alimentary canal I shall notice, is the common harvest-^man (Pha- langium Opilio) : in this, though the stomach and lower intestine are remarkably simple, yet their coecal appen- dages are numerous and singular; the former, which has no distinct gullet, is pear-shaped**; and the latter, tapering downwards, and truncated at the end*^; con-

9

* Ramdohr, lUd, 17:2.