Chapter IV: Letter XXXVIII (2)

But one of the most remarkable structures, in this respect, is to be seen in the larva and pupa of the dragon-flies (_Libellulina_). I have before noticed the _number_ of their _tracheæ_, but I shall here describe their whole internal respiratory apparatus. I must observe that _Reaumur_, _Cuvier_, and most modern writers on the physiological department of Entomology, have affirmed that they respire the _water_, and that they receive it for that purpose at their anal extremity: but M. Sprengel, from having observed in the larvæ abdominal spiracles, is unwilling to admit this as a fact[290]; and De Geer also seems to hesitate upon it, especially as he discovered that the animal seemed to absorb the water to aid it in its _motions_[291]. But when we consider that it is by the action of a _pneumatic_ apparatus that the absorption and expulsion of the water takes place, and that the animal when it has been taken out of that element, upon being restored to it, immediately has _eager_ recourse to this action[292], we shall feel inclined rather to adopt the opinion of those great physiologists Reaumur, Lyonet, and Cuvier, and admit that it absorbs water for the purpose of _respiration_. I shall now explain how this takes place. The pieces both internal and external that close the anal orifice have been before described; the others employed in the 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 direction, so that the five last segments of the abdomen become entirely empty, and form a chamber to receive the water that enters by it. When the water is to be expelled, the whole mass of air-vessels which had receded towards the trunk, is pushed forwards, and forms a piston that again expels the water in a jet. It consists of an infinite number of _bronchiæ_, entangled with each other, which proceed from the middle and posterior end of the _tracheæ_. 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 _tracheæ_, which decompose the water, and absorb the air contained in it. He also discovered that each of these tubes gave birth to another outside the _rectum_, which connected itself with one of the six great longitudinal _tracheæ_; two of which are of enormous size, and appear to serve as reservoirs, since they furnish air by transverse branches to two other tubes; they have each a recurrent branch, which follows the course of the intestinal canal, and furnishes it with an infinity of _bronchiæ_[293]. These _tracheæ_ are found in the perfect insect. The principal ones in some send forth many branches, terminating in vesicles, which in shape resemble the seed-vessels of some species of _Thlaspi_, while others appear to form a file of oblong ones[294]. Near each of their spiracles also is a vesicle which appears to be a reservoir[295].

But this kind of structure is not confined to insects strictly _aquatic_. Even such species of _terrestrial_ ones as live upon aquatic plants, and are, consequently, necessarily or accidentally often a considerable time under water, are furnished with some apparatus by means of which they can exist in this element for a considerable period. For example, most of the Weevils (_Rhyncophora_) die in a short time if immersed in water; yet the species of the genera _Tanysphyrus_, _Bagous_, and _Ceutorhynchus_ which feed on aquatic plants, can exist for days under water, as I have ascertained by experiment. _C. leucogaster_ and another of the same tribe, swims like a _Hydrophilus_, and will live a long time 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 vesicle 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 spiracles of the _metathorax_ in _Vespa_, &c.[296] In _Sphinx Ligustri_ the _bronchiæ_ terminate in oblong vesiculoso-cellular bodies, almost like lungs[297]; in _Smerinthus Tiliæ_ these are preceded by a simple vesicle bound with spiral fibres[298]. M. Chabrier thinks that these air-bladders of insects, amongst other functions, give more fixity and force to the muscles for flight[299].

Many physiologists have seen an analogy between the _spiral_ vessels of _plants_ and the _tracheæ_ of _insects_; and some of great name, as Comparetti, Decandolle, and Kieser, have thought that in some instances they terminated in the _oscula_ or cortical pores: but Sprengel contends that they are not accurate in this opinion[300]. In fact, the principal analogy seems to be in the _spiral_ structure of both these vessels.

* * * * *

Having considered the different organs of respiration both external and internal, I shall make a few further observations upon this function. We know little 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,--namely, the _spiracles_, or their representatives. 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 expire through the mouth, anus, or pores of the skin, are founded on an erroneous assumption. This physiologist, having observed on the surface of submerged insects numerous bubbles of air, concluded that they had passed through the above orifices[301]: but Bonnet found by various experiments carefully conducted, that this appearance was caused by air which adhered to the skin and its hairs, and that when the access of this was precluded by carefully moistening the skin with water previously to immersion, this accumulation of air-bubbles on its surface did not take place[302]. And in a variety of instances he observed large ones issue from all the spiracles, especially the anterior ones. These bubbles sometimes were alternately emitted and absorbed without quitting the spiracle[303], and at others were darted with force to the surface of the water, where they appeared to burst with noise[304]. This author is of opinion that the _first_ and _last_ pair of these organs are of most importance to respiration[305]. Reaumur subsequently owned that Bonnet's arguments had shaken his opinion[306]; and some observations of his own, with respect to the respiration of the _bot_ of the _ox_, go to prove that expiration and inspiration are not by the _same_ spiracles; for he found that the air in this animal was _expired_ by the eight little _lower_ orifices before mentioned[307], from which he clearly saw the air-bubbles issue--the _upper_ one he conjectures receives the air[308]. As the only communication that this grub has with the atmosphere is by its _posterior_ extremity, it follows, reasoning from analogy, that the anterior respiratory plates of Dipterous larvæ, which may be regarded as representing the spiracles of the trunk in insects in general, are destined for the escape of the air, after it has parted with its oxygen, received by the anal ones[309]. So that there seems very good ground for M. Chabrier's opinion that _inspiration_ is ordinarily by the _abdominal_ spiracles, and _expiration_ by those of the _trunk_ of insects[310]. He seems to have been led to the adoption of this opinion, not so much by experiments similar to that of Reaumur just stated, but by observing that in many instances these two sets of spiracles differ from each other, the latter having a _convex_ and the former a _concave_ mouth or bed[311]. In some cases, however,--for instance during flight,--he supposes the spiracles of the trunk may _receive_ as well as _emit_ the air[312]: he likewise is of opinion, and it seems not improbable, that by means of these openings in the trunk, from the rush 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 _collar_; while in the latter he attributes it to the spiracles in the _metathorax_ behind the wings attended by a poiser[313]. I incline, however, to M. Dufour's opinion[314],--that the vocal spiracles in the _Hymenoptera_, as well as in the _Diptera_, are those _behind_ the wings. Perhaps both theories may be right; for if you take any common humble-bee, you will find that, in the hand, it produces one kind of sound when its wings are motionless, and another more complex and intense when they vibrate. In numerous instances, however, there is no very striking _external_ difference between the spiracles of the _trunk_ and those of the _abdomen_: this observation applies more particularly to the caterpillars of _Lepidoptera_; but whether these receive the air by those of the abdomen, and return it by those of the trunk, has not yet been ascertained; and indeed, too little is at present known upon the subject, and too few facts have been collected, to admit of dogmatizing.

The _external signs_ of respiration in insects are not universally to be discovered. The alternate contraction and expansion of the abdomen is, however, very visible in some beetles, bees, the larger dragon-flies, and grasshoppers. In one of the latter, _Acrida viridissima_, Vauquelin observed that the inspirations were from fifty to fifty-five times in a minute in atmospheric air, and from sixty to sixty-five when in oxygen gas[315]. But M. Chabrier has given the most satisfactory account of these signs: The abdomen, says he, is the principal organ of inspiration; it can dilate and contract, lengthen and shorten, elevate and depress itself. In flight, in elevating its extremity at the same time with the wings, it contracts itself, pushes the air into the trunk, and diminishes the weight of the body by the centrifugal ascending force[316]. In the majority 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 the _Dynastidæ_, _Galeodes_, &c. by the longitudinal folded membrane that unites the dorsal and ventral segments--in the _Libellulina_ by similar _ventral_ folds; and in _Cimbex_ by membranous pieces in the first dorsal segment, which De Geer observed was elevated and depressed at the will of the animal[317].

Air is as essential to insects in their _pupa_ as in their _larva_ or _perfect_ states. Lyonet, however, Musschenbroek, Martinet, and some other physiologists, have doubted whether _quiescent_ pupæ breathed[318]; but Reaumur and De Geer seem to have proved that they do[319]: and if thrown into water, the same proof of respiration, by the emission and retraction of a bubble of air takes place, as in the larvæ; and De Geer found that if one be transferred under water from one spiracle to another, it will be absorbed by it[320]. Indeed, unless these pupæ had breathed, where would have been the necessity for the spiracles with which all are furnished? It is remarkable, however, that all these spiracles do not seem of equal importance in this respect. Reaumur found that if the _posterior_ spiracles only were closed with oil, the insect suffered no injury; but that if the _anterior_ ones were similarly treated, it infallibly died[321]. The respiration however of pupæ seems more perfect in those that have recently assumed that state, than in those that are more advanced towards the imago; in which at first, from Reaumur's experiments[322], it appears that the posterior spiracles were stopped; and in others still older, from Musschenbroek's[323], even the anterior ones. Those quiescent pupæ that during that state remain _submerged_, respire air. De Geer has given an interesting record of this, in the case of _Hydrocampa stratiotata_. This insect spins a double cocoon, the outer one thin, and the inner one of a close texture. In the pupa there are three pair of conspicuous spiracles on the second, third, and fourth segments of the abdomen, which are placed on cylindrical tubes, and they appear to have no other air-vessels. The respiratory gills of the larva having vanished, like some others of the same genus, they know how to surround themselves with an atmosphere of air in the midst of the water, so that the interior of their inner cocoon is impervious to the latter element--how they renew the air has not been ascertained. Though they respire air, water is equally necessary, for the animal died when kept out of water[324].

The great majority of insects respire in much the same manner in all their states, particularly as to their _external_ organs; for when the larva breathes by the lateral spiracles, the pupa and imago usually do the same. The converse of this, however, by no means holds; for it not unfrequently happens that the two latter breathe by means of lateral spiracles, though they received the air in their larva state by an apparatus altogether different. Thus the larvæ of many _Diptera_ breathe by an anal tube, while the pupa and imago follow the general system. Sometimes a tribe of insects breathe by an apparatus quite different in all their states, as we have seen to be the case with the common gnat[325], which has an _anal_ respiratory _tube_ in its _first_ state, _thoracic_ respiratory _horns_ in its _second_, and the _ordinary_ lateral _spiracles_ in its _third_.

Changes also take place in their _internal_ organs. In the larvæ the respiratory apparatus, especially the tracheal tubes, is often much larger and more ramified than in the imago; and as the former is the principal _feeding_ state, there seems good ground for Mr. B. Clark's opinion--that the respiration is intimately connected with the conversion of the food[326]. In the _imago_, there appears to be more provision for storing up the air in vesicular reservoirs, than in the _larva_. Wonderful is the mode in which some of the changes in the internal structure, which these variations indicate, must necessarily take place. They are, however, probably not more singular than those which less obviously occur in the air-vessels of all insects in their great change out of the larva into the pupa state. But having before enlarged on this subject, I need not repeat my observations[327].

The access of air is as necessary to insects even in their _egg_ state[328], and in many cases its presence seems provided for with equal care, by means as beautiful as those Sir H. Davy and Sir E. Home have shown to occur in the oxygenation of the eggs and fœtuses of vertebrate animals[329]. 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 while in the ovaries[330],--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 judging from analogy, 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 plants, and some of them embedded in the very substance of a leaf or twig[331], 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 utility in the same point of view. In the case of the _Trichoptera_ and other insects[332] whose eggs are dropped into the water enveloped in a mass of jelly, this substance perhaps serves for aërating the included 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[333]. It is not improbable that the singular rays that terminate the eggs of _Nepa_[334] may in some way be connected with the aëration of the egg.

To what I have before remarked with regard to the _vital heat_ of insects[335], I may under this head very properly add a few further observations. I there stated, that the temperature of these animals is usually that of the medium they inhabit, but that bees, and perhaps other gregarious ones, furnish an exception to this rule[336]. A confirmation of this remark is afforded by Inch, a German writer, who, upon putting a thermometer into a bee-hive in winter, found 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 (_Cantharis vesicatoria_,) 4° or 5° higher. A thermometer, standing in the air at 14° R., put into a glass vessel with _Acrida viridissima_, in nine minutes rose to 17°, and a similar result was observed with respect to other insects[337]. Dr. Martine says that caterpillars have but two degrees of heat above that of the air they live in[338]. Coleopterous insects are said to move slowly and with difficulty when the thermometer sinks to 36°, to become torpid at 34°, and to lose muscular irritability at a lower degree[339]. I have before observed that some insects will bear to be frozen into an icicle, and yet survive[340]: they share this power with reptiles, fishes, and amphibia. But, however small the excess of it in some insects above that of the medium they inhabit, it proves that they possess the power of _generating_ heat. Whether, like the warm-blooded animals, they generally possess that of _resisting_ heat by perspiration, &c. is not so clear. Yet the heat to which some can bear to be exposed, basking at noon, as Dr. Clarke informs us[341], on rocky and sandy places, exposed to the full action of the sun, appears sufficient, if not resisted by some principle of counteraction, to roast them to a cinder. That bees perspire is well known, but probably not singly.

When the respiration of insects is suspended by immersion in any fluid, it is often resumed, even when it has been long and they are apparently dead, if they be brought into contact with the atmosphere. Reaumur found this to be the case with bees[342]; and Swammerdam tells us that the maggot of the cheese-fly (_Tyrophaga Casei_) lived six or seven days in rain-water[343]: he found it so difficult to kill the larva of _Stratyomis Chamæleon_, 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[344].

That the suspended animation and subsequent death of most terrestrial insects when thrown into water is caused by the want of _air_, is evident from this,--that the same effect ensues if the spiracles be covered with any oily or fatty matter. In this case too, their vital powers soon become suspended: they revive, if the suffocating matter be soon removed; and if this be not done, infallibly perish. This fact was known to the ancients, for Pliny observes that bees die if dipped in oil or honey[345]. One exception to this law has been before mentioned[346]: a similar contrivance secures the cheese-maggot from having its respiration interrupted by its moist and greasy food; the grub also of _Sarcophaga carnaria_, and of other _Muscidæ_ probably, has its posterior spiracles placed in a plate at the bottom of a kind of fleshy pouch, which has the shape of a hollow, truncated, and reversed cone. This pouch the grub can close whenever it pleases, so as to cover its spiracles[347]. And numerous other larvæ, both of _Diptera_ and _Coleoptera_ that devour unclean and oily food, have doubtless some protection of this kind for their spiracles and respiratory plates.

I am, &c.

FOOTNOTES:

[144] _Anat. Compar._ iv. 296.

[145] Plin. _Hist. Nat._ _l._ xi. _c._ 3. Even Aristotle seems to have given into the common opinion. _De Respirat._ _c._ 3, 9. &c.

[146] _Philos. Trans._ v. 2011. Works, 4to. i. 79, 112.

[147] Aristot. _Hist. Animal._ _l._ viii. _c._ 27.

[148] _On Air and Fire_, 148, 155.

[149] _Tracts_, 208.

[150] _Mem. on Respirat._ 75.

[151] _Ann. de Chimie_, xii. 273.

[152] F. L. A. Sorg, _Respirat. Insect. et Verm._ Ellis, _Inquiry into Chang. prod. on Atmosph. Air by Respirat._ &c. 69.

[153] _Ann. de Chimie_, xii. 273.

[154] Sprengel, _Commentar._ &c. 27--.

[155] PLATE XXIII. FIG. 2. and PLATES VIII. IX. XVI. XXIX. _c´_,_h´´_, _m´´_, _A´´_, _D´´_.

[156] Moldenhawers (_Anat. der Pflanz._ 314--.) affirms that the spiracles of most insects are quite closed: but Sprengel (_Commentar._ § 8.) has satisfactorily refuted that opinion.

[157] PLATE XXIII. FIG. 2.

[158] Sprengel, _Commentar._ § 7.

[159] _Ibid._ _t._ iii. _f._ 30.

[160] PLATE XXIX. FIG. 23.

[161] Ibid. 8.

[162] Sprengel, 7. _t._ iii. _f._ 30.

[163] _Ibid._ _t._ ii. _f._ 22. _t._ iii. _f._ 29.

[164] PLATE XXIX. FIG. 29.

[165] Ibid. FIG. 16. Sprengel, _Ibid._ 9. _t._ 1. _f._ 4-6.

[166] _Ibid._ 9. _t._ i. _f._ 9.

[167] PLATE XXIX. FIG. 16. _a._

[168] Sprengel, _Ibid._ _t._ iii. _f._ 27.

[169] Sprengel, _Commentar._ 7--.

[170] Sprengel, from whom I have borrowed this quotation, expresses the time by "_scripulo horæ_." This word is of uncertain meaning, being scarcely ever applied to _time_; but as it means the twenty-fourth part of an ounce, Faber conjectures it may mean the same portion of an _hour_.

[171] Sorg, _Disquisit. circa respirat. insect._ 27, 46, 66. Sprengel _ubi supr._ 11--.

[172] Chabrier _sur le Vol des Ins._ _c._ l. 454.

[173] PLATE XXIX. FIG. 28. _A´´_.

[174] _Ibid._ FIG. 23.

[175] Sepp. I. iv. _t._ ii. _f._ 3.

[176] _Ibid._ _t._ xiv. _f._ 3.

[177] _Ibid._ _t._ v. _f._ 6, 7.

[178] _Ibid._ _t._ i. _f._ 7, 8.

[179] _Ibid._ _t._ x. _f._ 6, 7.

[180] _Ibid._ v. _t._ i. _f._ 3.

[181] _Sphinx Labruscæ_ Merian _Surinam._ 34.

[182] PLATE XXIX. FIG. 28. _A´´_.

[183] Swammerd. _Bibl. Nat._ _t._ xxvii. _f._ 5. Compare Sturm _Deutsch. Fu._ i. _t._ v. _f._ r.

[184] PLATE XXIX. FIG. 12. _c´_.

[185] De Geer, i. 81. _t._ v. _f._ 10. _f._

[186] _Sur le Vol des Ins._ _c._ i. 459.

[187] Reaum. iv. 246. _t._ xix. _f._ 8. _s._

[188] In this tribe, which I forgot to remark before, (see VOL. III. p. 549--.) there seems both _prothorax_ and _collar_.

[189] VOL. III. p. 550, 559. &c.

[190] PLATE VIII. FIG. 14. h´´.

[191] PLATE XXIX. FIG. 14, 15. m´´.

[192] Ibid. FIG. 15. a.

[193] Ibid. FIG. 14, 15. b.

[194] Ibid. FIG. 25. _k´´_.

[195] Chabrier _sur le Vol des Ins._ _c._ iii. _t._ vi. _f._ 4. _Sa, Sp._

[196] PLATE IX. FIG. 21. _m´´_.

[197] PLATE VIII. FIG. 9.

[198] VOL. III. p. 705--.

[199] VOL. III. p. 708.

[200] Sprengel, _Comment._ 3.

[201] _Ibid._

[202] vi. 398.

[203] De Geer, ii. 635.

[204] _Fourmis_, 22.

[205] _Osservaz. &c. sullo Iulus fœtid._ 14--.

[206] They are particularly visible in an undescribed East Indian species, (_S. alternata_ K. M. S.) with scuta alternately black and yellow.

[207] PLATE XXIX. FIG. 20. _A´´_.

[208] De Geer, vii. _t._ vi. _f._ 3.

[209] VOL. I. p. 254--.

[210] De Geer vi. 67. _t._ iii. _f._ 10. _ss._ 14. Mr. W. S. MacLeay (_Philos. Mag. N. Ser._ No. 9. 178.) says that in this grub the longitudinal trunks of the Tracheæ send off at equal distances lateral branches just as if there were spiracula to correspond with them. This is evidently a preparatory step to the formation of those that ultimately appear in the perfect insect.

[211] De Geer 66. _t._ iii. _f._ 13.

[212] PLATE XIX. FIG. 11. _a._

[213] Reaum. iv. 375--. _t._ xxvi. _f._ 7, 8.

[214] _Ibid._ 555. _t._ xxxv. _f._ 10. _ss._

[215] _Ibid._ 519--. _t._ xxxvii. _f._ 3, 4.

[216] PLATES XVI. FIG. 9. _a b._ XIX. FIG. 9, 10, 12, 13. _a._ XXIX. FIG. 3-7.

[217] PLATE XIX. FIG. 9. _a._

[218] PLATE XIX. FIG. 9. _b._

[219] Compare Swamm. _Bibl. Nat._ i. 154. _t._ xxxi. _f._ 5. Reaum. iv. 601--. _t._ xliii. De Geer vi. 317--. _t._ xvii. _f._ 2-8.

[220] Swamm. _Ibid._ _t._ xxxi. _f._ 7, 8.

[221] Reaum. iv. 607.

[222] PLATE XIX. FIG. 12. _a._

[223] Reaum. iv. _t._ xxxii. _f._ 2. _e._

[224] Mr. W. S. MacLeay (_Philos. Mag. N. Ser._ n. 9. 179.) asserts that what Reaumur (iv. 487. _t._ xxx. _f._ 6. _ll_) calls the first pair of legs of this grub, are the usual palmated stigmata which occur on the humerus of the larvæ of _Muscidæ_. It does not appear whether he has himself examined this grub, but Reaumur (443) states that it has _seven pairs of legs all armed with claws_. If this is correct, it is not properly a palmated organ.

[225] Reaum. iv. _t._ xxx. _f._ 10.

[226] Reaum. iv. _t._ xxx. _f._ 447--.

[227] _Ibid._ 456. _t._ xxxi. _f._ 1-7.

[228] PLATE XIX. FIG. 13. _a._

[229] _Bibl. Nat._ ii. 44.

[230] PLATE XIX. FIG. 10. _a._

[231] Reaum. v. _t._ iv. _f._ 6. _s, u._

[232] VOL. II. p. 275--.

[233] De Geer vi. 395--. _t._ xxiv. _f._ 16. 18. _d._

[234] v. _t._ vi. _f._ 1, 2.

[235] De Geer iii. 367. _t._ xviii. _f._ 1, 2, 9.

[236] _Ibid._ vi. 36. 194--. _t._ ii. _f._ 2, 3. _s._

[237] PLATE XVI. FIG. 9. _a. b._

[238] De Geer ii. 539--. _t._ xi. _f._ 12, 16, &c.

[239] De Geer i. 526--. _t._ xxxvii. _f._ 2-6.

[240] _Ibid._ iv. 362--. _t._ xiii. _f._ 16-19.

[241] VOL. I. p. 282--. II. 365--.

[242] See Reaum. vi. _t._ xlii.--xlvi. and PLATE XXIX. FIG. 3-5.

[243] Reaum. _Ibid._ _t._ xlv. _f._ 2.

[244] PLATE XXIX. FIG. 5. De Geer ii. 624--.

[245] Ibid. FIG. 4. De Geer _Ibid._ 647--.

[246] Ibid. FIG. 3. De Geer _Ibid._ 653--.

[247] Ibid. FIG. 6. De Geer _Ibid._ 727--.

[248] Reaum. vi. 465.

[249] _Ibid._ _t._ xlii. _f._ 4, 5. De Geer ii. 623.

[250] _Ibid._ 648. _t._ xvii. _f._ 11, 12.

[251] VOL. III. p. 154. De Geer ii. 697--. _t._ xxi. _f._ 4, 5, 12.

[252] De Geer _Ibid._ 666--. _t._ xix. _f._ 6.

[253] Reaum. vi. 393. _t._ xxxvi. _f._ 8, 9. _t._ t.

[254] Reaum. vi. 395. _t._ xxxvi. _f._ 8-9. _c. c._

[255] PLATE XXIX. FIG. 21.

[256] Marcel de Serres (_Mem. du Mus._ 1819. 137, &c.) calls the _tubular tracheæ_ that _receive_ the air, _arterial tracheæ_, and the _vesicular_ ones which act as _reservoirs_, _pulmonary tracheæ_.

[257] PLATE XXIX. FIG. 1. 2.

[258] Treviranus _Arachnid._ 7--. _t._ l. _f._ 1. _r. f._ 10. Comp. _N. Dict. d'Hist. Nat._ xxx. 419. Latreille calls these gills _Pneumo-branches_.

[259] Treviranus _Ibid._ 24. PLATE XXIX. FIG. 1.

[260] PLATE XXI. FIG. 3. _a b._

[261] Ibid. _a._

[262] Ibid. _b._

[263] Sprengel _Commentar._ _t._ i. _f._ 1.

[264] _Ibid._ _f._ 10.

[265] _Ibid._ _t._ ii. _f._ 15.

[266] Malpigh. _De Bombyc._ _t._ iii. _f._ 3.

[267] _Ibid._ _t._ iv. _f._ 1.

[268] Lyonet _Anat._ 101.

[269] Lyonet _Anat._ 101.

[270] Sprengel (_ubi. supr._ 16.) says that he never found more than _two_; but as Lyonet affirms that he has very often separated them (102), his accuracy cannot be questioned.

[271] Lyonet _Anat._ 103.

[272] _Ibid._ Cuv. _Anat. Comp._ iv. 438. This author says that the _intermediate_ tunic is the spiral thread (437).

[273] Lyonet 102.

[274] Ibid. 104. Sprengel _Commentar._ 17.

[275] Lyonet 104. Sprengel _Commentar._ 17.

[276] Lyonet 102. Malpigh. _De Bombyc._ 12. Reaum. i. 130.

[277] Swamm. _Bibl. Nat._ _t._ ii. _f._ 7.

[278] Lyonet 411.

[279] Professor Kidd (_Philos. Trans._ 1825. 235.) conjectures that the tracheæ, as well as air-vessels, may possibly be blood-vessels; but this hypothesis is inconsistent with the fact recently discovered by Dr. Carus, of a circulation, by other means, in larvæ. See Carus _Introd. to Comp. Anat._ &c. ii. 400.

[280] _N. Dict. d'Hist. Nat._ xvii. 541. Reaum. vi. 397. PLATE XXIX. FIG. 8. shows _three_ of them at _a_.

[281] _Essay on the Bots, &c._ 23. _t._ i. _f._ 7, 32, &c.

[282] _Ibid._ 49. Valisnieri i. 101. _t._ vi. _f._ 4. &c.

[283] _Bibl. Nat._ i. 149. a. _t._ xxix. _f._ _a._ Cuv. _Anat. Comp._ iv. 439. Malpigh. _De Bombyc._ _t._ iii. _f._ 2.

[284] _Sur le Vol des Ins._ c. ii. 336. note 1.

[285] Swamm. _Bibl. Nat._ _t._ xvii. _f._ 9. Cuvier _Ibid._ 440.

[286] PLATE XXIX. FIG. 10. _a._

[287] Ibid. _b._

[288] De Geer vi. 374.

[289] Reaum. v. 40. _t._ vi. _f._ 4, 7.

[290] Sprengel _Comment._ 4.

[291] De Geer ii. 667, 675.

[292] Reaum. vi. 394--.

[293] Reaum. vi. 394--. Cuv. _Anat. Comp._ iv. 440--. _N. Dict. d'Hist. Nat._ xvii. 540--.

[294] PLATE XXIX. FIG. 9. _a, b._ Reaum. vi. 418--. 450.

[295] Cuv. _Anat. Comp._ iv. 441.

[296] VOL. III. p. 583.

[297] Sprengel _Comment._ 17. _t._ iii. _f._ 24.

[298] _Ibid._ _t._ i. _f._ 11.

[299] _Sur le Vol des Ins._ c. ii. 336. note 1.

[300] Sprengel _Comment._ 13--. These _oscula_ or pores in the straw of _Triticum hybernum_, as figured by Mr. Bauer's admirable pencil, (Sir J. Banks _On the Blight, &c._ _t._ ii. _f._ 3.) exactly resemble the spiracles of insects.

[301] Reaum. i. 136.

[302] Bonnet _Œuvr._ iii. 39--.

[303] _Ibid._ 43.

[304] _Ibid._ 50.

[305] _Ibid._ 69.

[306] De Geer ii. 117.

[307] See above, p. 50.

[308] Reaum. iv. 520.

[309] Mr. B. Clark thinks that he has discovered spiracles in this larva in the usual situation, (_Essay on the Bots, &c._ 48. _t._ ii. _f._ 3.) but they are probably analogous to the spiraculiform tubercles of _Œ. Ovis_. Reaum. iv. 566. _t._ xxxv. 17-19. t. Vallisnieri (_Esperienz. &c._ 136) notices them.

[310] _Sur le Vol des Ins._ c. i. 423.

[311] _Ibid._ 454. and c. iv. 66. note 1.

[312] _Ibid._ c. i. 453.

[313] _Ibid._ 459, 456.

[314] _Ibid._ 459.

[315] _Annal. de Chim._ xii.

[316] _Sur le Vol des Ins._ c. i. 423, 454. c. iii. 344. c. iv. 66.

[317] De Geer ii. 946--.

[318] Lesser, L. i. 124. note *. Lyonet _Anatom._ pref. xii. De Geer ii. 132.

[319] Reaum. i. 399--. De Geer i. 37--.

[320] _Ibid._ 40.

[321] Reaum. i. 400.

[322] _Ibid._

[323] De Geer ii. 129.

[324] De Geer i. 531--. _t._ xxxvii. _f._ 13. s. Compare Reaum. ii. 396--.

[325] See above, p. 51--.

[326] In _Linn. Trans._ iii. 302.

[327] VOL. III. p. 195--.

[328] Spallanzani found that the eggs of insects placed under the exhausted receiver of an air-pump, or in any small closed vessels, did not hatch, though every other condition for their development was present. _Opusc. de. Phys._ i. 141.

[329] _Philos. Trans._ 1820. 213.

[330] _Bibl. Nat._ i. 204. b. _t._ xix. _f._ 5.

[331] VOL. I. p. 446--. III. p. 76.

[332] Ibid. 68--.

[333] _Philos. Trans._ 1820. 218.

[334] VOL. III. p. 94.

[335] VOL. II. p. 228--.

[336] Ibid. p. 211.

[337] Inch, c. iv. _Ideen zu Einer Zoochemie_, 68--.

[338] _On Thermom._ 141.

[339] Carlisle in _Philos. Trans._ 1805. 25.

[340] VOL. II. p. 229.

[341] _Travels_ ii. 482.

[342] Reaum. v. 540.

[343] Swamm. _Bibl. Nat._ ii. 65. a.

[344] _Ibid._ 48. a.

[345] _Hist. Nat._ _l._ xi. _c._ 19.

[346] Swamm. _Bibl. Nat._ ii. 64. a.

[347] Reaum. iv. 428. _t._ xxix. _f._ 2. _c, s._