Though in general there is only a pair of tracheæ, yet in some larvæ a larger number have been discovered. In those of the Libellulinæ there are six. According to M. Cuvier, Reaumur, who mentions only four, overlooked the two lateral ones that are connected with the spiracles[280]. The reason of this and other parts of their internal structure I shall explain under the next head. In the grub of the gad-flies of the horse (Gasterophili,) Mr. B. Clark discovered eight longitudinal tracheæ,—six arranged in a circle and two minute ones, which appeared to him to terminate in a pair of external nipples (spiracles) in the neck of the animal[281]. This is a singular anomaly, as the other Œstridæ have only a pair of tracheæ[282].
iii. Respiratory Sacs or Pouches. Besides their tracheæ and bronchiæ, many insects are furnished with reservoirs for the air, under the form of sacs, pouches, or vesicles. These are commonly formed by the bronchial tubes being dilated at intervals, especially in the abdomen, into oblong inflated vesicles; from which other bronchial tubes diverge, and again at intervals expand into smaller vesicles, so as to exhibit no unapt resemblance—as Swammerdam has observed with respect to those of the rhinoceros-beetle—to a specimen of Fucus vesiculosus. Cuvier compares them in the Lamellicorn beetles in general to a tree very thickly laden with leaves[283]; and Chabrier observes that they particularly occur in the intestinal canal[284]. This structure of the pulmonary organs may be seen also in the common hive-bee, and other Hymenoptera; but the vesicles are less numerous, and those at the base of the abdomen much larger than the rest[285]. These vesicles, by a very rough dissection, may be distinctly seen in the abdomen of the cockchafer, which appears to be almost filled with them. Not being composed of cartilaginous rings like the air-tubes, but of mere membrane, if a pin pierces one, the air that inflates it escapes, and it collapses. In the larva of a little gnat (Corethra culiciformis) the tracheæ appear to proceed from a pair of oblong vesicles of considerable size[286] in the trunk, and towards the anus they form two other smaller ones[287],—upon piercing the former, De Geer observed a considerable quantity of air to make its escape[288]. Another species, probably of the same genus, described by Reaumur, exhibits something similar[289].
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.