Thetracheæandbronchiæconsist ofthreetunics[270]: thefirstor external one is a thickish membrane, strengthened by a vast number of fibres or vessels, which form round it a number of irregular circles; thesecondis a membrane more thin and transparent, without a vascular covering[271]; thethirdis formed of a cartilaginous thread running in a spiral direction, which may be easily unwound[272]. This structure gives a great elasticity to these organs, so that they are capable of considerable tension, after which they return to their usual length[273]. TheBronchiæare cylindrical or slightly conical, insensibly diminishing in size as they leave the trunk, in which they originate. In larvæ, after losing their spiral fibre, they appear to terminate in membrane, but in perfect insects they pass into vesicles[274]. In theCossusthetracheais flattened, and in every segment, except the first and two last, is bound by a fleshy cord four or five times as thick as its threads. Where this occurs, there is a slight constriction,—probably here is a sphincter, by the contractionof which Lyonet supposes thetracheamay be shut when it is necessary to stop the passage of the air, and direct it to any particular point[275]. The structure here described is admirably adapted for the purpose it is intended to serve; for had these vessels been composed ofmembrane, they could not possibly have been prevented from collapsing; but by the intervention of a spiral cartilaginous thread this accident is effectually guarded against, and the necessary tension of the tubes provided for. However violent the contortions of the insect, however small the diameter of these vessels, they are sure to remain constantly open, and pervious to the air. And by this circumstance they may be always distinguished from the other organs of the animal, and likewise by their pearly or silvery hue, for from being constantly filled with air, these tubes, when viewed under a powerful microscope in a recently dissected insect, present a most beautiful and brilliant appearance, resembling a branching tree of highly polished silver or pearl:—though sometimes they are blue, or of a lead colour, and sometimes assume a tint of gold. In the dead insect the larger tubes soon turn brown, but the finer ones preserve their lustre several weeks[276]. The ramifications of the tracheal tree may be seen without dissection through the transparent skin of the common louse[277]and most of the thin skinned larvæ.
You will not expect to view in this way the minuter ramifications of thebronchiæ, when I have mentioned their number and incredible smallness. Nothing butthe scalpel of a Lyonet and the most powerful lenses are adequate to trace the extremities of these vessels; and even with every help, they at last become so inconceivably slender as to elude the most piercing sight. That illustrious anatomist found that the twotracheæof the larva of theCossusgave birth to 236 bronchial tubes, and that these ramify into no less than 1336 smaller tubes, to which, if 232, the number of the detached bronchiæ, be added, the whole will amount to 1804 branches[278]. Surprising as this number may appear, it is not greater than we may readily conceive to be necessary for communicating 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 vertebrate animals, thebronchiæare not only carried along the intestines 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 traversing the legs and wings, the eyes, antennæ, and palpi, and accompanying the most minute nerves through their whole course[279]. How essential to the existence of the animal must the element be that is thus anxiously conveyed 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 hathGodwrought, and this is the work of his hands.
Though in general there is only apairoftracheæ, yet in some larvæ a larger number have been discovered.In those of theLibellulinæthere aresix. According to M. Cuvier, Reaumur, who mentions onlyfour, 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 discoveredeightlongitudinaltracheæ,—sixarranged in a circle andtwominute 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 otherŒstridæhave only apairoftracheæ[282].
iii.Respiratory Sacs or Pouches.Besides theirtracheæandbronchiæ, 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 ofFucus 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 otherHymenoptera;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) thetracheæappear 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 thenumberof theirtracheæ, but I shall here describe their whole internal respiratory apparatus. I must observe thatReaumur,Cuvier, and most modern writers on the physiological department of Entomology, have affirmed that they respire thewater, 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 itsmotions[291].But when we consider that it is by the action of apneumaticapparatus 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 haseagerrecourse 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 ofrespiration. 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 evidentlyrespiratoryorgans. 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 ofbronchiæ, entangled with each other, which proceed from the middle and posterior end of thetracheæ. M. Cuvier in the interior of therectumof 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 oftracheæ, which decompose the water, and absorb the air contained in it. He also discovered that each of these tubes gave birth to another outside therectum, which connected itself with one of the six great longitudinaltracheæ; 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 ofbronchiæ[293]. Thesetracheæare 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 ofThlaspi, 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 strictlyaquatic. Even such species ofterrestrialones 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 generaTanysphyrus,Bagous, andCeutorhynchuswhich feed on aquatic plants, can exist for days under water, as I have ascertained by experiment.C. leucogasterand another of the same tribe, swims like aHydrophilus, 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 thecollarin the humble-bee, where Chabrier supposes the vocal spiracles are situate; and also at the mouth of the spiracles of themetathoraxinVespa, &c.[296]InSphinxLigustrithebronchiæterminate in oblong vesiculoso-cellular bodies, almost like lungs[297]; inSmerinthus Tiliæthese 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 thespiralvessels ofplantsand thetracheæofinsects; and some of great name, as Comparetti, Decandolle, and Kieser, have thought that in some instances they terminated in theosculaor cortical pores: but Sprengel contends that they are not accurate in this opinion[300]. In fact, the principal analogy seems to be in thespiralstructure 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 insectsrespire, except that they breathe out the air by the same kind of organs by which they receive it,—namely, thespiracles, 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 thefirstandlastpair 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 thebotof theox, go to prove that expiration and inspiration are not by thesamespiracles; for he found that the air in this animal wasexpiredby the eight littlelowerorifices before mentioned[307], from which he clearly saw the air-bubbles issue—theupperone he conjectures receives the air[308]. As the only communication that this grub has with the atmosphere is by itsposteriorextremity, it follows, reasoning from analogy, that the anterior respiratory plates of Dipterous larvæ, which may be regarded as representing the spiracles of the trunk ininsects 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 thatinspirationis ordinarily by theabdominalspiracles, andexpirationby those of thetrunkof 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 aconvexand the former aconcavemouth or bed[311]. In some cases, however,—for instance during flight,—he supposes the spiracles of the trunk mayreceiveas well asemitthe 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 thecollar; while in the latter he attributes it to the spiracles in themetathoraxbehind the wings attended by a poiser[313]. I incline, however, to M. Dufour's opinion[314],—that the vocal spiracles in theHymenoptera, as well as in theDiptera, are thosebehindthe 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 strikingexternaldifference between the spiracles of thetrunkand those of theabdomen: this observation applies more particularly to the caterpillars ofLepidoptera; 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.
Theexternal signsof 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 theDynastidæ,Galeodes, &c. by the longitudinal folded membrane that unites the dorsaland ventral segments—in theLibellulinaby similarventralfolds; and inCimbexby 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 theirpupaas in theirlarvaorperfectstates. Lyonet, however, Musschenbroek, Martinet, and some other physiologists, have doubted whetherquiescentpupæ 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 theposteriorspiracles only were closed with oil, the insect suffered no injury; but that if theanteriorones 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 remainsubmerged, respireair. De Geer has given an interesting record of this, in the case ofHydrocampa 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 theirexternalorgans; 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 manyDipterabreathe 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 ananalrespiratorytubein itsfirststate,thoracicrespiratoryhornsin itssecond, and theordinarylateralspiraclesin itsthird.
Changes also take place in theirinternalorgans. 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 principalfeedingstate, there seems good ground for Mr. B. Clark's opinion—that the respiration is intimately connected with the conversion of the food[326]. In theimago, there appears to be more provision for storing up the air in vesicular reservoirs, than in thelarva. 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 theireggstate[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 importancewhich 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 andHydrophiliare deposited, may probably, from Count Rumford's experiments, be of utility in the same point of view. In the case of theTrichopteraand 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 ofNepa[334]may in some way be connected with the aëration of the egg.
To what I have before remarked with regard to thevital heatof 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 Germanwriter, 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 withAcrida 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 ofgeneratingheat. Whether, like the warm-blooded animals, they generally possess that ofresistingheat 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 immersionin 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 ofStratyomis 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 ofair, 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 ofSarcophaga carnaria, and of otherMuscidæprobably, 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, andreversed cone. This pouch the grub can close whenever it pleases, so as to cover its spiracles[347]. And numerous other larvæ, both ofDipteraandColeopterathat devour unclean and oily food, have doubtless some protection of this kind for their spiracles and respiratory plates.
I am, &c.
We learn from the highest authority, that thebloodis thelifeof the animal[348]: every object of creation, therefore, that is gifted with animal life, we may conclude, in some sense, has blood, which in this large sense may be defined—The fluid that visits and nourishes every part of a living body[349]. But theGreat Authorof nature has varied themachineryby which this nutritive fluid is formed and distributed, gradually proceeding from the mostsimpleto the mostcomplexstructure; in which he seems to have seen it fit toinvertthe process observable in the systems of sensation and respiration, where the ascent is from the mostcomplex, to the mostsimplestructure. In the lowest members of the animal creation, the blood seems the portion they imbibe of the fluid medium in which they reside, which when chylified, distributes new molecules to all parts of their frame[350]. In others, as in insects, it is formed by the chyle that transpiresthrough the intestinal canal into the general cavity of the body, where it receives oxygen from the air-vessels, and is fitted for nutrition[351]. In these animals it is accompanied by a long dorsal vessel, the first step towards aheart, which alternately contracts and dilates with an irregular systole and diastole, but appears to have no vascular system connected with it, though in their preparatory states it has anextra-vascularcirculation which ceases in the perfect insect. Again: in others, as theTubicoles,Annelida, &c., a realcirculationhas been discovered; that is to say, a system of veins and arteries, but unaccompanied by a muscular heart[352]. In theArachnidaandBranchiopod Crustaceathe 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[353]. It has therefore now become a trueheart, and there is a regularcirculation; and in theDecapod Crustaceathe dorsal vessel is contracted into an oval form, and placed nearly in the centre of the trunk[354]. In the great majority of invertebrate animals the blood iswhite, but in theAnnelida, to which Class the common dew-worm belongs, a curious anomaly takes place—for it isred[355]. Thus a gradual ascent is made to the circulating system of the vertebrate and red-blooded animals. In all, however, thebloodis the principal instrument of nutrition and accretion; and is on that account properly so denominated, though not connected with a circulating system.
Having given you this general outline of the meansby which the blood is distributed in the different Classes of animals, I shall now confine myself to the case of insects andArachnida, beginning with theformer.
I. 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 appearance is real, that there is actually present in the back of most insects, placed immediately under the skin and furnished with numerous air-vessels, a longitudinal vessel[356]originating in the head near the mouth[357], running parallel with the alimentary canal nearly to the anus, containing a fluid which is propelled in regular pulsations of from 20 to 100 per minute, more or less as the weather is colder or warmer[358], causing a sensible alternate systole and diastole from the anal extremity towards the head. In theCossusthese pulses were observed by Lyonet to begin in theeleventhsegment, from which they passed from segment to segment, till they arrived at thefourth, where they terminated[359]. This vessel is what Malpighi, who first discovered it, termed aheart, or rather series of hearts[360]; but which Reaumur, who injected it, regarded as a simplearterywithout striking contractions[361]: but to steer clear of any hypothesis, I shall merely call it thedorsal vessel(Pseudo-cardia). When carefully taken out of the body it is found to be a membranous tube, appearing to be closedat each end[362], in many larvæ of equal diameter every where, but in perfect insects usually widest at theanalextremity[363], and attenuated into a very slender filament towards the head. In some insects, however, as in the larva of the chamæleon-fly (Stratyomis Chamæleon), it is attenuated atbothends, and in theEphemerais alternately constricted and dilated as Malpighi describes that of the silkworm[364], a dilated portion belonging to each segment[365]. In theCossus, and probably others, after thethirdsegment, it is furnished with nine pair, the three posterior pair being the largest, of triangular transverse bundles of muscular fibres, which Lyonet denominates itswings[366], the action of which produces its systole and diastole, and their propagation from the tail towards the head[367]. Under the last pair of these wings it is strengthened by a large number of circular muscular fibres[368]. I have stated it asappearingto be closed at each extremity, because Cuvier and most writers have so regarded it, and probably it is so closed in the perfect insect; but from Lyonet's words it should seem that, in the larva of theCossus, he considered it as open and expanded at its anterior end[369]. He seems also to suspect, that, by means of what he calls the frontal ganglions, a fluid is derived from the dorsal vessel to the spinal marrow. He likewise describes a large nerve as passing through it and becoming recurrent[370]. Carus, as we shall soon see, has also proved that this tube is not closed in larvæ.
Thefluidwhich 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[371]. Examined under the microscope it appears filled with a prodigious number of transparent globules, of incredible minuteness[372]. 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 theCossusyields a mass of it of the size of a grey pea[373].
From the situation of this dorsal vessel, which is precisely the same with that of the heart inArachnidaand the BranchiopodCrustacea, and from the systole and diastole which keep its fluid contents in constant motion, who can wonder that the physiologists who first discovered it, reasoning analogically, maintained that it was a trueheart? But modern comparative anatomists, and those of the highest name, from the absence of a vascular system for a circulation, have contended that it is not a true heart, but an organ appropriated to other purposes: a third hypothesis, and intermediate between these two, has very recently been promulgated, that the organ in question, namely, is a real heart, and in the preparatory states of insects, the centre of a real circulation, which, in the imago state, ceases with the full development of the wings; but that this circulation isextravascular, or without peculiar vessels analogous to veins and arteries.
I shall now enlarge a little upon each of these hypotheses, beginning with the first or original one.
No one will deny that the argument from analogy is strongly in favour of this: I need not therefore dwell upon it, but proceed to others. Swammerdam, to whose exactness in observing, and scrupulous accuracy, every reader of his immortal work will bear testimony, expressly asserts that he has seen vessels issuing from the dorsal vessel in the silkworm, and even succeeded in injecting them with a coloured fluid[374]. Now it seems extremely improbable that so practised and expert an anatomist should have been deceived, especially upon a point which would naturally excite his most earnest and undivided attention. Without thisrecordedexperiment, perhaps, it might be thought, though this was very unlikely, that he had mistakenbronchiæfor veins and arteries: but how could they have beeninjectedfrom the supposed heart? Another great physiologist, Reaumur, in the caterpillar of the saw-fly of the rose (Hylotoma Rosæ) observed, besides thedorsalvessel, aventralone of similar form, in which also was a pulsation, but slower than that of the other. This he supposes may be the principal trunk of the veins[375]. Bonnet thought he discovered a similar vessel in a large caterpillar, but withall his attention could perceive no motion in it[376]. Reaumur also fancied he perceived in the grub ofMusca vomitoria, 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 contraction and dilatation for several minutes,—this experiment was repeated with the same result upon several grubs[377]. 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 aPhryganeaL. (as Lyonet did in those of a flea[378]); and in the transparent thigh ofOrnithomia aviculariahe discovered a pulse like that of an artery[379]. Baker, whose only object was to record what hesaw, speaks of thecurrentof the blood being remarkably visible in the legs of some smallbugs[380]: what he meant by that term is uncertain, but they could not bespiders, which he had just distinguished. This author has likewise seen a green fluid passing through the vessels of the wings of grasshoppers[381]; 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[382]; but this last fact may be independent of a circulation.
But though these arguments, which I have stated in their full force, appear strong, and at first sight conclusive,those which may be urged for the more modern opinion—that no circulation exists in insects, properly so called,—appear to have still greater weight. Lyonet, whose piercing eye and skilful hand traced the course of so many hundred nerves andbronchiælong after they became invisible to the unassisted eye, and which were a thousand times smaller than the principal 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 communicating with it[383]. And Cuvier, whose unrivalled 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 therefore ought not to be called by that name: that it is rather a secretory vessel, like many others of that kind in those animals. As to the nature of the fluid that it secretes, and its use, he thinks it impossible, from our present information on the subject, to form any satisfactory conclusion[384]. Marcel de Serres informs us—which further seems to prove that it can be no real heart—that this vessel may be totally removed without causing the immediate death of the insect[385]. This opinion receives additional confirmation from the mode in whichrespirationis performed in insects. Inthose animals that have a circulation, this takes place by means oflungsorgills;—thus we find, even in theCrustaceaandArachnidaso nearly related to insects, that the organs of this function are truegills; whereas in insects, though in some of their states their respiratory tubes are branchiform, yet they arenotgills, 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 are furnished with aliver, as is the case with theArachnidaand even many aggregate animals that have a heart; but in insects there are only hepatic ducts. M. Cuvier has also proved that theconglomerate glands, which exist 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 float in the interior of the body: from this circumstance, he is led to conclude that their nutrition is byimbibitionor immediate absorption, as in thePolypiand other zoophytes, the chyle transpiring through the alimentary canal, and running uniformly to all parts of the body[386].
These arguments appear so satisfactory, that Physiologists in general seem to have been convinced by them that no circulation, at any time, takes place in insects, and that their supposed heart is merely a secretory vessel, though of what kind they were at a loss toconjecture[387]. But, convincing as they seem, they appear to have been founded inerror, and on the idea that acirculation, as well as aheart, necessarily implies avascularsystemconsisting of veins and arteries; for by the recent discoveries of M. Carus, it has been satisfactorily proved that insects in their preparatory states, have anextravascularcirculation, the arterial and venose currents not being confined byparietes. The observations upon which M. Carus' hypothesis is founded, were made in the Autumn of 1826; and an abstract of their results presented to the Union of German Naturalists and Physicians, which then held its meeting at Dresden, many of the members of which, as MM. Oken, Husche, Heyne, Purkinje, Otto, Weber, and Müller, had ocular proofs of the reality of the phenomena.
His first observations were made on the larva ofAgrion Puella, which swims by means of three vertical laminæ attached to the tail; which, when the wings first appear as rudiments, begin to be exsiccated and are finally detached. Each of these laminæ, in its natural vertical position, presents an inferior abdominal and a superior dorsal edge, has two tracheæ running along its centre with ramifying bronchiæ, and consists of granular substance contained between two strata of the external integuments. A current of blood-globules enters eachlamina somewhat nearer to its abdominal than to its dorsal edge, and running through the greater part of its length suddenly turns and bends its course back towards the body, somewhat nearer to the dorsal than to the abdominal margin of the lamina. The channel thus formed in the midst of the granular substance is perfectly transparent, except where it is occupied by the blood-globules, or crossed by the bronchiæ. The parietes of the channel are not strictly defined, nor formed by any thing like the coats of a vessel, the blood circulating through the granularParenchyma; a circumstance however which is not peculiar to this case, but also occurs generally in the first states of the circulation, as it presents itself for instance in the embryo ofFishes, and in thefigura venosaof the incubated egg[394]. The blood-globules are elongated like a grain of wheat, considerably larger than those of the human blood, and float in a fluid which is invisible because of its transparency, but the existence of which is proved by the variations in the position of the globules in the current, sometimes following its direction, at others crossing it transversely, or more or less obliquely.
When the animal is vigorous, the current is uninterrupted, although its velocity is accelerated at regular intervals; and that not only in the excurrent (arterial), but also in the recurrent (venous) part of its course through the lamina. When the animal becomes exhausted, or the laminæ exsiccated, the circulation is interrupted, and in the same manner, as under the same circumstances, in the larvæ of frogs and lizards; the disturbance displaying itself not merely by a cessation of the process, but alsoby retrograde movements of the currents, or by oscillatory motions of the blood-globules.
In proportion as the wings are developed, the circulation in the laminæ diminishes, and ultimately ceases, preparatory to the detachment of the laminæ themselves. At the same time, however, it presents itself under a new form in the wings. In these the excurrent or arterial stream takes its course along the inner margin of the wing, and the recurrent or venous returning along the outer; whilst, occasionally, other transverse currents take their course through the net-work of the wing from its inner to its outer margin. As the wings are further developed, the circulation in them, like that in the caudal laminæ, gradually becomes weaker and ultimately ceases[395].
The next observations were made on the transparent larva of a neuropterous insect (probably aSemblisorSialis), in which the pulsations of the dorsal vessel were distinctly seen at its posterior extremity, from which they were propagated towards the anterior; these two divisions of that vessel appearing to bear to each other the relation of aheartandaorta. There were no traces of other vessels, though regular and rapid currents of blood-globules, exterior to the tracheæ, proceeded from the head towards the posterior extremity of the body, where each of these currents entered the heart, which again propelled its contents with accelerated velocity through the anterior part of the dorsal vessel towards the head. The lateral currents also were accelerated upon each contraction of the heart, proving that they must communicate with the dorsal vessel at the anterior part of the body, though theopacity of the head rendered it impossible to ascertain the mode ofanastomosis. An excurrent and returning current were also traced to each of the legs[396]. But the phenomena of the circulation was most distinctly visible in the larva ofEphemera vulgata, even more distinctly than it is possible to trace it in the larvæ of frogs and newts. In this animal the circulation, with the help of the microscope, is at once visible in the three last segments of the body; and with a little attention is discoverable not only in the three terminalcaudulæ, and in the upper joints of the legs, but also in the head, and particularly the roots of the antennæ. In the posterior part of the body there are on each side two currents of blood, not bounded by parietes, situate on each side of the intestinal canal, the inner one being the most considerable. The external one communicates with the internal by several intermediate branches; from this probably the streams are detached, which in the form of loops are seen at the upper joints of the legs, though it is not possible precisely to ascertain this, nor even whether these lateral currents continue distinct in the thorax, which probably they do. At the ninth abdominal segment these currents which flow posteriorly from the head, change their direction, and are inflected so as to enter the pulsating heart, from which the current again flows towards the head. Before they enter the heart they give off three streams, one for each of the threecaudulæ. The currents in thesecaudulæpresent the phenomena of the circulation with peculiar distinctness, and are particularly remarkable from the circumstance, that the excurrent and recurrent streams, though closely approximated without any visible separation,flow without disturbing each other. The excurrent stream is accelerated in correspondence with the pulsations of the heart; the recurrent on the contrary being always somewhat more sluggish, and the first to stagnate and cease when the strength of the animal is impaired. In the anterior part of the head currents can be discovered, forming loops like those of the legs, at the roots of the antennæ; each current proceeding from the cranial surface, and in returning taking its course towards the region of the larynx[397].
M. Carus has likewise observed currents of blood in the larvæ of water-beetles (HydrophilusandDytiscus)[398]; but at present he appears to have detected it in no terrestrial larva. Whether this is occasioned by their opacity, or it exists only in the ovum, as he seems to suspect[399], must be left for determination to future observers; it is scarcely probable, however, that the larvæ ofDytisciandHydrophilishould differ from otherColeopterain their circulation.
The endeavours of M. Carus to discover any proofs of a circulation in insects in their last state, except in the wings at their first development, were without success[400]. He observes that the fact of the currents of fluids in larvæ not being defined by vascular parietes, enables us to comprehend the rapidity and facility with which the traces of the circulation are lost in the perfect insect. On the other hand, that the existence of a circulation at one period, and its cessation at another, elucidate many circumstances connected with the physiology of these animals: for instance, the contrast between therapid growth and transformations of the larvæ, and the stationary existence of the imago, &c. Lastly he remarks, that the phenomena of this circulation do not throw any light on the obscure subject of the mode of nutrition in perfect insects; which therefore must still be supposed to be effected according to the idea of Cuvier, without the intervention of vessels[401].
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 in 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 transparent, 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 the case also in the imago[402]. If this be true, and there is no reason to doubt his accuracy, when they are more advanced, it resumes its old course, as Lyonet observed, from the tail to the head[403]. 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 pulsations differ in quickness in different portions of the heart[404]. If its course were really always the same, and in one direction, 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. InSyrphus Pyrastriand other aphidivorousflies, this dorsal vessel, instead of the usual form which it had in the larva, assumes a very peculiar appearance. 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 transmission 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 included fluid does not run in the dorsal vessel in a regular course, but is propelled at intervals by drops, as if from a syringe, first from the wide end towards the trunk, and then in the contrary direction, forming a very interesting 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 conveyed forward by the outward ones, and backward by the intermediate one[405]: 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 author observed the above remarkable structure not only in theSyrphi, but in many of their affinities, and thinks that it is also widely diffused amongst theMuscidæ[406].
I must now say something upon what I conceive to be the realbloodof insects; for I think no one will object to that name being given to their nutritive fluid, especiallyin the larva, 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 and office[407]; only that in these animals, as Cuvier has observed, at least in their perfect state, the blood, for want of a circulating system, not being able to seek the air, the air goes to seek the blood[408]. The dispersion of this fluid appears to be universal, so that all the parts and organs contain it in a greater or less degree[409]. In many insects, if you break only an antenna or a leg, a drop of fluid flows out at the wound. In larvæ, the fluid which bathes[410], or visits, all the internal parts and organs is not only sufficient for their nutriment, but a large quantity of seemingly superfluous blood remains that is not wanted for this purpose. This is expended in the production of the caul orepiploon(Corps graisseuxReaum.), which laps over and defends all the viscera of the animal, and goes principally to the formation of the imago[411]. I have said that Cuvier conceives nutrition in insects to take place byimbibitionor immediate absorption; that is, I suppose, the different parts and organs thus constantly bathed in the blood, imbibe from it the particles necessary for their constant accretion. M. Chabrier seems to think that it is the compression and dilatation of the trunk that duly distributes the nutritive fluid[412]; Lyonet compares the nutrition of insects by their fibres from this fluid, when formed into thecorps graisseux, to that of plants that draw their support by their roots from the earth[413]. Much obscurity, however, at present rests upon this subject—much for future investigation to explore; but in all the works of theMost Highthere is always something inscrutable, something beyond the reach of our senses and faculties, which teaches us humbly to adore his infinite perfections.