This reasoning, in opposition to Dr. Virey's theory, that the changes of instinct depend on the altered structure of the nervous system, becomes greatly strengthenedwhen we advert to the higher classes of animals, which surely in any investigation of the nature of instinct ought to be closely kept in view; for the faculty, though often less perfect in them than in insects, is still of thesame kind, and may consequently be expected to follow the same general laws. In a young swallow, for example, all its instincts are not developed at once any more than in an insect. The instinct which leads it to migrate does not appear for some months after its birth, and that of building a nest still later. But we have not the slightest ground for believing that these new instincts are preceded by any change in the structure of the great sympathetic nerve, or of any other portion of the nervous system: and the same may be said as to the sexual instincts developed in quadrupeds some years subsequent to their birth. If, then, these remarkable changes in the instinct of the higher classes of animals can take place independently of any visible change in the nerves, what substantial reason can be assigned why they may not also in the class of insects?
On the whole, I think you will agree with me, that there is nothing in Dr. Virey's hypothesis which should lead me to alter the opinion I have already so strongly expressed in a former letter[135], as to the insufficiency of the mechanical theories of instinct hitherto promulgated, adequately to explainallthe phenomena; and unless they do this they are evidently of small value. Such theories as I have there adverted to may often seem to be supported by a few insulated facts, but with others, far more numerous, they are utterly at variance; and, to omit many other instances, I am strongly inclined todoubt the possibility of satisfactorily explaining thevarietyof instincts exercised by a bee[136], or theextraordinarydevelopment of new ones in particular circumstances only[137], on any merely mechanical grounds.
And after all, even suppose it could be demonstratively shown thateveryinstinct is as clearly dependent on secondary causes, as I have formerly admitted thatsomedoubtless seem to be, yet what would this teach us as to the essential nature of instinct? We have advanced indeed a step; but still, as I have before observed in referring to the theories of Brown and Tucker, we have only placed the world upon the tortoise, and instinct, as to itsessence, which is what we want to detect, is as mysterious as ever: just as, though we can clearly prove that the mind is acted upon by the senses, yet this throws no light upon the essential nature of the mind, which we are forced to admit is inscrutable, as if to teach us humility, and prevent our vainly fancying, that though allowed to discover some of the arcana of nature, we shall ever be able to penetrate into her inmost sanctuaries.
That Dr. Virey should regard instinct in insects as purely mechanical was the natural consequence of his denying them any portion of intellect; but his opinion cannot I think be consistently assented to, if it be the fact, as I have just shown[138], that they are not wholly devoid of the intellectual principle. Whatever is merely mechanical, must, under similar circumstances, always act precisely in the same way. An automaton once constructed, whilst its machinery remains in order, will invariably perform the same actions; and Des Cartes, whenhe had constructed his celebrated female automaton, imagined that he had irrefragably proved his principle, that brutes are mere machines. But if, instead of losing himself in the wilds of metaphysical speculation, he had soberly attended to facts, he would have seen that the instinct of animals can be modified and counteracted by their intellect, and consequently cannot be regarded as simply mechanical. Though the instinctive impulse of an empty stomach powerfully impel a dog to gratify his appetite, yet, if he be well tutored, the fear of correction will make him abstain from the most tempting dainties: and in like manner a bee will quit the nectary of a flower, however amply replenished with sweets, if alarmed by any interruption. The ants on which Buonaparte amused himself with experiments at St. Helena, though they stormed his sugar-basin when defended by a fosse of water, controlled their instinct and desisted when it was surrounded with vinegar[139]: and in the remarkable instance communicated to Dr. Leach by Sir Joseph Banks, the instinct of a crippled spider so completely changed, that from a sedentary web-weaver it became a hunter[140]. There is evidently, therefore, no analogy between actions strictly mechanical and instincts, which, though they may often seem to be excited by mechanical causes, are liable to be restrained or modified by the connexion of the instinctive and intellectual faculties[141]; and while we are ignorant how this connexion takes place, it is obviously impossible to reason logically on the subject.
In thus denying that any existingmechanicaltheory of instinct is satisfactory, I by no means intend to assertthat instinct is purelyintellectual. I have already given you my opinion[142], that it is not the effect of any immediate agency of the Deity; nor am I prepared to assent to the doctrine of a writer, who has in some respects written ably on the subject in question, who says, that "the Divine Energy does in reality act notimmediately, butmediately, or through the medium ofmoralandintellectual influencesupon the nature or consciousness of the creature, in the production of the various, and in many instances truly wonderful, actions which they perform[143]." The same objection applies to this as to so many other metaphysical theories, that it is not adequately supported byfacts; and all theories not so supported are injurious to science in proportion as their plausibility is greater, by leading the student to relax in that observation of nature and attentive study of the instincts of animals, on which alone sound hypothesis on this subject can be ultimately founded.
I shall conclude these remarks on the nature of instinct with a few observations as to the circumstances in which insects may be supposed to be guided by this faculty, and those in whichintellectseems to direct them. The bee, when it takes its flight to a field where flowers abound, is governed by intellect in the use of its senses; for these are given to it asguides: and when it arrives there, they direct it to the flowers, and enable it to ascertain which contains the treasures it is in search of; but having made this discovery, its instinct teaches it to imbibe the nectar and load its hind legs with pollen.—Again: its senses, aided by memory, enable it to retraceits way to the hive, where instinct once more impels it in its various operations. So that when we ascribe a certain degree of intellect to these animals, we do not place them upon a par with man; since all the most wonderful parts of their economy, and those manipulations that exceed all our powers, we admit not to be the contrivance of the animals themselves, but the necessary results of faculties implanted in their constitution at the first creation by theirMaker. I may further repeat, that the mere fact of being endowed with the external organs of sense, proves a certain degree of intellect in insects. For if in all their actions they were directed merely by their instinct, they might do as well without sight, hearing, smell, touch, &c. but having these senses and their organs, it seems to me a necessary consequence, that they must have a sufficient degree of intellect, memory, and judgement, to enable them advantageously to employ them.
There is this difference between intellect in man, and the rest of the animal creation. Their intellect teaches them to follow the lead of their senses, and make such use of the external world as their appetites or instincts incline them to,—andthis is their wisdom; while the intellect of man, being associated with an immortal principle, and being in connexion with a world above that which his senses reveal to him, can, by aid derived from heaven, control those senses, and bring under his instinctive appetites, so as to render them obedient to the το ἡγεμονικον, or governing power of his nature:and this is his wisdom.
I am, &c.
"Life and flame have this in common," says Cuvier, "that neither the one nor the other can subsist withoutair; all living beings, from man to the most minute vegetable, perish when they are utterly deprived of that fluid[144]." The ancients, however, not perceiving insects to be furnished with any thing resemblinglungs, took it for granted that they did notbreathe; though Pliny seems to hesitate on the subject[145]. But the microscopic and anatomical observations of Malpighi, Swammerdam and Lyonet, and the experiments of more modern physiologists, have incontestably proved that insects are provided with respiratory organs, and that the respiration of air is as necessary to them as to other animals. They can exist indeed for a time in irrespirable air; and immersion in hydrogen or carbonic acid gases is not, as I have often ascertained, so instantly fatal to them as it would be to vertebrate animals; but like them, theyspeedily perish in air altogether deprived of its oxygen, or placed in situations to which all access to this essential element is excluded. Their respiration too of atmospheric air produces the same change in it with that of the vertebrate animals, the oxygen disappearing, and carbonic acid gas being produced in its place. Boyle had long since ascertained, that when bees, flies, and other insects were placed under an exhausted receiver, they often perished[146]: and the same effect was even observed by the ancients to ensue, when their bodies were by any means covered with oil or grease, which necessarily closed the orifices of their respiratory organs[147].
But for the first series of experiments ascertaining the necessity of a supply of air to insects, and their conversion of it into carbonic acid, we are indebted to the illustrious Scheele[148]; and his experiments have been repeated and confirmed by Spallanzani, Vauquelin, and other chemists. The former found, that when caterpillars and maggots were confined in vessels containing only about eleven cubic inches of atmospheric air, though furnished with sufficient food, they soon died, and sooner when the space was more confined[149]. He ascertained too, that a larva weighing only a few grains consumed, in a given time, as much oxygen as an amphibious animal a thousand times as voluminous[150]. A male grasshopper (Acrida viridissima) in six cubic inches of oxygen lived but eighteen hours, and the female placed in eight cubic inches of atmospheric air, only thirty-six hours. Theusual tests in both instances detected the conversion of the oxygen present into carbonic acid[151]. Precisely the same result was obtained by Sorg and Ellis, who, having placed a number of flies in nine cubic inches of atmospheric air, found them all dead by the third day, the oxygen intirely vanished, and a quantity of carbonic acid nearly equal in bulk produced[152].
It is ascertained too, that insects like other animals require in the process of respiration not merely oxygen, but such a mixture of it with nitrogen or azote as composes atmospheric air: for Vauquelin found that a grasshopper placed in six cubic inches of oxygen lived only half as long (eighteen hours) as another placed in eight inches of atmospheric air; its breathing was much more laborious, and it died when not more than one-twentieth of the oxygen had been converted into carbonic acid[153]. That a large quantity ofoxygenpenetrates all parts of insects, is evident also from theacidprevalent in the fluids of most of them, as likewise from the wonderful power of their muscles. Thatazoteis also received, seems probable from theammoniawhich has been extracted from the fluids of many, and from the rapid putrescence of these animals[154].
The mode, however, in which the respiration of insects is carried on, differs greatly from that which obtains in the higher animals. They have no lungs, no organs confined to a particular part of the body, by means of which the whole of the blood is regularly exposed to theaction of the inspired air. They do not breathe through themouth, but through numerous orifices calledspiracles, and the respiratory vessels connected with these are conducted to every part of the body. In some indeed, that we have included under the denomination of insects, as theArachnida, an approach is made to the branchial respiration of fishes.
The respiratory apparatus of insects may be considered undertwoprincipal heads:—viz. the orifices or spiracles, and otherexternalorgans by which the air is alternately received and expelled; and theinternalones, by which it is distributed. Each of these is well worthy of your attention.
I. Theexternalrespiratory organs of insects may be divided intothreekinds.Spiracles;Respiratory plates; andbranchiformandother pneumatic appendages.
i.Spiracles[155](Spiracula), or breathing pores, are small orifices in the trunk or abdomen of insects, opening into thetracheæ, by which the air enters the body, or is expelled from it[156]. They may be considered principally as to theircompositionandsubstance;shape;colour;magnitude;situation; andnumber.
1.Compositionandsubstance. Perhaps you may not be aware that the structure of these minute apertures is not so simple as at the first view it may seem; but when you recollect that by them the insectbreathes, you will suspect that provision may be made for their opening and shutting. A spiracle therefore, speaking analogically,may be regarded in numerous cases as amouthclosed bylips. In caterpillars and many other insects, the substance of the crust where it surrounds the spiracle, is elevated so as to form a ring round it. The lips, properly speaking, are formed of a single cartilaginous piece or platform, with a central longitudinal cleft or opening, when closed often extending the whole length of the piece[157]; but in some appearing always open and circular: of the former description are those covered by the elytra in the common cockchafer; and of the latter, those that are not so covered: in some, as in the antepectoral pair of the mole-cricket, there appear to be no lips, the orifice being merely closed with hairs[158]. Though the aperture is usually in the middle of the platform, in the female ofDytiscus marginalis, it is nearer the posterior side, the anterior or upper lip being the longest. In the majority, the mouth or cleft is nearly as long as the spiracle; yet in the puss-moth (Cerura Vinula) it is shorter[159]. Some spiracles, however, are unilabiate, or have onlyonelip. This is the case withGonyleptesand perhaps others[160]. The lips are usually horizontal, but sometimes they dip so as to make the spiracle appear open.
With regard to thesubstanceof these organs, it is more or less cartilaginous, and probably elastic; the surface frequently appears to be corrugate or plaited; this is very distinctly seen in the stag-beetle and the cockchafer: in the last insect, under a powerful magnifier, we are told that the lips appear to consist of parallel cartilaginous processes, separated by a cellular web[161]. In some speciesofCopristhe corrugations form a perplexed labyrinth; in the caterpillar of the puss-moth the plaits are so narrow as to look like rays[162]; and in someDynastidæthe lips approach to a lamellated structure. Again, inHydrophilus caraboidestheupperlip, and inDytiscus circumflexus,bothlips seem formed of elegant plumes[163]: a similar ornament distinguishes the inner edge of the lips in the caterpillar of the great goat-moth (Cossus ligniperda) and others[164]. In the grub of the rhinoceros-beetle (Oryctes nasicornis) the margin of the lower or inner lip is decorated by pinnated rays, which enter the cellular membrane that covers the upper lip[165]: in this larva, and that likewise of the cockchafer, the two lips are formed of different substances; in the last the upper or outer one consists of a perforated cellular membrane, through which the air can pass, while the lower or inner one is a cartilaginous valve that closes the orifice[166]: in the former this valve is surmounted by a boss[167]. In the pupa ofSmerinthus Populi, a hawk-moth not uncommon, and of some dragon-flies (Libellula depressa), the margin of the two lips is crenated, probably with notches which alternate, that the mouth of the spiracle may shut more accurately[168]. The substance is unusually thick in the spinose caterpillars of butterflies; and in the pupa of one,Uria Proteus, it is villose.
Under the present head I may observe, that in some cases, as in the puss-moth, and the larva of the commonwater-beetle (Dytiscus marginalis), the spiracles are closed by a semifluid substance, which however, according to Sprengel, is permeable to the air[169]. The animal, where these organs are furnished with lips, has doubtless, by means of a muscular apparatus, the power ofopeningandshuttingthem: this is done, we are told, by elevating and depressing, or rather by contracting and relaxing them. Sorg counted in one case (Oryctes nasicornis)twenty, and in another (Acrida viridissima)fifty, of these motions to take place in little more thantwominutes[170]: but the quickness and force of this motion is not always uniform; for the same physiologist observed, that inCarabus auratus, when feeding or moving its body rapidly, the contraction of the spiracles took place at very short intervals; but when it was fasting, and its motions were slow, the intervals were longer[171]: it is probable also, that the temperature may accelerate or retard the motion. In the summer I examined a specimen ofPhyllopertha horticola, that had indeed been somewhat injured, with this view: the pulses of the abdomen, which alternately rose and fell, were at about the rate of the pulse of a man in health, sixty in a minute, and the spiracles appeared to me to keep pace with this motion: later in the year, when the temperature was lower, as I was walking, I took a specimen of some grasshopper (Locusta).Upon viewing it under a lens, I observed one of the convex pectoral spiracles open and shut, and the interval between two breathings appeared nearly half a minute.
2. With regard to theirshape, spiracles vary considerably. In general we may observe that the abdominal ones are usually flat, while those of the trunk are often convex[172]. Sometimes they are very narrow and nearly linear, as in many pupæ ofLepidoptera, and those in themetathoraxof the sand-wasps (Ammophila) and affinities; at others they are wider and nearly elliptical, as inLucanusand many Lamellicorn beetles: again, inCopristhey are circular; inCordylia Palmarumovate; inDytiscusoblong[173]; inGoerius olenslunulate; inGonyleptesnearly of the shape of a horse-shoe[174]; and probably many other forms might be traced, if a thorough investigation with this view were undertaken.
3. Thecolourof spiracles will not detain us long. In the caterpillars ofLepidopterathis is often so contrasted with that of the rest of the body, as to produce a striking and pleasing effect. Thus when the body is of adarkcolour, they are usually of apaleone[175]; or if the body ispale, they aredark[176], or surrounded with a dark ring[177]. This contrast is often rendered more striking by their position with regard to the partial colours that often ornament caterpillars: in those whose sides are decorated by a longitudinal stripe, the spiracles are often planted in it[178]; or just above it[179]; or between two[180]: insome hawkmoths the intermediate ones are set in white or pale spots, which gives great life to the appearance of the animal. In general, in perfect insects the most prevalent colour is buff, or reddish-yellow. In the larva of the great water-beetle these organs resemble the iris of the eye, being circular with concentric rings alternately pale and dark[181].
4. Thesizeof spiracles varies considerably. Those in the larva last mentioned are so minute as to be scarcely visible except under a lens, while those behind the fore-legs in the mole-cricket are a full line in length, and those in thepleuraofAcrocinus accentifer, a Brazilian Capricorn beetle, are more than twice as long. In the same species they are often found of different sizes;—thus theanalpairs in the water-beetle lately alluded to, I mean in the perfect insect, are much larger than the rest[182], probably that the animal may imbibe a larger quantity of air when it rises to the surface of the water, where it suspends itself by thetail. In those Lamellicorn beetles in which the terminal part of the abdomen is not protected by the elytra, thecoveredspiracles are the largest.
5. Under the next head, thesituationof spiracles, I shall not only consider the part of the body in which they are situated, but likewise theirpositionin the crust; to which last, as it will not detain us long, I shall first call your attention. Their position in this respect is most commonlyoblique: but in the abdomen of the above water-beetle they aretransverse, and in a larva I possess, probably of anElater, they arelongitudinal. In spinosecaterpillars these organs are generally planted between two spines, one being above and the other below. Thelateralline of the body most commonly marks their situation; but in many cases they becomeventral, and in othersdorsal. The most important circumstance, however, connected with the present head is their appropriation to particular segments or parts of the body, for, like the ganglions of the spinal marrow, they are distributed to almost every segment. Let us take a summary view of their arrangement in this respect.
No insect has any spiracle in thehead; but in caterpillars and many otherlarvæthere is a pair in thefirstsegment of the trunk. This is also to be found in the other states, but is not easily detected in thepupæofLepidoptera: in theColeopteraorder, in the grub of the Lamellicorn beetles, it is extremely conspicuous, and planted in the side of the first segment[183]; in other Coleopterous grubs it is not so readily found, but probably its station is somewhere behind the base of the arms, where it is very visible in that of theStaphylinidæ. In theimagoof insects of this order, this antepectoral spiracle has been overlooked, and indeed is not soon discovered: to see it clearly, the manitrunk should be separated from the alitrunk; and then if you examine thelowerside of the cavity, you will see a pair of, usually, large spiracles planted just above the arms, in the ligament that unites these two parts of the trunk to each other: in the common rove-beetle, however, (Goerius olens)you may easily see it without dissection[184]. In theOrthopterait is situated behind the arms, as inGryllotalpa: or between themand theprothorax, as inBlatta: in theHemipteraandNeuropteraprobably the situation is not very different. In theLepidopterathis pair of spiracles is planted just before the base of the upper or primary wings[185]: a similar situation, I suspect, is appropriated to it in theTrichoptera, but covered by a tubercle or scale. Something similar has been noticed by M. Chabrier, in the same situation and circumstances, in the collar ofHymenoptera[186]. In numerousDipterathis breathing pore is planted on each side between the collar and thedorsolumabove the arms[187], and inHippoboscain the collar itself[188].
InLepidopterous,Coleopterous, and some other larvæ, the two segments of the body corresponding with the alitrunk in the perfect insect, are without spiracles, neither have they in this state, though pneumatic organs have been discovered[189], any real ones in that part: but not so theremainingorders, all of which have these organs in that section of the trunk. To begin with theOrthoptera:—inBlattathere seems to be a long narrow one behind the intermediate leg; in theGryllotalpathere is one in the posterior part of thepleura; and inLocusta, above both the intermediate and hind legs[190]. It is probable, that in general those that havenospiracles in the manitrunk havefourin the alitrunk, which seems the natural number belonging to the trunk. In many of the HeteropterousHemipterain theparapleurathere is an open spiracle without lips[191], to which, as in that beautiful bugScutellera Stockeri, a channel sometimes leads. The space in which this spiracle is planted in other genera of bugs (Pentatoma&c.) is covered with a kind of membranous skin, often much corrugated[192]. In the aquatic insects of this section, and many terrestrial ones, asReduvius, &c. this spiracle is obsolete. There is another circumstance, possibly connected with their respiration, relating to many of the bugs, which may be mentioned here. If you examinePentatoma rufipes, a very common one, you will find between thescapulaandparapleuraa long orifice or chink; this upon a closer inspection, under a good magnifier, you will see completely filled with minute stiff hairs or bristles, which fringe the posterior margin of thescapula[193]. In a Brazilian species ofLygæus(sexmaculatusK. M. S.) with incrassated posterior thighs, these hairs are replaced by lamellæ which have the aspect ofgills. A red, vertical, convex spiracle, with its orifice towards the head, and terminating posteriorly in a kind of conical sac, is situated towards the hinder part of thepleurain the giant water-scorpion (Belostoma grandis[194]); this seems analogous to one lately mentioned in the mole cricket. In the other section of this Order it is not easy to decipher the parts of the under side of the alitrunk. InFulgora,Cicada, and many others of its genera, there appears to be more than one opening into the chest; but whether they are of a pneumatic nature or not, can only be ascertained by an inspection of the living animal. There is a very visiblespiracle over each of the four last legs of theLibellulina[195], but in the remainder of theNeuropteraOrder they have eluded my search. In theHymenopteraandDipterathey are nearly in the same situation, being placed behind the wings on each side of themetathorax; in the latter Order with the poiser near them on the inner side[196]: in this also, the spiracles of thetrunkare withoutlips, except in the larvæ, but are often merely an orifice, sometimes fringed with hairs; this is particularly conspicuous inSyrphus, in which these orifices are very large, and in some species closed by an elegant double fringe of white hairs. This is doubtless to prevent the entrance of any particles of dust or the like.
We are next to consider the situation of the spiracles of theabdomen: these which are supposed to be appropriated exclusively toinspiration, are usually more numerous than those of the trunk, by which it is probable thatexpirationis performed, and have principally attracted the notice of Entomologists: they are either dorsal, lateral, or ventral. InDytiscus,Copris, &c. amongst the beetles, all the spiracles aredorsal; in the larvæ ofColeopteraandLepidopterathey arelateral; and in the HeteropterousHemipterathey are usuallyventral: inDynastesthey are commonly found of all three descriptions;—thethreefirst beingdorsal, thetwonextlateral, and thelastpairventral[197]. In some instances, as inPerga Kirbii, and probably otherHymenoptera, these organs are planted in that portion of the dorsal segment which turns under, as was observed in a former letter[198],and becomes ventral. Generally there is apairof spiracles toeachsegment, and in those insects that have a hypochondriack joint[199]there is often a spiracle in it. The last segment of the abdomen is always without these orifices, as is the basal one inVelia,Ranatra, and some other bugs. A singular anomaly distinguishes theLibellulina: they appear to have noabdominalspiracles[200], yet I have seen the abdomen ofLibellula depressawhen reposing, contract and dilate alternately, from whence it follows that this part is concerned in respiration. Sprengel says that the larvæ in this tribe have seven or nine on each side[201], and Reaumur speaks of them as discoverable in the pupa[202]. I have carefully examined the pupa-skin of most of the genera ofLibellulina, under a powerful magnifier, but have not succeeded in discovering any thing like these organs in the abdomen. TheEphemeraand probably the otherNeuropterahave abdominal spiracles[203]. M. Latreille observed one on each side of the base of the scale on the footstalk of the abdomen in ants[204]. Generally the abdominal spiracles may be described as planted in thecrustof the insect; but in many cases their station is in the membranous folds, which I have therefore named thepulmonarium, that sometimes separate the dorsal from the ventral segments: these folds allow of a considerable distention of the abdomen, which is probably necessary when all the air-vessels are full. In a gravidIchneumonI once saw it enlarged to more than twice its natural size by means of this membrane, through which the eggs were distinctlyvisible.—Before I bid adieu to this subject, I must say a few words upon the situation of the organs in question in themyriapods. InIulus, in each segment is a pair of orifices which have usually been regarded as spiracles, but M. Savi found that these orifices opened into vesicles containing a fetid fluid, and upon a very close examination he discovered the real spiracles above the base of the legs, in connexion withtracheæ[205]. In some of the larger species ofScolopendrælarge open spiracles in the same situation are extremely visible[206].Cermatiapresents a singular anomaly:—a single series of spiracles of the usual form, each planted in a cleft of the posterior margin of the dorsalscuta, runs along the back of the animal[207]: unless we may suppose that, like the seeming spiracles ofIulusjust mentioned, these are merely orifices by which it covers itself with some secretion.
6. A few words upon thenumberof spiracles.—If you examine the common dog-tick (Ixodes Ricinus), you will find onlyoneof these organs on each side of the abdomen[208]; theLibellulina, as we have seen, have onlyfour, all in the trunk; in theDynastidæ,Melolontha, and the larva ofDytiscus, there arefourteen;sixteenin theCopridæ;eighteeninDytiscus, and probably the majority ofColeoptera, both larva and imago, andLepidoptera; and a pair to each segment except the last, in theMyriapods.
ii.Respiratory plates(Respiratoria). The nearest approachto spiracles is made by those remarkable plates that are found in such larvæ ofDiptera, as in that state inhabit substances that might impede or altogether stop the entrance or exit of the air by the ordinary spiracles, such as dead or living flesh, dung, or the like. TheCreatortherefore, as he has seen it good for wise reasons[209]to commission certain insects to feed on unclean food, has fitted them for the offices that devolve upon them, and has placed their orifices for breathing in plates at each extremity of the body. There are usually two of these plates at the head, and two at the tail. In the grub of the common flesh-fly (Sarcophaga carnaria), at the junction of the first segment of the body with the second, two of these plates are planted, which are concave and circular, with a denticulated margin; in the cavity near the lower side is a round spiracle. These plates the animal can withdraw within the body, so as to prevent this spiracle from being stopped up by any greasy substance[210]. The posterior extremity of this grub is truncated, and has a large and deep cavity surrounded by several fleshy prominences: at the bottom of this are two oval brown plates, in each of which arethreeoval spiracles, placed obliquely: by the contraction of the fleshy prominences, this cavity also can be closed at the will of the animal[211]. In some cases, several stiff rays orspines replace the prominences[212]. InEchinomyia grossaand others the anal plates appear not to be perforated, being surmounted only by a central boss[213]; but this, most probably, as in the case ofŒstrus Ovis[214], is avalvethat closes the respiratory orifices. In the gad-fly of the ox (Œ. Bovis) there are no plates at theanteriorextremity of the body; but those planted in theotherend are very remarkable, and demand particular attention. Each is separated by a curved line into two unequal portions; the smallest of which is contiguous to the convex belly, and the largest to the concave back of the animal. This last is distinguished by two hard, brown, kidney-shaped pieces, a little elevated with the concave sides turned towards each other: in this sinus is asingle, small, white spot, which appears to be a spiracle: in the smallest portion areeightminute circular orifices, arranged in a line[215]. As the only communication which this grub has with the atmosphere is at itsanalextremity, it has no occasion for respiratory organs at theother. The gad-fly of the horse (Gasterophilus Equi, &c.) which has no communication at all with the external air, breathing that which is received into the stomach, has these plates atbothends of the body.
iii.Respiratory Appendages[216]. These may be divided intotwokinds; those by which the animal hasimmediatecommunication with theatmosphere, and those by which it extractsairfromwater.
1. To begin with thefirst. These are often found in insects which, during their two first states, live in the water. No better example, nor one more easy to be examined, of this structure, can be selected, than the gnat (Culex). You must have occasionally observed in tubs of rain-water, numerous little wriggling worm-like animals, which frequently ascend to the surface; there remain a while, and then bending their head under the body rapidly sink to the bottom again. These are the larvæ of some species of the genus just named; and if you take one out of the water and examine it, you will perceive that it is furnished near the end of its body with a singular organ, which varies in length according to the species, and forms an angle with the last segment but one[217]. The mouth of this organ is tunnel-shaped, and terminates in five points like a star; and by this it is usually suspended at the surface of the water, and preserves its communication with the atmosphere: in its interior is a tube which is connected with thetracheæ, and terminates in several openings, visible under a microscope, at the mouth of the organ. The points or rays of the mouth when the animal is disposed to sink in the water, are used to close it, and cut off its communication with the atmosphere. When the animal is immersed, a globule of air remains attached to the end of the tube, so that it is in fact of less specific gravity than that element, and it is not without some effort that it descends to the bottom; but when it wishes to rise again, it has only to unclose the tube, and it rises without an effort to the surface, and remains suspended for any length of time. Its anal extremity is clothed with bunches ofhairs, which are furnished with some repellent material which prevents their becoming wet[218]: it is this repellent quality that probably causes a dimple or depression of the surface, which if you look narrowly you will discover round the mouth of the tube[219].
When the gnat undergoes its first change and assumes the pupa, instead of asinglerespiratory appendage it is furnished with apair, each in shape resembling a cornucopia, and, what is remarkable, placed near the opposite extremity of the body, for they proceed from the upper side of the trunk[220]. By these tubular horns, which Reaumur compares to asses' ears[221], they respire, and are suspended at the surface.
Other respiratory tubes or horns are more complex. The rat-tailed grub of a fly (Helophilus pendulus), like the gnat, breathes by a tube: but as if theCreatorwilled to show those whose delight it is to investigate his works, by how many varying processes he can accomplish the same end, this respiratory organ is of a construction totally different from that we have been considering. It is not fixed to the side of the tail, but is a continuation of the tail itself, and is composed of two tubes, the inner one, like the tube of a telescope, being retractile within the other[222]. The extremity, which is very slender, and through which the air finds admission by a pair of spiracles, terminates in five diverging hairs or rays, which probably maintain itin equilibrioat its station at the surface[223]. As these larvæ seek their foodamongst the mud at the bottom of shallow pools, in which they are constantly employed, they require an apparatus capable of being lengthened or shortened, to suit the depth of the water, that they may maintain their necessary communication with the atmosphere; and for this purpose asingletube would not have been sufficient: thereforeProvidencehas furnished them withtwo, and both are extremely elastic, consisting of annular fibres, so as to admit their being stretched to an extraordinary length. Reaumur found that these animals could extend their tails to neartwelvetimes their own length. The mechanism by which the terminal piece is pushed forth or retracted, is very curious, though extremely simple. Two large paralleltracheæ, the direction of which is from the head[224]of the grub to its tail, occupy a considerable portion of its interior: near the origin of the tail, where they are very ample, they suddenly grow very small, so as to form a pair of very slender tubes, but so long that, in order to find room in a very contracted space, they form numerous zigzag folds attached to the terminal tube; when this issues from the outer tube they consequently begin to unfold, and when it is intirely disengaged, they are become quite straight and parallel to each other. Reaumur has figured them as being united at thebaseof the inner tube[225]; most probably, however, they do not here stop short, but, as inother instances, proceed to the end, and terminate in the two spiracles mentioned above: he conjectures that when the animal has occasion to push forth its respiratory apparatus, it injects into these vessels part of the air contained in the body of thetracheæ, which of course would cause them to unfold and push forth the tube[226]. When this insect assumes the pupa, instead of its anal respiratory organ it hasfourrespiratory horns in the trunk near the head[227].
The larva of the chamæleon-fly (Stratyomis Chamæleon) is furnished with a respiratory organ of a still different and more elegant structure, exhibiting some resemblance to thetentaculaof what are called sea anemones. In this larva the last joint of the body is extremely long, and terminates in an orifice to receive the air, which is surrounded by a circle of about thirty diverging rays, consisting of beautifully feathered hairs or plumes[228]. This apparatus serves the same purpose with that above described of the larva of the gnat. The feathery hairs are so prepared as to repel the water, and thus to suspend the animal by its tail at the surface, and preserve a constant access of air. When it has occasion to sink, it turns these hairs in and shuts the orifice, carrying down with it an air-bubble that shines like quicksilver, and which Swammerdam conjectures enables it again to become buoyant when it wants to breathe[229].
In the red aquatic larva of a small gnat (Chironomus plumosus) there aretwoanal respiratory subcylindrical horns, with the orifice fringed with hairs[230]; and in anothergnat Reaumur discoveredfour[231]. The larva ofTanypus maculatus, whose remarkablelegsI formerly noticed[232], exhibits in theinteriorof its trunk two long, oval, opaque bodies, which De Geer conjectures may be air-reservoirs; these, when the animal assumes the pupa, according to every appearance becomeexternal, and are placed on the back, precisely where the respiratory horns of aquatic pupæ are usually situated,—they appear to terminate in a transparent point[233]. The pupa of aTipulaobserved by Reaumur, instead oftwohas onlyoneof these respiratory organs, in the form of a very fine hair proceeding from the anterior end of the trunk, and considerably longer than the animal itself[234].
It is observable that aquatic insects that come to the surface of the water for air, receive it at the anus, often carrying it down with them as a brilliant bubble of quicksilver. This is generally done by means of spiracles in perfect insects, but in the water-scorpion tribe in that state respiration is by means of a long hollow tube, consisting of two concavo-convex pieces which apply exactly to each other. This is found in both sexes, and therefore cannot be anovipositor, as some have thought[235].
These respiratory organs, however, are not invariably confined toaquaticlarvæ and pupæ, for those of some aphidivorous flies have anal ones, and the pupa ofDolichopus nobilitatus, or a fly nearly related to it, which isterrestrial, has likewise a pair of long sigmoidal ones on the back of the trunk[236]. The pupa also of the rat-tailedlarva just noticed as havingfourhorns, resides under theearth, the insect being onlyaquaticin its grub state.
2. I am next to consider those respiratory appendages by which aquatic insects, since they do not come to the surface for that purpose, appear to extract air for respiration from thewater; so that they may be looked upon in some degree as analogous to thegillsof fishes: there is, however, this difference between them—in fishes, the blood is conveyed in minute ramifications of the arteries to the surface of the branchial laminæ, through the membranes of which they abstract the air combined with the water; but as insects have no circulation, the process in them must be different, and their branchiform appendages may be regarded as presenting someanalogyrather than anyaffinityto those of fishes. The first approach to this structure is exhibited by the pupa of a gnat lately mentioned (Chironomus plumosus); for on each side of the trunk this animal has a pencil consisting of five hairs elegantly feathered, which, when they diverge, form a beautiful star; its anus also is furnished with a fan-shaped pencil of diverging hairs[237].
On most of the abdominal segments of the larvæ and pupæ of theTrichopteraare a number of white membranous floating threads, arranged in bundles,fouron each segment, two above and two below, and traversed longitudinally by several air-vessels orbronchiæ, which run in a serpentine direction, growing more slender as they approach the extremity, and in some places sending forth very fine ramifications,—these are their respiratory organs[238]. The caterpillar also of a little aquaticmoth (Hydrocampa stratiotata) at first sight appears to be covered on each side with hairs, but which examined under a microscope are found to be branching flattish filaments, each furnished with tubes from thetracheæ. These caterpillars have also the semblance of spiracles, but apparently found in the usual situation[239]. The larva of a little beetle often mentioned in my letters (Gyrinus Natator), is furnished on each side of every abdominal segment with a long, hairy, slender, acute, conical process, of the substance of the segment, through each of which an air-tube meanders; the last segment but one hasfourof these processes, longer than the rest[240].
Laminoseor foliaceous respiratory appendages distinguish the sides of the abdomen of the larvæ and pupæ of theEphemeræ, whose history you found so interesting[241]. In them these organs wear much the appearance ofgills. In the different species they vary both in their number and structure. With regard to their number, some have onlysixpair of them, while others haveseven. In theirstructurethe variations are more numerous, and sometimes present to the admiring physiologist very beautiful forms[242]. They usually consist of two branches, but occasionally are single, with one part folding over the other, as in one figured by Reaumur, which precisely resembles the leaf of some plant, the air-vessels orbronchiæin connexion with thetracheæbranching and traversing it in all directions, like the veins of leaves[243]. The double onesdiffer in form. In the larva and pupa ofEphemera vulgatathere aresixof these double false gills on each side of the abdomen, the three last segments being without them; each branch consists of a long fusiform piece, rather tumid and terminating in a point, which is fringed on each side with a number of flattish filaments, blunt at the end. An air-vessel from thetracheaenters the gill at its base; is first divided into two larger branches, each of which enters a branch of the false gill. These branches send forth on each side numerous lesser ramifications, one of which enters each of the filaments[244]. In another species (E. vespertina) each false gill presents the appearance of a pair of ovate leaves with a long acumen, and the air-vessels represent the midrib of the leaf, with veins branching from it on each side[245]; and, to name no more, inE. fusco-grisea, one branch represents the leaf of aBegonia, the sides not being symmetrical, with its veins, while the other consists only of numerous branching filaments[246]. In other aquatic larvæ, as in that of the common May-fly (Sialis lutaria), these appendages consist of several joints[247].
By the above apparatus these aquatic animals are enabled to separate the air from the water, as the fish by their gills; but how this separation is made has not been precisely explained. The false gills in many species are kept in continual and intense agitation. When they move briskly to one side, Reaumur conjectures they may receive the air, and when they return back they mayemit it[248]. This brisk motion probably disengages it from the water. In many species, when in repose, they are laid upon the back of the animal[249], but in others they are not[250].
The larvæ of theAgrionidæappear to respire like those of theEphemeræ, &c. by means of long foliaceous laminæ or false gills filled with air-vessels; but instead of beingventral, they proceed from theanus. They are three in number, one dorsal and two lateral, perpendicular to the horizon, of a lanceolate shape, beautifully veined, with a longitudinal middle nervure, from which others diverge towards the margin, which are probablybronchiæ. They are used by the animal, which swims like a fish, as fins, but it does not appear to imbibe the water like the otherLibellulinæ, nor to propel itself by ejecting it,—a circumstance which furnishes an additional argument for the more received opinion, that this action in them is for the purpose of respiration as much as for motion[251].
The larvæ and pupæ of theLibellulinæ, receive the water and air that they respire by a large anal aperture, which is closed at the will of the animal by five hard, moveable, triangular, concavo-convex pieces, all very acute and fringed with hairs. These pieces are placed so that there is one above, which is the largest of all; one on each side, which are the smallest, and two below; when these are closed they form together a conical point[252]. Sometimes only three of these pieces are conspicuous[253]:three other cartilaginous pieces, resembling the valves of a bivalve shell, close the passage within the pointed pieces[254]. At this orifice the water is received; and when, by an internal process to be described afterwards, it has parted with its oxygen, is again expelled.
Under this head I shall mention a fact which may be connected with respiration of the insects concerned. In dissecting a moth related toCatocala Pronuba, but I do not recollect the particular species,—at the base of the abdomen of the male I discovered two bunches of long fawn-coloured parallel hairs, planted each in an oval plate, plane above, but below convex and fleshy; while the plates remained attached to the insect, they appeared to have a distinct pulsation. The hairs, which are about half an inch long, diverge a little, and form a tuft not very unlike a shaving-brush[255]. I have not since met with this species, but I have preserved the brush and scale. Somewhere in Bonnet's works, but I do not recollect where, I have since found mention of a similar fact in another moth.
II. Having considered theexternalrespiratory organs of insects, by which the air isreceived, we are next to consider theinternalones, by which it isdistributed. These aregills;tracheæandbronchiæ; andsacsorpouches[256].
i. Gills (Branchiæ[257]). Having lately described whatmay be denominatedfalsegills, or branchiform appendages, I shall now call your attention to what may be denominatedtrueones, which are peculiar to theArachnidaClass: but what is remarkable, the animals that breathe by them are very rarely inhabitants of the water, so that their functions cannot be perfectly analogous to those of fishes.
In theScorpion, on each side of the four first ventral segments a spiracle may be discovered, which has nolipas in other insects, but is merely a circularorifice. These orifices do not lead totracheæorvesicles, but totrue gills, which are situated below a muscular web which clothes the internal surface of the crust. Each gill consists of many semicircular very thin plates, of a dead milky white, which are connected together at the dorsal end like the leaves of a book. There appear to be more thantwentyof these leaves, which when strongly magnified look transparent and destitute of any vessels. Each gill is fastened at the back to the spiracle[258]. In thespidersalso, gills are discoverable, but differently circumstanced. On the under side of the abdomen, near the base, is a transverse depression, on each side of which is a longitudinal opening leading to a cavity, which is covered from above by a cartilaginous plate. In this cavity is situated a true gill, which is white, triangular, and covered with a fine skin; the leaves of this gill are far more numerous and much finer and softer than those of the gills of the scorpion. On account of their softness they have often the appearance of a slimy skin; but theirlaminated structure shows itself very clearly in old specimens, and in such as have been immersed in boiling water[259].
ii.TracheæandBronchiæ[260]. Parallel with each side of the body of mostinsectsand extending its whole length, runtwocylindrical tubes[261], which communicate with the spiracles[262], and from which issue, at points opposite to those organs, other tubes which ramifyad infinitum, and are distributed to every part of the body[263]. The first of these tubes are called thetracheæand the latter thebronchiæ. This structure appears, however, not to be universal: it is to be found in caterpillars and manyDipterouslarvæ; but in that of the rhinoceros-beetle and other Lamellicorns, thebronchiæbranchdirectlyfrom the spiracle, the bottom or interior mouth of which is lined by a membrane from which they proceed[263]: something similar has been observed to take place in many insects in other states, as the common cockchafer[264]; in the pupa ofSmerinthus Populi[265]; in theCicadæ[266]; in the Locust tribe[267]; and many others. In theCossus, or larva of the great goat-moth, thetracheacommences with the first spiracle, and finishes a little beyond the last, after which it diminishes considerably in diameter, and terminates in several branches orbronchiæ, which proceed to the anal extremity of the body[268]. Thebronchiæwhich originate from thetracheæin thevicinity of each spiracle, may be considered as consisting in general ofthreepackets;—dorsalones, which are distributed to the back and sides of the animal;visceralones, which enter the cavity of the body, and are lost amongst the viscera and the caul; andventralones, which dipping from thetracheæoverrun the lower part of the sides and belly[269].