Volume 4 — LETTER XXXVIII.

INTERNAL ANATOMY AND PHYSIOLOGY OF INSECTS CONTINUED.

RESPIRATION.

"Life and flame have this in common," says Cuvier, "that neither the one nor the other can subsist without air; all living beings, from man to the most minute vegetable, perish when they are utterly deprived of that fluid[144]." The ancients, however, not perceiving insects to be furnished with any thing resembling lungs, took it for granted that they did not breathe; though Pliny seems to hesitate on the subject[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, they speedily 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. The usual 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 of oxygen penetrates all parts of insects, is evident also from the acid prevalent in the fluids of most of them, as likewise from the wonderful power of their muscles. That azote is also received, seems probable from the ammonia which 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 the action of the inspired air. They do not breathe through the mouth, but through numerous orifices called spiracles, 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 the Arachnida, an approach is made to the branchial respiration of fishes.

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

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

i. Spiracles[155] (Spiracula), or breathing pores, are small orifices in the trunk or abdomen of insects, opening into the tracheæ, by which the air enters the body, or is expelled from it[156]. They may be considered principally as to their composition and substance; shape; colour; magnitude; situation; and number.

1. Composition and substance. Perhaps you may not be aware that the structure of these minute apertures is not so simple as at the first view it may seem; but when you recollect that by them the insect breathes, you will suspect that provision may be made for their opening and shutting. A spiracle therefore, speaking analogically, may be regarded in numerous cases as a mouth closed by lips. In caterpillars and many other insects, the substance of the crust where it surrounds the spiracle, is elevated so as to form a ring round it. The lips, 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 of Dytiscus 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 only one lip. This is the case with Gonyleptes and perhaps others[160]. The lips are usually horizontal, but sometimes they dip so as to make the spiracle appear open.

With regard to the substance of these organs, it is more or less cartilaginous, and probably elastic; the surface 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 species of Copris the 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 some Dynastidæ the lips approach to a lamellated structure. Again, in Hydrophilus caraboides the upper lip, and in Dytiscus circumflexus, both lips 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 of Smerinthus 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 common water-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 of opening and shutting them: 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 than two minutes[170]: but the quickness and force of this motion is not always uniform; for the same physiologist observed, that in Carabus 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 of Phyllopertha 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 their shape, 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 of Lepidoptera, and those in the metathorax of the sand-wasps (Ammophila) and affinities; at others they are wider and nearly elliptical, as in Lucanus and many Lamellicorn beetles: again, in Copris they are circular; in Cordylia Palmarum ovate; in Dytiscus oblong[173]; in Goerius olens lunulate; in Gonyleptes nearly 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. The colour of spiracles will not detain us long. In the caterpillars of Lepidoptera this is often so contrasted with that of the rest of the body, as to produce a striking and pleasing effect. Thus when the body is of a dark colour, they are usually of a pale one[175]; or if the body is pale, they are dark[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]: in some 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. The size of spiracles varies considerably. Those in the larva last mentioned are so minute as to be scarcely visible except under a lens, while those behind the fore-legs in the mole-cricket are a full line in length, and those in the pleura of Acrocinus accentifer, a Brazilian Capricorn beetle, are more than twice as long. In the same species they are often found of different sizes;—thus the anal pairs in the 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 the tail. In those Lamellicorn beetles in which the terminal part of the abdomen is not protected by the elytra, the covered spiracles are the largest.

5. Under the next head, the situation of spiracles, I shall not only consider the part of the body in which they are situated, but likewise their position in the crust; to which last, as it will not detain us long, I shall first call your attention. Their position in this respect is most commonly oblique: but in the abdomen of the above water-beetle they are transverse, and in a larva I possess, probably of an Elater, they are longitudinal. In spinose caterpillars these organs are generally planted between two spines, one being above and the other below. The lateral line of the body most commonly marks their situation; but in many cases they become ventral, and in others dorsal. 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 the head; but in caterpillars and many other larvæ there is a pair in the first segment of the trunk. This is also to be found in the other states, but is not easily detected in the pupæ of Lepidoptera: in the Coleoptera order, 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 the Staphylinidæ. In the imago of 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 the lower side of the cavity, you will see a pair of, usually, large spiracles planted just above the arms, in the ligament that unites these two parts of the trunk to each other: in the common rove-beetle, however, (Goerius olens)you may easily see it without dissection[184]. In the Orthoptera it is situated behind the arms, as in Gryllotalpa: or between them and the prothorax, as in Blatta: in the Hemiptera and Neuroptera probably the situation is not very different. In the Lepidoptera this 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 the Trichoptera, but covered by a tubercle or scale. Something similar has been noticed by M. Chabrier, in the same situation and circumstances, in the collar of Hymenoptera[186]. In numerous Diptera this breathing pore is planted on each side between the collar and the dorsolum above the arms[187], and in Hippobosca in the collar itself[188].

In Lepidopterous, 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 the remaining orders, all of which have these organs in that section of the trunk. To begin with the Orthoptera:—in Blatta there seems to be a long narrow one behind the intermediate leg; in the Gryllotalpa there is one in the posterior part of the pleura; and in Locusta, above both the intermediate and hind legs[190]. It is probable, that in general those that have no spiracles in the manitrunk have four in the alitrunk, which seems the natural number belonging to the trunk. In many of the Heteropterous Hemiptera in the parapleura there is an open spiracle without lips[191], to which, as in that beautiful bug Scutellera 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, as Reduvius, &c. this spiracle is obsolete. There is another circumstance, possibly connected with their respiration, relating to many of the bugs, which may be mentioned here. If you examine Pentatoma rufipes, a very common one, you will find between the scapula and parapleura a long orifice or chink; this upon a closer inspection, under a good magnifier, you will see completely filled with minute stiff hairs or bristles, which fringe the posterior margin of the scapula[193]. In a Brazilian species of Lygæus (sexmaculatus K. M. S.) with incrassated posterior thighs, these hairs are replaced by lamellæ which have the aspect of gills. A red, vertical, convex spiracle, with its orifice towards the head, and terminating posteriorly in a kind of conical sac, is situated towards the hinder part of the pleura in the giant water-scorpion (Belostoma grandis[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. In Fulgora, 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 visible spiracle over each of the four last legs of the Libellulina[195], but in the remainder of the Neuroptera Order they have eluded my search. In the Hymenoptera and Diptera they are nearly in the same situation, being placed behind the wings on each side of the metathorax; in the latter Order with the poiser near them on the inner side[196]: in this also, the spiracles of the trunk are without lips, except in the larvæ, but are often merely an orifice, sometimes fringed with hairs; this is particularly conspicuous in Syrphus, 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 the abdomen: these which are supposed to be appropriated exclusively to inspiration, are usually more numerous than those of the trunk, by which it is probable that expiration is performed, and have principally attracted the notice of Entomologists: they are either dorsal, lateral, or ventral. In Dytiscus, Copris, &c. amongst the beetles, all the spiracles are dorsal; in the larvæ of Coleoptera and Lepidoptera they are lateral; and in the Heteropterous Hemiptera they are usually ventral: in Dynastes they are commonly found of all three descriptions;—the three first being dorsal, the two next lateral, and the last pair ventral[197]. In some instances, as in Perga Kirbii, and probably other Hymenoptera, 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 a pair of spiracles to each segment, 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 in Velia, Ranatra, and some other bugs. A singular anomaly distinguishes the Libellulina: they appear to have no abdominal spiracles[200], yet I have seen the abdomen of Libellula depressa when 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 of Libellulina, under a powerful magnifier, but have not succeeded in discovering any thing like these organs in the abdomen. The Ephemera and probably the other Neuroptera have 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 the crust of the insect; but in many cases their station is in the membranous folds, which I have therefore named the pulmonarium, 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 gravid Ichneumon I once saw it enlarged to more than twice its natural size by means of this membrane, through which the eggs were distinctly visible.—Before I bid adieu to this subject, I must say a few words upon the situation of the organs in question in the myriapods. In Iulus, 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 with tracheæ[205]. In some of the larger species of Scolopendræ large open spiracles in the same situation are extremely visible[206]. Cermatia presents a singular anomaly:—a single series of spiracles of the usual form, each planted in a cleft of the posterior margin of the dorsal scuta, runs along the back of the animal[207]: unless we may suppose that, like the seeming spiracles of Iulus just mentioned, these are merely orifices by which it covers itself with some secretion.

6. A few words upon the number of spiracles.—If you examine the common dog-tick (Ixodes Ricinus), you will find only one of these organs on each side of the abdomen[208]; the Libellulina, as we have seen, have only four, all in the trunk; in the Dynastidæ, Melolontha, and the larva of Dytiscus, there are fourteen; sixteen in the Copridæ; eighteen in Dytiscus, and probably the majority of Coleoptera, both larva and imago, and Lepidoptera; and a pair to each segment except the last, in the Myriapods.

ii. Respiratory plates (Respiratoria). The nearest approach to spiracles is made by those remarkable plates that are found in such larvæ of Diptera, as in that state inhabit substances that might impede or altogether stop the entrance or exit of the air by the ordinary spiracles, such as dead or living flesh, dung, or the like. The Creator therefore, 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 are three oval 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 or spines replace the prominences[212]. In Echinomyia grossa and 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 of Œstrus Ovis[214], is a valve that closes the respiratory orifices. In the gad-fly of the ox (Œ. Bovis) there are no plates at the anterior extremity of the body; but those planted in the other end 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 a single, small, white spot, which appears to be a spiracle: in the smallest portion are eight minute circular orifices, arranged in a line[215]. As the only communication which this grub has with the atmosphere is at its anal extremity, it has no occasion for respiratory organs at the other. 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 at both ends of the body.

iii. Respiratory Appendages[216]. These may be divided into two kinds; those by which the animal has immediate communication with the atmosphere, and those by which it extracts air from water.

1. To begin with the first. 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 the tracheæ, 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 of hairs, 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 a single respiratory appendage it is furnished with a pair, 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 the Creator willed 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 it in equilibrio at its station at the surface[223]. As these larvæ seek their food amongst 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 a single tube would not have been sufficient: therefore Providence has furnished them with two, and both are extremely elastic, consisting of annular fibres, so as to admit their being stretched to an extraordinary length. Reaumur found that these animals could extend their tails to near twelve times their own length. The mechanism by which the terminal piece is pushed forth or retracted, is very curious, though extremely simple. Two large parallel tracheæ, 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 the base of the inner tube[225]; most probably, however, they do not here stop short, but, as in other instances, proceed to the end, and terminate in the two spiracles mentioned above: he conjectures that when the animal has occasion to push forth its respiratory apparatus, it injects into these vessels part of the air contained in the body of the tracheæ, 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 has four respiratory 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 the tentacula of 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 are two anal respiratory subcylindrical horns, with the orifice fringed with hairs[230]; and in another gnat Reaumur discovered four[231]. The larva of Tanypus maculatus, whose remarkable legs I formerly noticed[232], exhibits in the interior of 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 become external, 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 a Tipula observed by Reaumur, instead of two has only one of these respiratory organs, in the form of a very fine hair proceeding from the anterior end of the trunk, and considerably longer than the animal itself[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 an ovipositor, as some have thought[235].

These respiratory organs, however, are not invariably confined to aquatic larvæ and pupæ, for those of some aphidivorous flies have anal ones, and the pupa of Dolichopus nobilitatus, or a fly nearly related to it, which is terrestrial, has likewise a pair of long sigmoidal ones on the back of the trunk[236]. The pupa also of the rat-tailed larva just noticed as having four horns, resides under the earth, the insect being only aquatic in its grub state.

2. I am next to consider those respiratory appendages by which aquatic insects, since they do not come to the surface for that purpose, appear to extract air for respiration from the water; so that they may be looked upon in some degree as analogous to the gills of 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 some analogy rather than any affinity to 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 the Trichoptera are a number of white membranous floating threads, arranged in bundles, four on each segment, two above and two below, and traversed longitudinally by several air-vessels or bronchiæ, 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 aquatic moth (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 the tracheæ. 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 has four of these processes, longer than the rest[240].

Laminose or foliaceous respiratory appendages distinguish the sides of the abdomen of the larvæ and pupæ of the Ephemeræ, whose history you found so interesting[241]. In them these organs wear much the appearance of gills. In the different species they vary both in their number and structure. With regard to their number, some have only six pair of them, while others have seven. In their structure the variations are more numerous, and 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 or bronchiæ in connexion with the tracheæ branching and traversing it in all directions, like the veins of leaves[243]. The double ones differ in form. In the larva and pupa of Ephemera vulgata there are six of 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 the trachea enters 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, in E. fusco-grisea, one branch represents the leaf of a Begonia, 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 may emit 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 the Agrionidæ appear to respire like those of the Ephemeræ, &c. by means of long foliaceous laminæ or false gills filled with air-vessels; but instead of being ventral, they proceed from the anus. 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 probably bronchiæ. They are used by the animal, which swims like a fish, as fins, but it does not appear to imbibe the water like the other Libellulinæ, 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 the Libellulinæ, 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 to Catocala 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 the external respiratory organs of insects, by which the air is received, we are next to consider the internal ones, by which it is distributed. These are gills; tracheæ and bronchiæ; and sacs or pouches[256].

i. Gills (Branchiæ[257]). Having lately described what may be denominated false gills, or branchiform appendages, I shall now call your attention to what may be denominated true ones, which are peculiar to the Arachnida Class: but what is remarkable, the animals that breathe by them are very rarely inhabitants of the water, so that their functions cannot be perfectly analogous to those of fishes.

In the Scorpion, on each side of the four first ventral segments a spiracle may be discovered, which has no lip as in other insects, but is merely a circular orifice. These orifices do not lead to tracheæ or vesicles, but to true 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 than twenty of 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 the spiders also, 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 their laminated structure shows itself very clearly in old specimens, and in such as have been immersed in boiling water[259].

ii. Tracheæ and Bronchiæ[260]. Parallel with each side of the body of most insects and extending its whole length, run two cylindrical tubes[261], which communicate with the spiracles[262], and from which issue, at points opposite to those organs, other tubes which ramify ad infinitum, and are distributed to every part of the body[263]. The first of these tubes are called the tracheæ and the latter the bronchiæ. This structure appears, however, not to be universal: it is to be found in caterpillars and many Dipterous larvæ; but in that of the rhinoceros-beetle and other Lamellicorns, the bronchiæ branch directly from 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 of Smerinthus Populi[265]; in the Cicadæ[266]; in the Locust tribe[267]; and many others. In the Cossus, or larva of the great goat-moth, the trachea commences with the first spiracle, and finishes a little beyond the last, after which it diminishes considerably in diameter, and terminates in several branches or bronchiæ, which proceed to the anal extremity of the body[268]. The bronchiæ which originate from the tracheæ in the vicinity of each spiracle, may be considered as consisting in general of three packets;—dorsal ones, which are distributed to the back and sides of the animal; visceral ones, which enter the cavity of the body, and are lost amongst the viscera and the caul; and ventral ones, which dipping from the tracheæ overrun the lower part of the sides and belly[269].

The tracheæ and bronchiæ consist of three tunics[270]: the first or external one is a thickish membrane, strengthened by a vast number of fibres or vessels, which form round it a number of irregular circles; the second is a membrane more thin and transparent, without a vascular covering[271]; the third is 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]. The Bronchiæ are 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 the Cossus the trachea is 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 contraction of which Lyonet supposes the trachea may 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 of membrane, 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 the bronchiæ, when I have mentioned their number and incredible smallness. Nothing but the 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 two tracheæ of the larva of the Cossus gave 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, the bronchiæ are 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 hath God wrought, and this is the work of his hands.

Though in general there is only a pair of tracheæ, yet in some larvæ a larger number have been discovered. In those of the Libellulinæ there are six. According to M. Cuvier, Reaumur, who mentions only four, overlooked the two lateral ones that are connected with the spiracles[280]. The reason of this and other parts of their internal structure I shall explain under the next head. In the grub of the gad-flies of the horse (Gasterophili,) Mr. B. Clark discovered eight longitudinal tracheæ,—six arranged in a circle and two minute ones, which appeared to him to terminate in a pair of external nipples (spiracles) in the neck of the animal[281]. This is a singular anomaly, as the other Œstridæ have only a pair of tracheæ[282].

iii. Respiratory Sacs or Pouches. Besides their tracheæ and bronchiæ, many insects are furnished with reservoirs for the air, under the form of sacs, pouches, or vesicles. These are commonly formed by the bronchial tubes being dilated at intervals, especially in the abdomen, into oblong inflated vesicles; from which other bronchial tubes diverge, and again at intervals expand into smaller vesicles, so as to exhibit no unapt resemblance—as Swammerdam has observed with respect to those of the rhinoceros-beetle—to a specimen of Fucus vesiculosus. Cuvier compares them in the Lamellicorn beetles in general to a tree very thickly laden with leaves[283]; and Chabrier observes that they particularly occur in the intestinal canal[284]. This structure of the pulmonary organs may be seen also in the common hive-bee, and other Hymenoptera; but the vesicles are less numerous, and those at the base of the abdomen much larger than the rest[285]. These vesicles, by a very rough dissection, may be distinctly seen in the abdomen of the cockchafer, which appears to be almost filled with them. Not being composed of cartilaginous rings like the air-tubes, but of mere membrane, if a pin pierces one, the air that inflates it escapes, and it collapses. In the larva of a little gnat (Corethra culiciformis) the tracheæ appear to proceed from a pair of oblong vesicles of considerable size[286] in the trunk, and towards the anus they form two other smaller ones[287],—upon piercing the former, De Geer observed a considerable quantity of air to make its escape[288]. Another species, probably of the same genus, described by Reaumur, exhibits something similar[289].

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

But this kind of structure is not confined to insects strictly aquatic. Even such species of terrestrial ones as live upon aquatic plants, and are, consequently, necessarily or accidentally often a considerable time under water, are furnished with some apparatus by means of which they can exist in this element for a considerable period. For example, most of the Weevils (Rhyncophora) die in a short time if immersed in water; yet the species of the genera Tanysphyrus, Bagous, and Ceutorhynchus which feed on aquatic plants, can exist for days under water, as I have ascertained by experiment. C. leucogaster and another of the same tribe, swims like a Hydrophilus, and will live a long time in a bottle filled with water and corked tight. Other insects also, that are not at all aquatic, have pneumatic pouches. A striated or channeled vesicle I have found under the lateral angles of the collar in the humble-bee, where Chabrier supposes the vocal spiracles are situate; and also at the mouth of the spiracles of the metathorax in Vespa, &c.[296] In Sphinx Ligustri the bronchiæ terminate in oblong vesiculoso-cellular bodies, almost like lungs[297]; in Smerinthus Tiliæ these are preceded by a simple vesicle bound with spiral fibres[298]. M. Chabrier thinks that these air-bladders of insects, amongst other functions, give more fixity and force to the muscles for flight[299].

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

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

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

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

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

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

The access of air is as necessary to insects even in their egg state[328], and in many cases its presence seems provided for with equal care, by means as beautiful as those Sir H. Davy and Sir E. Home have shown to occur in the oxygenation of the eggs and fœtuses of vertebrate animals[329]. It is only necessary to view the admirable net-work of air-vessels which Swammerdam discovered spread over the surface of the eggs of the hive-bee while in the ovaries[330],—a provision which, from analogy, we may conclude obtains generally; from the importance which nature has attached to the oxygenation of the germ while in the matrix. And judging from analogy, we may infer that the access of this element is as carefully secured after the egg is laid, as before. The eggs of most insects being of a porous texture, often attached to the leaves of plants, and some of them embedded in the very substance of a leaf or twig[331], are in a situation for the abundant absorption of oxygen: and the pouch of silk in which the eggs of spiders and Hydrophili are deposited, may probably, from Count Rumford's experiments, be of utility in the same point of view. In the case of the Trichoptera and other insects[332] whose eggs are dropped into the water enveloped in a mass of jelly, this substance perhaps serves for aërating the included embryo, in the same way with the jelly surrounding the eggs of the frog, dog-fish, &c. It would be desirable to ascertain whether the former jelly be of the same nature as the experiments of Mr. Brande have shown the latter to be[333]. It is not improbable that the singular rays that terminate the eggs of Nepa[334] may in some way be connected with the aëration of the egg.

To what I have before remarked with regard to the vital heat of insects[335], I may under this head very properly add a few further observations. I there stated, that the temperature of these animals is usually that of the medium they inhabit, but that bees, and perhaps other gregarious ones, furnish an exception to this rule[336]. A confirmation of this remark is afforded by Inch, a German writer, who, upon putting a thermometer into a bee-hive in winter, found it stand 27° higher than in the open air; in an anthill, he found it 6° or 7° higher; in a vessel containing many blister-beetles (Cantharis vesicatoria,) 4° or 5° higher. A thermometer, standing in the air at 14° R., put into a glass vessel with Acrida viridissima, in nine minutes rose to 17°, and a similar result was observed with respect to other insects[337]. Dr. Martine says that caterpillars have but two degrees of heat above that of the air they live in[338]. Coleopterous insects are said to move slowly and with difficulty when the thermometer sinks to 36°, to become torpid at 34°, and to lose muscular irritability at a lower degree[339]. I have before observed that some insects will bear to be frozen into an icicle, and yet survive[340]: they share this power with reptiles, fishes, and amphibia. But, however small the excess of it in some insects above that of the medium they inhabit, it proves that they possess the power of generating heat. Whether, like the warm-blooded animals, they generally possess that of resisting heat by perspiration, &c. is not so clear. Yet the heat to which some can bear to be exposed, basking at noon, as Dr. Clarke informs us[341], on rocky and sandy places, exposed to the full action of the sun, appears sufficient, if not resisted by some principle of counteraction, to roast them to a cinder. That bees perspire is well known, but probably not singly.

When the respiration of insects is suspended by immersion in any fluid, it is often resumed, even when it has been long and they are apparently dead, if they be brought into contact with the atmosphere. Reaumur found this to be the case with bees[342]; and Swammerdam tells us that the maggot of the cheese-fly (Tyrophaga Casei) lived six or seven days in rain-water[343]: he found it so difficult to kill the larva of Stratyomis Chamæleon, which he first immersed twenty-four hours in spirits of wine, and then put them several days in water, without killing them,—that he lost his patience, and dissected them alive. He tried to drown them also in vinegar, in which they held out more than two days[344].

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

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