Chapter III: Letter XXXVIII (1)

_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].