Part 5

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.

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