CHAPTER IV.

RESPIRATION OF THE LARVA.

We well know, from the various melancholy accidents which have taken place, that unless human beings have a constant supply of pure fresh air they must perish. Many years ago, a number of unfortunate persons were shut up in a narrow cell, called the Black Hole, at Calcutta, where they could scarcely find room to stand, much less obtain air to breathe. In one night the greater part of them died. More recently, as some poor Irish were being conveyed in a steamer in stormy weather, the captain, out of prudence and mistaken kindness, ordered the hatches to be all battened down, so that fresh air was prevented from getting in. A shocking spectacle was beheld soon afterwards:--in the close, suffocating cabin lay a number of dead bodies, of men, women, and children, all destroyed by the want of air. Now, precisely the same fatal results take place if we treat larvæ in the same manner; from which we learn that undoubtedly breathing is not only one of their functions, but a most necessary and important one.

This may be easily proved. If the reader will procure about a dozen caterpillars of any common kind, and put them into a dry phial, corking them closely up, and if he leaves them in their prison of glass a sufficient time, all will die, even though he may have supplied them, on putting them in, with an abundance of food. Clearly, therefore, they do not die of starvation, and we should be naturally disposed to say they must have died of suffocation; that is, they died because the function of breathing could not take place, as there was not the necessary change of the air contained in the phial. But suppose that other caterpillars were procured and put into another phial, with a sufficiency of green food, the mouth being covered over with a piece of muslin, or fine lace, what would then be the result? Simply that they would live and thrive for as long a period as they received their proper quantity and quality of food, and would pass through all their stages of existence as comfortably as if in the open air. The reason would be because the open fibres of the muslin, or lace, do not interfere in the least with the requisite change of air for respiration.

Again, if after corking up the bottle for a short time, we were to shake out all the caterpillars from it, and then put into it a little water, in which quick-lime has been slaked--sold by chemists under the name of lime-water,--we should find it become quite white and milk-like. This would be, because, as the physiological chemist well knows, the function of breathing causes carbonic acid gas to be poured out of the body, no matter whether it is the body of an insect or an elephant; and this gas has the property of turning lime-water of the colour mentioned. Hence we have a second and convincing proof that larvæ breathe--we need scarcely remind the reader that caterpillars are larvæ--and more than this, that the function of breathing in them resembles, so far as the discharge of this peculiar gas is concerned, the same function, whether carried on in the human body or in that of the most mighty and majestic of the beasts of the field.

The great chemist, Scheele, has experimented, somewhat in the manner we have recommended, upon the breathing of larvæ. He found that when caterpillars and maggots, to use the customary terms for these varieties of larvæ, were confined in vessels containing only about eleven cubic inches of atmospheric air, and so stopped as to preclude the ingress of any fresh air whatever, although furnished with an abundant supply of food, they soon died. When he put them into a still smaller vessel, they died the sooner. In fact, precisely in proportion to the purity, or to the amount of the air contained in the space in which he confined them, were their lives prolonged or shortened. He found on examining the air that it contained, as above stated, carbonic acid gas.

We may, therefore, consider the question as settled--that larvæ do breathe. Strange it may appear to some, but neither in the larva nor in any other form of its existence do insects breathe by the _mouth_. How, then, it may be asked? We shall now describe their breathing apparatus very briefly. Along each side of the body runs a delicate tube for the conveyance of air, called a _trachea_, or when both are spoken of, the _tracheæ_; these channels run underneath the muscles and skin, and open to the air by little branch tubes, the mouths of which are called _spiracles_. Besides these branch tubes which open to the air, the large tubes send off a great number of smaller ones, which extend to all parts of the body, penetrating even to the legs and wings. In some insects, the spiracles, or breathing holes, are defended by a pair of flaps, which swing to and fro, so as to let in the air, and close upon it after each inspiration. By this simple means the breathing of insects is carried on; the air enters the tubes, passes along them, and is then conveyed to every part of the body; after which, having fulfilled its duties, it escapes again. In man, and the higher animals, there is a separate place for the air in the body, which we call the lungs, and here the blood is conveyed to the air, in order to be purified. But, as we have just seen, in insects it is very different, for in them the air is conveyed by innumerable minute pipes to every portion of the frame. Such is the breathing mechanism common in larvæ which live in the air.

It will be readily imagined that those larvæ which dwell in the water must be furnished somewhat differently, in order to enable _them_ to breathe. And here, as we shall now find, there are some very curious arrangements, in order to effect this object. They may be considered under two divisions. First, contrivances for breathing air while the larva is immersed in the water; and, second, apparatus for extracting the dissolved oxygen gas necessary for breathing from the water.

Let us select a few of the most singular instances under the first of these divisions. "No better example," write the entertaining authors of the Introduction to Entomology, "can be selected than the gnat. You must have occasionally observed in tubs of rain-water, numerous little wriggling worm-like animals, which frequently ascend to the surface, there remain awhile, and then bending their head under the body, rapidly sink again to the bottom. 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 the body, with a singular organ, which varies in length according to the species, and forms an angle with the last segment but one. The mouth of this organ is funnel-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 this mouth are used to close it when the animal is disposed to sink in the water, and thus 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 water, 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. The extremity of the larva is clothed with bunches of hairs, which are furnished with some repellent material which prevents their becoming wet. 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."

In summer time, the reader can scarcely fail of the gratification of watching these most curious creatures, for they abound in ditches, pools, and the stagnant waters of artificial fountains. They form a very curious object under the microscope, and particularly in the gas microscope, or solar microscope, where the image of the larvæ is thrown, as in the case of the magic-lantern, upon a white sheet, or wall. Their curious writhings to and fro, the energy they display in lashing about their tails, so as to throw the few drops of water in which they are imprisoned into mimic whirlpools, their occasional dartings down to the bottom, and subsequent gentle rise up to the surface, together with the terror into which countless minor occupants of the same watery prison with themselves are thrown, when one of these comparative giants plunges into the midst of them, form a spectacle as unique as it is interesting and diverting. A sharp eye, and a phial and cork, are the alone requisites for finding out these singular beings on most fine days in summer. But a little while ago the writer was amused to find, on a sunny day in June, the water of a small fountain playing before his study window, thickly populated with these active larvæ, and it became an amusement which seldom wearied, to watch their fantastic evolutions, and to trace their changes from the larva through the ulterior stage up to the perfect insect. To this, however, we shall probably have occasion to revert before the conclusion of this work.

Another instance of a curious contrivance for assisting the breathing of a larva, whose body is wholly immersed in water, may also be found near home. Most probably the majority of our readers have seen, or at any rate are well acquainted with the apparatus by which the diver descends and works under water. Lest some should not, we may mention that it consists of a water-tight dress, which covers the diver from head to foot, while his head is enclosed in a helmet, or hood, furnished with a pipe or hose, which is long enough to reach to the surface of the water, and is there generally connected with an air-pump, which forces down fresh air, and so enables the man to breathe almost as comfortably as if he were walking on land, instead of at the bottom of the sea. Let it be conceived now that there were no air-pump at the top of the hose, but simply a buoy of cork to keep it floating, and that the diver had the power of shortening or lengthening the hose at his pleasure, we should then have a tolerably fair representation of the contrivance with which Infinite Wisdom has supplied the humble creatures of whom we are about to speak.

By carefully examining the water running from manure heaps, or water which has become putrid, and is much loaded with mud and ooze, the reader will pretty generally be successful in discovering a number of the larvæ of a two-winged fly of the shape represented in the cut. When discovered, the singularity of their structure will amply repay him for the trifling unpleasantness attending the search after them. After washing them in a little fair water, and putting them into a tall ale-glass, or German beer-glass, which must be three parts filled with water, they will be in the best condition for our examination. When Réaumur first beheld them, he gave them the name of _Vers à queue de rat_; that is, larvæ with a rat's tail; and when the reader looks at them, as shown in the representation annexed, he will be inclined to think with us, that a more appropriate title could not have been found.

On watching them they will be seen to wriggle about until they are comfortably settled, darting from place to place, as if uneasy in their new position and in the pure element. By-and-by, however, they become reconciled to their lot, and lie perfectly quiet at the bottom of the glass in the position represented. From the tail of each will now be seen to rise a beautifully delicate and minute tube, which comes up to the surface of the water, and here, whatever be the movements of the larva below, it remains floating and free. Just as the diver may, while his companions above are pouring down torrents of fresh air, move about, and perform various duties under, water, so the larva may move from one position to another, and assume almost any variety of attitude; but its little air-pipe still retains its perpendicular direction, and does not quit the surface of the fluid.

A singular experiment may now be tried with them. Let a little water be poured into the glass by means of a funnel, the end of which dips under the water so as not to give them unnecessary disturbance, and by this means the level of the water in the glass becomes insensibly raised half an inch or so. In a few seconds it will be found that these little tails have all lengthened themselves, and are again at the surface of the water, even at its higher level. More water may be poured in until perhaps the level is raised three or four inches, and still the same thing takes place, the larvæ still prolong their curious tails, until it seems as if they would stretch to an indefinite length, for they are already eight or ten times longer than the body! But, as we might jestingly say--the longest _tail_ comes to an end at last; for, on continuing to pour more water in, the larvæ stretch their tails to the utmost, and then, finding the level of the water still above their reach, they become uneasy, and all quit the bottom of the glass and attach themselves to its sides.

Let us now examine more minutely this very singular apparatus. By a good lens we may readily do this, as the organ is very transparent, permitting its internal structure to be beautifully seen. Placing the creature in a convenient position for our examination, and in a good light, it will be presently seen that the tail is composed in reality of two tubes, of which one slides up and down within the other, just like the tubes of a telescope (see cut). By this means the larva is provided with an instrument capable of being made to reach to a great length, and yet also possessing the capability of being packed up again in a very small compass. The proper breathing tube is the small inner one; at its extremity is a pair of spiracles, or breathing pores, and surrounding these are placed five minute bristles, which are supposed to act somewhat as a buoy, and to retain the tail in its position. Where the tail joins on to the body of the larva, two tubes thrown into folds are placed, which are connected with the breathing tubes of the tail, at one end, and with the breathing pipes which run along the body of the larva at the other. When the tail is pushed out to its farthest, these zig-zag tubes are stretched quite straight; when it is drawn in again, they are thrown into folds. Both these appearances are shown in the plate. It has been supposed that the larva causes the tube to be pushed out by forcing air into it, and thus causing the zig-zag tubes to straighten themselves. Perhaps the tail is drawn in by the same tubes contracting again, and thus folding together and dragging back the tube to which they are connected.

We need scarcely stop to mention how useful is this admirable contrivance to the insect. Having to seek for their food at the bottom of ditches, drains, or puddles, they are necessarily exposed to the risk of suffocation, unless fresh air can be conveyed to them; and it is difficult for us to imagine how any apparatus could have been contrived which would have adapted itself to all the varying depths of water in which the insect must be constantly living, as it changes from place to place. He must have been a clever engineer who could have successfully met this difficulty. None of our present diving apparatus does so. The organ we have been considering, the creation of an Infinite Mind, small and despicable as it may seem in our eyes, fulfils perfectly every function for which it was formed. It admits of free movement from place to place, it admits, moreover, of free change from one level to another in the fluid by which the larva is surrounded, and it is at the entire disposal of the insect, which can, without the smallest inconvenience, accommodate it to the various circumstances in which it may be placed.

Possibly the same phial in which was brought home the last-named larvæ with the rat's tail, will furnish us with an equally elegant instance of larva-respiration under water, in the case of the larva of a fly, called the chameleon fly. This little creature, if it can be found, and though not as common as the last, it may yet be detected, in summer by the edge of ponds, &c., has truly elegant habits. The great Swammerdam, who first described it, was lost in admiration at it. The extremity of its body is furnished with a coronet of about thirty elegantly feathered hairs. These are under the control of the insect, and are capable of being folded up, so as to enclose a minute bubble of air. When the larva is weary of remaining at the top of the water, and wishes to dive to the still and cool depths below, it causes these hairs so to fold up and close upon each other as to include the air-bubble, so that it cannot escape. Thus furnished with a pearl in its tail, it plunges downward, and thus breathes under water until the purity of the captive air-bubble is impaired, when it returns to the surface to repeat the same operations. The appearance of the little bubble shining, with a brilliant silvery lustre under the water, is very pleasing, and accounts for the enthusiastic expressions used by Swammerdam, in speaking of the visible manifestation of the skill of God in this insect. The fact of an insect coming to the surface for air, and stealing away a portion, then diving down again into the waters, is not uncommon in insect history, although few are possessed of so elegant a contrivance for effecting it as the larva in question.

We must pass on to mention an instance or two of larvæ which, from the peculiarity of their respiration, come under the second of our divisions, that is, they live in the water, and do not breathe air, but obtain the gas necessary for their respiration _from the water_, which holds it in a state of solution. We shall first speak of the larva of the common May-fly, the perfect insect of which is so well known to, and highly prized by, anglers, under the various names of "duns" and "drakes," than which in some streams no insect that flies is a better bait to the unwary fish. Should the reader wish to examine them for himself, they may be found in May or June, hiding themselves during the day in the mud, or underneath stones, or in little holes in the banks, or taking a sub-aqueous tour upon the bed of the brook, while the clear and purling waters roll smoothly on above them. Their little under-water cells are shown in the cuts, together with the larva.

Their organs for breathing the water, if the expression, which is in some measure correct, may be admitted, are very like the gills of a fish. But instead of forming a part of the structure of the mouth and head, as those of the fish, in the larva they are placed along its body, and are arranged in pairs. A delicate little air-tube enters into each of these gills, and there branches out into a great many directions; this tube is connected with the large trachea, or breathing vessel of the insect, running along its body. The air entering these gills, and thus being exposed to a large surface of water containing dissolved gas, loses its impurities, and absorbs the pure gas from the water, whence it is then conveyed to the various parts of the body. In many species of these larvæ the whole series of gills are moved to and fro incessantly, and so quickly as to be scarcely perceptible to the eye. Some of them use them as oars, moving with inconceivable rapidity, and being capable of darting both backwards and forwards with great quickness. They have also the power of leaping or springing in the water to a considerable distance; and feed on minute aquatic larvæ, and on vegetable matters.

The only other instance of this kind that we shall quote will be that of the terrible larva of the dragon-fly, whose deeds of insect murder came before us on a former occasion. The apparatus provided for them is very singular, and has been described with his usual accuracy by the talented Réaumur. It exists in the tail of the insect, and may be readily watched in action by adopting the following expedient:--Pour into a small glass, in which the larva has been put, and which is half full of water, a few drops of a solution of carmine, by means of a little tube introduced into the water, and made to drop its contents near the tail of the creature. Immediately it will be seen that the coloured liquid is rapidly sucked in by the tail of the insect, and then, after the expiration of a little time, is forced back again in a tiny jet, with considerable violence, to a distance of several inches.[H] Réaumur tells us, that if we hold the insect between the finger and thumb gently, and let fall a drop of water on its tail, we shall presently see it eagerly sucked in, the body of the insect becoming sensibly larger; and by-and-by it is thrown out again. Sometimes it raises its tail above the surface of the water, and, whether for sport or not, it is hard to say, squirts the water in a mimic jet to some distance.

The apparatus has been described as resembling the piston or plunger of a force-pump, which moves to and fro within a cavity inside the insect's body, and by that means draws in and forces out the water. The tail is defended by a sort of coronet, composed of five hard bristle-like pieces, which are under the control of the creature, and may be opened and closed at pleasure. The larva appears capable of inflicting a somewhat painful pinch by this contrivance, so that, what with its wonderful mask, with its powerful squirt, and with its tail thus armed, it would be difficult to select another insect better prepared for battle with creatures of every aquatic tribe than it. The action of the pumping apparatus sends it rapidly forward through the water, somewhat like a paddle-wheel, or at least upon the same principle as that contrivance, namely, the resistance offered by the water to the stroke of this apparatus.