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.
A Spiracle open. The same shut.
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.
Breathing apparatus of an aquatic Larva.
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.