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. [Illustration: _Breathing apparatus of an aquatic Larva._]