And this rule holds equally good in the case of the insects, which are devoid of lungs for the very sufficient reason that those organs are necessarily weighty, and consequently unsuitable to the requirements of beings which are in great measure creatures of air. In all animals intended for a more or less aerial existence every particle of superfluous weight must be dispensed with, in order that the strain upon the muscles of flight may be reduced to the least possible degree. Take the bats, and see how the skeleton has been attenuated until it scarcely seems capable of affording the necessary rigidity to the frame. Take the birds, and see how a large portion of the body is occupied by supplementary air-cells, which permeate the very bones themselves, and thus minimize the weight without detracting from the strength. And so also with the insects, but in a different manner. For in them the very lungs themselves are taken away, and replaced by a respiratory system of great simplicity, and yet of wonderful intricacy, which penetrates to every part of the structure, and simultaneously aerates the whole of the blood contained in the body. In other words, an insect is one large Lung. * * * * * If we take any moderately large insect, say a wasp or a hornet, we can see, even with the naked eye, that a series of small spot-like marks runs along either side of the body. These apparent spots, which are generally eighteen or twenty in number, are in fact the apertures through which air is admitted into the system, and are generally formed in such a manner that no extraneous matter can by any possibility find entrance. Sometimes they are furnished with a pair of horny lips, which can be opened and closed at the will of the insect; in other cases they are densely fringed with stiff, interlacing bristles, forming a filter, which allows air, and air alone, to pass. But the apparatus, of whatever character it may be, is always so wonderfully perfect in its action that it has been found impossible to inject the body of a dead insect with even so subtle a medium as spirits of wine, although the subject was first immersed in the fluid, and then placed beneath the receiver of an air-pump. The apertures in question, which are technically known as “spiracles,” communicate with two large breathing-tubes, or “tracheæ,” which extend through the entire length of the body. From these main tubes are given off innumerable branches, which run in all directions, and continually divide and subdivide until a wonderfully intricate network is formed, pervading every part of the structure, and penetrating even to the antennæ and claws.