The wings of insects vary considerably. Some are membranous and smooth; others are covered with down or scales; while in many the fore pair are solid chitinous wing-cases, useless for flight, and chiefly acting as protective covers to the hind pair, which, when the insect is at rest, lie folded up beneath them. The flying wings are traversed with branching tubes called nervures, which, while strengthening them, also perform an important function in the breathing of the insect, and are pierced with small openings; these openings are very distinct in some of the hymenoptera. Insects breathe by means of openings situated along the sides of the thoracic and abdominal segments called spiracles, opening out into branching air vessels called trachea, which pass into the interior, ramifying throughout the body and extending into every part and appendage, even to the tips of the antennae. The nervous system, the life and movement of the insect, consists of a double chain of ganglia, (_ganglion_, a knot,) nerve cells, which are connected with finer encircling nerve tissues, that radiate in all directions, returning to the ganglia, the latter regulating the nerve sensation. Therefore, as their perceptions are so much less confined to the brain than in vertebrate animals, they cannot feel to the same extent. Thus, you can frequently find a locust, beetle, or ant that has escaped from a bird, minus its abdomen, still crawling about, quite able to move all its remaining organs. You can even remove the long slender body of a dragon-fly, and carefully insert a bit of grass stalk of the same length and weight to balance the wings, and it will fly off quite readily; but of course they will not live long after such injuries. We know that many insects must have very keen perceptions of sound, or the movement of the air around them, for they will drop to the ground at the least alarm, before the bush upon which they are resting has been touched. Very little is understood about the organs of hearing, except in the case of grasshoppers and locusts where the ears have been located at the base of the abdomen or on the front leg; these in some species can be detected with an ordinary lens. It is considered by some writers that the hairs and spiracles upon the different parts of the body may transmit sound and act as ears. The organs of sound are very interesting, but can be better treated when dealing with the different groups. Usually, there are only two sexes of insects, males and females; but among those living in social communities, like the bees, ants, wasps, and termites, the majority of the inhabitants are neuters. These neuters are usually aborted females, which do all the work in the construction of the nest and look after the food supply of the rest of the community. FOSSIL INSECTS