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 insects; but it is evident that the majority of insects have found it more advantageous to have the fore and hind wings different in one way or another.

In the grasshoppers, it was observed (Fig. 63), the hind wings are expanded into broad membranous fans, while the fore wings are slenderer and of a leathery texture. The same is true of the roaches (Fig. 53), the katydids (Fig. 168 B), and the crickets, except in special cases where the fore wings are enlarged in the male to form musical organs (Fig. 39). In all these insects the hind wings are the principal organs of flight. When not in use they are folded over the body beneath the fore wings, which latter serve then as protective coverings for the more delicate hind wings. In the beetles (Figs. 137, 168C) the hind wings are much larger than the fore wings, and, as with the grasshoppers and their kind, they take the chief part in the function of flight. The beetles, however, have carried the idea of converting the fore wings into protective shields for the hind wings a little farther than have the grasshoppers; with them the fore wings are usually hard, shell-like flaps that fit together in a straight line over the back (Fig. 137 A), forming a case that completely conceals, ordinarily, the membranous hind wings folded beneath them. Neither the grasshoppers nor the beetles are swift or particularly efficient, but they appear to demonstrate that the ordinary insect mechanism of flight is more effective with one pair of wings than with two.

The butterflies and the moths use both pairs, of wings in flight; but with these insects, it is to be noted, the front wings are always the larger (Fig. 168 D). The buttertties, with four broad wings, fly well in their way and are capable of long-sustained flight, though they are comparatively slow goers. Some of the moths do much better in the matter of speed, but it is round that the faster flying species have the fore wings highly devcloped at the expense of the hind wings; and that the two wings on each side, furthermore, are voked together in such a manner as