INSECTS
they could glide through the air from the branches of one tree to another as well as can a modern flying squirrel by means of the folds of skin stretched along the sides of its body between the fore and the hind legs. If such lobes then became flexible at their bases, it required only a slight adjustment of the muscles already present in the body to give them motion in an up-and-down direction; and the wings of modern insects, in most cases, are still moved by a very simple mechanism which has involved the acquisition of few extra muscles.
It appears, however, that three pairs of fully-developed wings would be too many for mechanical efficiency. In the later evolution of insects, therefore, the prothoracic lobes were never developed beyond the glider stage, and in all modern insects this first pair of lobes has been lost. Furthermore, it was subsequently found that swift flight is best attained with a single pair of wings; and nearly all the more perfected insects of the present time have the hind pair of wings reduced in size and locked to the front pair to insure unity of action. The files have carried this evolution toward a two-winged condition so far that they have practically achieved the goal, for with them the hind wings are so greatly reduced that they no longer have the form or function of organs of flight, and these insects, named the Diptera, or two-winged insects, fly with one highly specialized and efficient pair of wings (Fig. 167).
The Paleodictyoptera became extinct by the end of the Carboniferous period, and their disappearance gives added support to the idea that they were the last survivors of an earlier type of insect. But they were by no means the primitive ancestors of insects, for, in the possession of wings alone, they show that they must have undergone a long evolution while wings were in the course of development; but of this stage in the history of insects we know nothing. The rocks, so far as has yet been revealed, contain no records of insect life below the upper
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