progress toward greater efficiency in the mechanism of flight, and that the acme in this Il'ne has been attained by the files and mosquitoes. The truth of this contention will become apparent when we compare the relative development of the wings and the manner or effective- ness of flight in the several principal orders of insects.
Fro. 16 7. A robber fly, showing the typical structure of any member of the order Diptera The files are two-winged insects, the hind wings belng reduced to a pair of knobbed stalks, the halteres (H1)
It is most probable that when insects first acquired wings the two pairs were alike in both size and form. The termites (Fig. 168 A) afford a good example of in- sects in which the two pairs of wings are still almost identical. Though the termites are poor flyers, their weak- ness of flight is hot necessarily to be attributed to the form of the wings, because their wing muscles are partially degenerate. The dragonflies (Fig. 58) are particularly strong flyers, and with them the two pairs of wings are
but little different in size and form; but the dragonflies
