progress toward greater efficiency in the mechanism of flight, and that the acme in this line 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 effectiveness of flight in the several principal orders of insects.
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Fig. 167. A robber fly, showing the typical structure of any member of the order Diptera
The files are two-winged insects, the hind wings being reduced to a pair of knobbed stalks, the halteres (Hl)
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 insects in which the two pairs of wings are still almost identical. Though the termites are poor flyers, their weakness 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
