INSECTS 



progress toward greater efficiency in the mechanism of 

 flight, and that the acme in this line has been attained by 

 the flies 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. 



Fig. 167. A robber fly, showing the typical structure of any 



member of the order Diptera 



The flies are two-winged insects, the hind wings being reduced 



to a pair of knobbed stalks, the halteres (HI) 



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 not necessarily to be attributed to the 

 form ot the wings, because their wing muscles are partially 

 degenerate. The dragonflies (Fig. <;8) are particularly 

 strong flyers, and with them the two pairs of wings are 

 but little different in size and form; but the dragonflies 



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