MOSQUITOES AND FLIES 



to insure their acting as a single wing (D). The moths 

 clearly show, therefore, as do the grasshoppers and the 

 beetles, the efficiency of a single pair of flight organs as 

 opposed to two. The moths, however, have attacked 

 from a different angle the problem of converting their in- 

 herited equipment of four wings into a two-wing mecha- 

 nism — instead of suppressing the flight function in one pair 

 of wings, they have given a mechanical unity to the two 

 wings of each side, thus attaining functionally a two- 

 winged condition. 



The wasps (Fig. 133) and bees, likewise, have evolved 

 a two-winged machine from a tour-wing mechanism on the 

 principle of uniting the two wings on each side. The bees 

 have adopted a particularly efficient method of securing 

 the wings to each other, for each hind wing is fastened to 

 the wing in front of it by a series of small hooklets on its 

 anterior vein that grasp a marginal thickening on the rear 

 edge of the front wing (Fig. 168 E). Moreover, the bees 

 have so highly perfected the unity in the design of the 

 wings that only on close inspection 'is it to be seen that 

 there are actually two wings on each side of the body. 



Finally, the flies, including all members of the order 

 Diptera, have boldly executed the master stroke by com- 

 pletely eliminating the second pair of wings from the 

 mechanism of flight. The flies are literally two-winged 

 insects (Figs. 167, 168 F). Remnants of the hind wings, 

 it is true, persist in the form of a pair of small stalks, 

 each with a swelling at the end, projecting from behind 

 the bases of the wings (Figs. 167, 168 F, HI). These 

 stalks are known as "balancers," or halteres, and in their 

 structure they preserve certain features that show them 

 to be rudiments of wings. 



The giving over of the function of flight to the front 

 pair of wings has necessarily involved a reconstruction in 

 the entire framework and musculature of the thorax, and a 

 study of the fly thorax gives a most interesting and in- 

 structive lesson in the possibilities of adaptive evolution, 



i3 l 9] 



