11(3 THE LIFE-STORY OF INSECTS [OH. 



mentioned in Chapter n (p. 15), or certain genera- 

 tions of virgin females are wingless, for example 

 aphids (pp. 18-19) and gall-flies (pp. 94-5). Insects 

 may thus become secondarily wingless, that is to say be 

 manifestly the offspring of winged parents, and such 

 wingless forms may on the other hand give rise to 

 offspring or descendants with well-developed wings. 

 Frequently, as in the case of the aphids, many 

 wingless generations intervene between two winged 

 generations. A striking illustration of this fact is 

 afforded by an aquatic bug, Velia currens, commonly 

 to be seen skating over the surface of running water. 

 The adults of Velia are nearly always wingless, but 

 now and then the naturalist meets with a specimen 

 provided with functional wings, the possession of which 

 enables the insect to make its way to a fresh stream. 

 Moreover there are whole orders of parasitic insects, 

 such as the lice and fleas, which, showing clear affinity 

 to orders of winged insects, are believed to be 

 secondarily wingless. These orders are designated 

 by Sharp 'Anapterygota.' And from the analogy of 

 the periodic loss and recovery of wings in various 

 generations of the same species, he has concluded 

 that the gap between the exopterygote and the 

 endopterygote method of development may have 

 been bridged by an anapterygote condition ; that the 

 ancestors of those insects with complete transfor- 

 mations were the wingless descendants of primitive 



