STRUCTURE, GROWTH AND ECONOMICS OF INSECTS 



15 



May flies and the males of scale insects have their hind wings very 

 much reduced or entirely wanting. In the flies (Diptera) the hind 

 wings are converted into halteres (balancers). 



2. Where the fore wings are either wholly or partially chitinized. 

 Examples of such thickening of the wings occur among the grass- 

 hoppers and crickets, the beetles 

 and the heteropterous bugs. 



3. Where the hind and fore 

 wings are strongly reduced or 

 completely atrophied, as in the 

 fleas (Siphonaptera), bird-lice 

 {Mallophaga) and sucking lice 

 {Siphnnculata) . 



Venation. — It has been found 

 that the system of veins in the 

 different orders of insects is 

 fundamentally alike, being de- 

 rived from the primitive type 

 fairly well seen in some stone- 

 flies and some cockroaches. 

 This fact becomes evident only 

 when a comparison is made of 

 the wings of the more general- 

 ized members of the different 

 orders, as specialization has 

 greatly modified their structure 

 in most genera. Comstock and 

 Needham have shown by a 

 study of the developing wings 



of nymphs and pupaj that the /'^- 20.--Several stages in the de- 

 "^ , .... . . velopment of the wings of a cabbage butter- 



principal longitudinal vems m fly. {After Met cer.) 



the more generahzed orders are 



formed about tracheae (Fig. 20). In the development of the wing 

 these tracheae grow out into the wing-bud, and later the veins are 

 formed about them. The cross veins, however, as a rule do not arise 

 in this manner, as tracheae are apparently absent. In the course of 

 development specialization has brought about changes in the venation, 

 recognized, first, by the addition of veins through branching of the prin- 



