CHAPTER IX 



The Two-winged Flies 



So far as numbers of individuals are concerned, the 

 Two-winged Flies of the order Diptera are among the most 



abundant of all 

 insects. These 

 are distinguished 

 by the fact that 

 there is but one 

 pair of wings, al- 

 though in most 

 species there is 

 a pair of curious 

 little projections 

 called halteres 

 or "balancers" 

 which represent 

 the second pair 

 of wings. The 

 flies have com- 

 plete transformations, and in the larval state commonly ex- 

 ist as footless maggots. The mouth parts are formed for 

 sucking, although there are often special modifications for 

 biting, in connection with the sucking apparatus. 



Fortunately, there are comparatively few species of Dip- 

 tera that are destructive to cultivated crops. Among these 

 few, however, are some of the most troublesome of all in- 

 jurious insects, such as the Hessian Fly, the Apple Maggot, 



