XII 

 DIPTERA 



Flies, Mosquitoes, etc. 



This is another very large order of insects. There are 

 perhaps 10,000 species in North America. The adults are 

 two-winged, as the name signifies.* These two are the fore 

 wings, and pertain to the mesothorax, while the hind wings 

 are never developed for flight, but are represented by a 

 pair of minute knobbed threadlike organs, called halteres 

 or balancers. The metathorax is small and the mesothorax is 

 correspondingly large, since it contains the flight muscles. 

 The head is generally freely poised on a conic prothorax. 

 The eyes are large, covering most of the surface of the head. 

 The antennae are variously formed. The mouthparts are 

 combined into a sucking beak, very different from that of 

 the bugs and the butterflies. As in the bugs, the labium 

 forms a sheath inclosing the modified jaws and these may 

 be developed as sharp puncturing stylets. They are so in 

 the mosquitoes and biting flies; but in the commoner flies, 

 the tube is soft and pliant, and bears a pair of flaplike lobes 

 (labellae) at its tip. The beak is more or less retractile. The 

 tarsi are 5-jointed. 



This group includes many of the worst pests of man and 

 beast and many parasitic insects. 



The group is one in which there are remarkable differences 

 in life history and habits. Let us illustrate the more special- 



* dis, two and pteron, wing. 



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