INSECTS 



showing how a primitive ancestral mechanism may be 

 entirely remodeled to serve in a new capacity. If the flies 

 had been specially "created," and not evolved, their 

 structure could have been much more directly fitted to 

 their needs. 



It is not only in the matter of wings and the method of 

 flight that the flies show they are highly evolved insects; 



B 



Fig. 169. The black horsefly, Tabanus atratus 



A, the entire fly. B, facial view of the head and mouth parts. Ant, antenna; 



E, E, compound eyes; Li, labium; Lm, labrum; Md, mandible; Mx, maxilla; 



MxPlp, maxillary palpus 



they are equally specialized in the structure of their mouth 

 parts and in their manner of feeding. The flies subsist on 

 liquid food. Those species that can satisfy their wants 

 from liquids freely accessible have the mouth parts formed 

 for sucking only. Unfortunately, however, as we all too 

 well know, there are many species that demand, and usu- 

 ally obtain, the fresh blood of mammals, including that 

 of man, and such species have most efficient organs for 

 piercing the skin of their victims. 



The most familiar examples of flies that "bite" are the 

 mosquitoes and horseflies. The horseflies (Fig. 169 A), 

 some of which are called also gadflies and deer flies, belong 

 to the family Tabanidae. An examination of the head of 



320] 



