THE FLIES, OR HEXAPOD INSECTS. 1 85 



classes of animals, and next to that of Mammalia, also the 

 most important. Although Flies develop a greater variety of 

 genera and species than all other abimals taken together, 

 yet these are all in reality only superficial variations of a 

 single type, which is entirely and constantly preserved in 

 its essential characteristics. In all Flies the three divisions 

 of the trunk — head, breast (thorax), and hinder body — are 

 quite distinct. The hinder body, or abdomen, as in the case 

 of spiders, has no articulated appendages. The central divi- 

 sion, the breast or thorax, has on its ventral side three pairs 

 of legs, on its back two pairs of wings. It is true that, in 

 very many Fhes, one or both pairs of wings have become 

 reduced in size or have even entirely disappeared ; but 

 the comparative anatomy of Flies distinctly shows that 

 this deficiency has arisen only gradually by the degenera- 

 tion of the wings, and that all the Flies existing at present 

 are derived from a common, primary Fly, which possessed 

 three pairs of legs and two pairs of wings. (Compare p. 256.) 

 These wings, which so strikingly distinguish Flies from all 

 other Arthropoda, probably arose, as has been already shown, 

 out of the tracheate gills which may still be observed in the 

 larvae of the ephemeral files (Ephemera) which live in water. 

 The head of Flies universally possesses, besides the eyes, 

 a pair of articulated feelers, or antennae, and also three 

 jaws upon each side of the mouth. These three pairs 

 of jaws, although they have arisen in all Flies from 

 the same original basis, by difierent kinds of adaptation, 

 have become changed to very varied and remarkable 

 forms in the various orders, and are therefore employed 

 for distinguishing and characterizing the main divisions 



