THE FLIES, OR HEXAPOD INSECTS, 185 
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 animals 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 ccntral 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 Flies, 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 
larvee of the ephemeral flies (Ephemera) which live in water. 
The head of Flies universally possesses, besides the eyes, 
a pair of articulated feelers, or antenne, 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 different 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 
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