INSECTS AND THEIR NEAR RELATIVES 9 



animals now described. The adult insects are readily recognized 

 from the other classes of arthropods, but many of the immature 

 forms, such as maggots, lack the typical characteristics of the 

 group. The segments of the body of an insect are grouped into 

 three distinct regions, the head, the thorax, and the abdomen. 

 The head bears a single pair of feelers, or antennae, the mouth-parts, 

 and the compound eyes. The thorax bears three pairs of jointed 

 legs and in the adult stage usually two pairs of wings, though in 

 the flies there is but a single pair and in a few orders wings are 

 lacking. The abdomen is usually without appendages in the adult 

 state, although on caterpillars and other immature stages prolegs, 

 or false legs, which are not segmented, are often found. 



COMPARATIVE STRUCTURE OF THE CLASSES OF ARTHROPODA 



