INSECTS 



187 



Arthropoda. Some of the insects, however, have quite as close 

 an affinity to the annelid worms, one genus, Peripatus, having 

 many annelid characteristics. Biologists, therefore, agree in 

 making Crustacea and insects independent phyla, with common 

 ancestors in annelid-like forms. 



Like Crustacea, the insect body is composed of somites which 

 are regionally fused to form more or less independent head, 



FIG. 81. A cockroach, from the ventral surface. 



thorax and abdomen. The head consists of five somites, the 

 thorax of three, and the abdomen of eleven or less, the number of 

 somites being highly variable in the abdomen, but fixed in head 

 and thorax. The head always bears compound eyes and, very 

 often, simple eyes in addition. It also carries one pair of anten- 

 nae, and two pairs of pre-maxillae (Fig. 81). In the cockroach, 

 the latter are united to form a labrum overhanging the mouth 

 (Fig. 82). The head also bears one pair of mandibles or jaws, 

 and two pairs of maxillae. In different orders of insects these 

 mouth parts are adapted for different modes of nutrition. For 



