INSECTS 



183 



Superficially the insects are so similar to Crustacea that form- 

 erly they were all classed together in the common phylum 

 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. 



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



Like Crustacea the insect body is composed of somites which 

 are regionally fused to form more or less independent head, 

 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 latter 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 antennae, 

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

 these latter are united to form a labrum overhanging the mouth 

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



