188 



HOMOLOGY 



biting and chewing in orthoptera (grasshoppers, cockroach, etc.), 

 coleoptera (beetles), hemiptera (bugs), and hymenoptera (ants, 

 bees and wasps) ; for sucking or licking, diptera (flies, mosqui- 

 toes, etc.), lepidoptera (butterflies, moths), and neuroptera 

 (dragon flies, etc.) . Just as we may trace homologies of the crus- 

 tacean appendages, so we may trace the homologous parts of 

 different insects in which the appendages are adapted for 

 different functions (Fig. 82). 



FIG. 82. Homologous mouth parts of cockroach (left), bee (center) and mos- 

 quito (right). (Combination of figures from Hertwig.) 



The thorax of the cockroach consists of three fused somites, 

 termed the pre-, meso-, and meta-thorax, and, as in the lob- 

 ster, it bears the most important organs. Each somite carries 

 one pair of legs, and these three pairs of legs are so constant in 

 the insects that the phylum is sometimes called the Hexapoda. 

 These legs are adapted for many different activities. 



The thorax also carries the wings. These are thin bags of 

 cuticle drawn out from the dorsal angles of the meso- and meta- 

 thorax, which become expanded and stiffened in the air, and 

 are the most characteristic of the external appendages of insects, 

 distinguishing them from all other animal forms. The wings, 

 like other appendages, are also subject to wide variations in 

 form and function. 



