184 



HOMOLOGY 



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

 mouth parts are adapted for different modes of nutrition. For 

 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 



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

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



different insects in which the appendages are adapted for 

 different functions (Fig. 82). 



The thorax of the cockroach consists of three fused somites 

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

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

 are the most characteristic of the external appendages of insects, 



