428 THE INVERTEBRATA 



them, known collectively as the ligula. Where there are four as in the 

 Cockroach, the two median glossae are defined from the outer para- 

 glossae. Each of these lobe? is further divided into two in Machilis 

 (Fig. 301). Arising from the floor of the mouth is a small sclerite, the 

 hypopharynx, which bears the salivary aperture. A pair of sclerites, 

 superlinguae, are normally fused to the sides of the hypopharynx. 



Fig. 302 indicates the similarity between the insectan and crus- 

 tacean mouth parts. Such an attempt at a comparison is only possible 

 with the more generalized mouth appendages of the Insecta. 



With the evolution of different feeding habits, the structure of the 

 mouth parts, just described, has been departed from in a variety of 

 ways. Comparative and embryological study, however, clearly reveals 

 a uniformity of plan throughout, and the student must realize that the 

 modifications to be met with in bugs, butterflies, bees and flies are 



Fig. 302. To show the resemblance between the insectan maxilla and labium 

 and the biramous limb of the Crustacea. From Imms, after Hansen. A, Bi- 

 ramous crustacean appendage. B, Insectan maxilla. C, Maxillipeds of a 

 Gammarid crustacean. D, Insectan labium, end., end.i endites; en. endo- 

 podite; ep. epipodite; ex. exopodite; sni. submentum. 



all referable to the basal plan as exemplified in the mouth parts of 

 Blatta or Machilis. 



The thorax is separated from the head by a flexible neck region 

 usually containing cervical sclerites, which, however, have not any 

 segmental value. It consists of three segments — the prothorax, which 

 carries a pair of legs but no wings, the mesothorax and the metathorax, 

 which each bear a pair of legs and, typically, wings. The legs are made 

 up of five main segments, the coxa and trochanter (both of which are 

 small), the femur and tibia (which form the greater part of the limb), 

 and the tarsus (which is usually further subdivided into a number of 

 joints, and ends in a pair of claws with a bifid cushion between them 

 called the pulvillus). Of the many adaptations exhibited by the legs of 

 insects the jumping type found in grasshoppers, the digging type in the 

 mole-cricket Grylloialpa, the swimming type in the water beetles like 



