154 COMPARATIVE ANATOMY 



prothorax, mesothorax, and metathorax. They are roofed 

 over dorsally by as many slightly arched chitinous plates or 

 terga, generally known as the pronotum, mesonotum, and 

 metanotum. Of these the pronotum is much the largest, and 

 projects forward so much that it not only conceals the neck, 

 but the head can also be withdrawn under it. Ventrally each 

 of the three thoracic segments is provided with a small but 

 distinct median skeletal piece, the sternum, and laterally there 

 are some smaller hard pieces, some of which may be equivalent 

 to the epimera of Crustacea, but others more probably belong 

 to the bases of the limbs. The pro-, meso-, and metathorax bear 

 each a pair of walking legs consisting of a stout basal joint, the 

 coxa, a very small articular joint, the trochanter, an elongated 

 and fairly stout joint, the femur, a more slender tibia, and a 

 tarsus composed of five short terminal joints terminated by a 

 very small movable piece bearing a pair of hooked claws. 

 The mesothorax and metathorax are further distinguished by 

 the possession of wings movably articulated to the anterior 

 corners of their terga. The wings are membranous expansions 

 of the integument, strengthened by a network of chitinous 

 ridges or nervures. The anterior or mesothoracic wings are 

 dark-coloured and horny ; they are not used in flight, and 

 when at rest are laid flat back over the metathorax and 

 abdomen, the left wing overlapping the right, so that together 

 they form a protective cover to the more delicate posterior 

 wings, and hence are generally known as the wing covers or 

 tegmina. The metathoracic wings are thin and membranous, 

 and when at rest are folded up longitudinally and laid back 

 over the abdomen covered by the tegmina. In Periplaneta 

 americana both sexes have wings extending back beyond the 

 posterior end of the body. In the male of P. orientalis the 

 wing covers only extend as far back as the posterior edge of 

 the fifth abdominal tergum, and the female is nearly wingless, 

 the tegmina being represented by a pair of oval scales articu- 

 lated to the mesonotum, and the metathoracic wings by lateral 

 extensions of the metanotum, marked with a few nervures 

 (fig. 38). The young forms of both sexes in both species of 

 Periplaneta are devoid of wings, but the lateral margins of the 

 meso- and metanotum are expanded and produced backwards. 

 It is clear from the study of the young forms and the rudi- 

 mentary wings of the female P. orientalis that the wings are 



