226 



COCKROACHES. PHYLUM ARTHROPODA 



EXTERNAL FEATURES 



In its main lines the anatomy of a cockroach resembles that 

 of a crayfish. The animal is segmented, the segments (somites) 

 being nnlike and grouped into three regions known as head, 

 thorax, and abdomen, but these do not correspond with the 

 parts similarly named in the crayfish. There is a thick cuticle, 

 not moulted by the adult, and some somites bear jointed limbs. 

 The thorax bears also two pairs of wings. At the sides of the head 



Fig. 162. — Insect cuticle. — From Wigglesworth, Biol. Rev., 1948. 23, 408. 



A , ideal section of the integument ; B, schematic section of the epicuticle. 



a, endocuticle ; b, exocuticle ; c, epicuticle ; d, bristle ; e, pore canals ; /, duct of dermal gland ; g, basement 



membrane ; h, epidermal (hypodermal) cell ; i, k, and I, special cells of epidermis ; m, blood cell ; 



n, dermal gland ; o, cement layer ; p, wax layer ; q, polyphenol layer ; r, cuticuUn layer ; 5, pore canal. 



lie a pair of large, unstalked, compound eyes. The coelom, of 

 which traces are found in development, disappears in the adult, 

 but there is a hsemocoelic perivisceral cavity containing blood. 



The insect cuticle is much more than a mere covering of 

 chitin. There are three layers (Fig. 162). The innermost and 

 thickest is the endocuticle, which is made of chitin and a protein 

 called arthropodin, intimately associated and perhaps chemically 

 combined. Outside this is the exocuticle ; it has basically the same 

 structure as the endocuticle, but the protein has been tanned, 

 a process which involves both oxidation and the introduction 

 of aromatic groups to the protein molecule. The result is a 



