THE INSECTA 



159 



longitudinal folds in the ileum, is thin and smooth in the colon, 

 thicker and finely setose in the rectum. The internal lining 

 of the rectum is thrown into six prominent longitudinal ridges 

 corresponding with the infoldings visible on the external 

 surface. 



The commencement of the hind-gut is marked by the 

 attachment of six bundles of very fine and long Malpighian 



stw. 



A. 



Fig. 39 



A . The principal ventral tracheal vessels of the cockroach (from Miall, slightly 

 altered), st.i, first stigma between the prothorax and mesothorax ; st.3, 

 third stigma of the left side ; st./o, tenth stigma of the left side. B. A dis- 

 section of a female cockroach from the dorsal side, cp, crop ; g, gizzard or 

 proventriculus ; he, hepatic caeca ; z", intestine ; ?/, Malpighian tubules ; cl, 

 colon ; r, rectum ; sg; salivary glands ; sr, salivary receptacle ; ov, ovaries. 

 C. The central nervous system of the cockroach, sp-g, supraoesophageal 

 (cerebral) ganglion ; sb.g, suboesophageal ganglion ; thi, th.2, tkj, the three 

 thoracic ganglia ; abi-ab6, the six abdominal ganglia. 



tubules. Each bundle comprises ten or more thread-like 

 tubes, so that as many as sixty to seventy are present in the 

 adult cockroach. The tubules are lined by a cubical glandular 

 epithelium, and their lumina are often blocked by crystals 

 containing uric acid. The tubules are, in fact, the functional 

 excretory organs of the cockroach, and it is obvious that they 

 differ very much in situation and structure from the green 

 glands or shell-glands of the Crustacea. In development the 



