4i2 THE ALIMENTARY CANAL OF THE IMAGO. 



In transverse sections the cavity of this part of the intestine 

 is diminished by the projection of four thick longitudinal folds 

 of the intestinal wall into its interior (PI. XXIV., Figs. 4 and 5). 



The Proctodeum or rectum, as has been already stated, consists 

 of three parts. The first part is dilated into a small sac lined 

 with cubical cells, covered by a thin cuticular layer near the 

 valve, but usually naked below. It is possible that the remains 

 of the food are subjected to trituration as they pass through 

 the distal portion of the distal intestine and the rectal valve, 

 and that the residuum of nutritive matter is absorbed in this 

 part of the rectum. 



The first part of the rectum is separated from the second 

 part by a well-marked sphincter muscle. 



The second part of the rectum is a large pouch the rectal 

 pouch. It is lined by a single layer of flattened epithelium, 

 and has a distinct cuticular intima. The rectal papilla project 

 into its cavity, which is usually occupied by the thick semi- 

 fluid excrement which is not unlike the urine of birds and 

 reptiles. 



The third portion of the rectum, the anal rectum, is tubular, 

 narrowing towards the anus. It is lined by a thick cuticular 

 intima continuous with the cuticular integument supported by 

 pavement epithelium. The muscular coat is very thick. The 

 lumen of this part of the intestine is reduced to a mere fissure 

 except during the passage of excrement. 



b. The Malpighian Tubes. 



The Malpighian Tubes are four in number, two on each side ; 

 they exhibit a moniliform appearance, and are usually of a 

 bright yellowish-brown colour. The two tubes on either side 

 unite at an acute angle and form a wide duct about 2 mm. in 

 length, which opens into the lower extremity of the proximal 

 intestine. 



The length of each of the Malpighian tubes is probably an 

 inch or more, but they form such complex coils that I have 

 found it impossible to unravel them. They lie on either side 

 of and around the coils of the proximal intestine. 



