30G Annals Entomological Society of America [Vol. XI, 



This fluid secreted doubtless serves as an aid in molting, as 

 was suggested under the discussion of the oesophagus, for the 

 intima of the whole hind-intestine, like that of the fore-intestine, 

 is shed at each molt. This fluid is developed along the whole 

 course of the fore-intestine, but is present only in the last 

 division of the hind-intestine. This may be due to the fact that 

 the rest of the hind-intestine is the principal seat of absorption ; 

 the intima is much thinner, and probably slips off more easily. 



A cross-section through the rectum is shown in figure 28; 

 the rectal epithelium after a molt, in figure 29; the rectal 

 epithelium with the fibrillar area developing, in figure 30; 

 the rectal epithelium after the formation of the pupal intima, 

 in figure 31; and the rectal invagination, in figure 32. 



Basement membrane. The basement membrane, which is 

 clear and well developed throughout this region, is continuous 

 with that of the body wall. 



Circular muscles. There are three distinct layers of very 

 strong circular muscles in the rectum, each layer forming a 

 complete ring around the alimentary canal. They have their 

 origin on the posterior face of the apodeme-like projection from 

 the intima at the anterior end of the rectum, and are inserted on 

 the cuticula around the proctodeal invagination. Each layer 

 originates and is inserted independently. 



Longitudinal muscles. Longitudinal muscles are entirely 

 lacking in the rectum. 



3. THE MALPIGHIAN VESSELS. 

 The General Course of the Vessels. 



From a morphological point of view, the Malpighian vessels 

 ■of an insect are ectodermal structures which arise during embry- 

 onic life as evaginations of the distal end of the hind-intestine. In 

 the larva of the alder flea-beetle, they are six in number, con- 

 stituting two series of four vessels and two vessels respectively. 

 The details of their course in the body cavity has already been 

 pointed out (Woods, 1916), and their relation to the colon is 

 discussed elsewhere in this paper (pages 298-302). It should be 

 clearly understood that they do not open into the colon, and 

 that their only connection with the lumen of the alimentary 

 canal is at the point of their evagination. 



