VII] INTERNAL ANATOMY 59 



any food absorption takes place. Commencing at the oeso- 

 phageal valve it consists of a straight tube expanded posteriorly, 

 extending to about the posterior limit of the sixth segment. 

 The expanded posterior region is commonly termed the " sto- 

 mach " and this term may conveniently be retained ; it is in 

 this hinder region of the mid-gut that the malarial parasite 

 develops. The wall of the mid-gut is surrounded by both 

 longitudinal and circular muscle layers and the latter throw 

 the surface into a series of folds so that this region of the 

 alimentary canal has an annulated appearance. The mid-gut 

 is extremely well supplied with tracheae, which branch in such 

 a manner that the whole surface is covered with fine air- 

 capillaries. The interior is lined by a layer of large cubical 

 cells, some of which are secretory in function. 



The ileum commences at the junction of the Malpighian 

 tubules with the mid-gut ; it is a thin-walled tube lined with 

 chitin raised up in the form of ridges. It passes insensibly 

 into the colon, which is merely a straight tube opening suddenly 

 into the large oval rectum. The latter contains six large ovoid 

 papillae which considerably diminish the size of the chamber. 

 These rectal papillae are abundantly supplied with tracheae 

 and their function is possibly respiratory. 



The rectum narrows just before the anus, which opens in 

 the last segment of the body, immediately beneath the genital 

 aperture. It is guarded by two short lateral papillae. 



The Malpighian tubules are five in number, opening into 

 the alimentary canal at the junction of the mid- and hind-gut. 

 The wall of each tubule is composed of large secretory cells 

 abundantly supplied with tracheae. These cells are excretory 

 in function, for the Malpighian tubules often contain uric acid 

 and other waste nitrogenous products. 



The Malpighian tubules are frequently the haunt of pro- 

 tozoal parasites, the lumen of the tubes being a common place 

 in which to find insect flagellates. 



The salivary apparatus ; commencing with the opening at 

 the tip of the hypopharynx, it consists of the following parts : 

 a median groove, or " salivary gutter," extending from the tip 

 to the base of the hypopharynx. This groove is arched over 



