120 
becomes gradually larger (colon). At the termination 
of the colon there is a slight constriction, after which 
the canal dilates again to form the rectum (Fig. 38). 
Into the rectum project six solid growths, the 
so-called rectal glands, which are, however, papillae. 
Posteriorly the rectum ends in the anus close above 
the genital canal. 
The appendages of the alimentary canal are :— 
The Salivary Glands .—The salivary glands consist 
of six tubular acini lying three upon either side. A 
duct can be seen traversing almost the entire length of 
each acinus. Shortly after leaving the acinus, the 
three unite to form a single duct. Beneath, and in 
contact with the lower surface of the suboesophageal 
ganglion, the ducts of each side unite to form a com¬ 
mon salivary duct which passes forwards and enters the 
chitinous first portion of the alimentary canal close to 
the base of the proboscis (Fig. 42). 
The Malpighian Tubules .—These are five in 
number and open into the first portion of the hind-gut 
immediately beyond the pylorus. Their blind ends 
are held in position in the neighbourhood of the rectum 
by tracheal branches. They pass forwards in loops 
above their origin, so that, in transverse section, as 
many as ten may be seen cut across. 
The Vascular System .—As in most insects where 
the respiratory system ramifies throughout the whole 
body, the vascular system is not well developed. A 
dorsal vessel or heart and an anterior prolongation of 
this (aorta) are the only closed blood-vessels. Apart 
from the dorsal vessel the blood circulates in large 
blood spaces, which lie between the lobes of the fat- 
body and among the muscles and viscera. 
The dorsal vessel passes close beneath the tergal 
plates throughout the abdomen. It is very thin walled, 
