I2O 



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, 



