390 THE INVERTEBRATA 



trophic membrane, which is, however, secreted by special cells of the 

 proventricular region (which may be ectodermal). Its function and 

 place in digestion is not understood. The hind gut begins where the 

 Malpighian tubules enter the alimentary canal and is usually divided 

 into a small intestme or ileum, a large intestine or colon, in both of 

 which the chitinous lining is sometimes folded and produced into 

 spines, and a short globular rectum. 



Though the digestive enzymes of insects belong to the same classes 

 as those of mammalia there are many significant differences. The 

 rapid growth of caterpillars is probably due to the possession of 

 enzymes better adapted for the penetration of cellulose cell walls 

 and digestion of vegetable protoplasm than mammals possess. Many 

 insects, particularly sucking and wood-boring forms, possess ali- 

 mentary diverticula in which bacteria or yeasts are housed, either 

 free in the lumen or inside the cells ; it is supposed that the vegetable 

 enzymes can be utilized in some way for the special needs of the 

 animal. The saliva of various insects shows great variety according to 

 their habits ; thus the larva of the tiger-beetle {Cicindela) ,the flesh-eating 

 larvae of flies, e.g. Sarcophaga, and the aquatic larva of Corethra, pour 

 their saliva, which contains a proteolytic enzyme, on their food and suck 

 up the products of digestion (external digestion). Bees, with their re- 

 liance on pollen and honey as food, have four different kinds of salivary 

 glands. These probably serve different purposes such as to invert 

 sugars, to ensure preservation of food by adding formic acid, and 

 to predigest pollen in the manufacture of ''bee bread" on which the 

 young are fed. In wood-boring larvae the secretion of a mandi- 

 bular gland softens the wood and thus assists mastication, while, in 

 caterpillars, silk production is the main function of labial glands. 



The principal excretory organs are the Malpighian tubules, opening 

 into the anterior end of the hind gut, and therefore are just as much 

 ectodermal structures as the nephridia of annelids. The proof of their 

 function is the presence of crystals, which can be identified micro- 

 chemically as uric acid, inside the cells and in the lumen of the tubule. 

 A mass, mainly of uric acid, is found in the hind gut of pupating insects, 

 having been deposited there by the tubules. But in addition nitro- 

 genous end products are found in the nephrocytes (cells found 

 commonly associated with the fat body and the pericardium), the 

 fat body and the hypodermis in quantities which increase with age, 

 so that it appears that the mechanism of the Malpighian tubules for 

 ridding the body of the insect of nitrogenous excreta is by no means 

 efficient. 



The circulatory system. There is, firstly , a heart, primitively consisting 

 of thirteen chambers, each corresponding to a segment, with a pair of 

 ostia at the base of each chamber. The blood is driven forward in 



