ASSIMILATION OF FOOD 123 



DIGESTION AND ABSORPTION 



Our knowledge regarding the nature and mechanism of the processes 

 by which food material ingested by the insect is converted into assimi- 

 lable substances and used up in nutrition is unfortunately very scanty. 

 Packard, writing fifteen years ago, devoted only six pages to the subject, 

 while Berlese's recent voluminous work contains little more. Neither 

 entomologists or parasitologists have given special attention to the case 

 of the digestion of blood, and even Schaudinn, who recognized the 

 importance of the matter, touched on it only incidentally, without giving 

 any account of the histological appearances. It must be remembered, 

 however, that the changing conditions met with in the alimentary tract 

 during digestion constitute the environment of ingested parasites, and 

 have therefore a special importance from the present point of view. 

 The following account is by no means as full as the importance of the 

 subject demands. 



The part of the alimentary tract which is specially concerned with 

 digestion and absorption is the mid-gut, or mesenteron, which is equipped 



for this purpose with a secreting epithelium, the cells 



r . . , , Function of 



ot which are usually columnar and have no cnitinous mesenteron 



intima. The secretion is passed out into the lumen of 

 the gut, where it mixes with the food, resolving it into soluble sub- 

 stances, which are then taken up by the cells and passed into the body 

 cavity. Phagocytosis of food particles does not take place. The fore- 

 gut, derived from the stomodaeum, serves as a channel to pass the food 

 from the mouth to the digestive chamber, and is modified in various 

 ways, as has already been pointed out, to regulate the flow, and in certain 

 cases to act as a food reservoir. In many mandibulate insects, such as 

 the cockroach, the part of the fore-gut immediately anterior to the 

 mid-gut is dilated to form a chamber termed the crop, into which open cer- 

 tain gland-like organs, the caeca. In this chamber a preliminary diges- 

 tion takes place, the inlet into the mid-gut being meanwhile shut off by a 

 strong sphincter muscle. In blood-sucking insects, however, digestion is 

 confined entirely to the mid-gut, and the crop serves at most as a 

 temporary reservoir for the food. Instead of appearing only as a simple 



dilatation of the oesophagus, it may be separated from 



,.,,.. Digestion in the 



it by a long duct, the receptacle itself being situated in crop 



the abdomen. As will be shown presently, the 



cells of the posterior part of the stomodaeum have also the function of 



providing a covering layer for the fluid food on its entry into the mid-gut. 



