DIGESTION 51 



chamber') of these larvae, and form an important, perhaps an essen- 

 tial, aid to digestion - being themselves digested later by the pro- 

 teolytic enzymes of their host. The same occurs in some Tipulid 

 larvae. But there are related beetles (Dorcus) which have a similar 

 fauna and yet are said not to digest cellulose. Other insects that feed 

 on wood harbour yeast-like organisms, often within the cells of the 

 gut; and it was natural to attribute to these, also, the function of 

 digesting cellulose. But, in fact, some beetles that have such 'sym- 

 bionts' are unable to digest cellulose, and others which are without 

 them can do so. Many blood-sucking insects, also, possess intra- 

 cellular bacteria or yeasts in their tissues; and the occurrence of these 

 in close association with the gut, in the case of the tsetse-fly (Glossina) 

 and the Pupipara, led to the suggestion that they were concerned in 

 the digestion of blood. On the face of it this was not a probable 

 hypothesis, for the blood proteins do not differ specially from those 

 of other tissues ; and it has been shown (at least in the tsetse-fly) to be 

 incorrect; for no digestion of the blood takes place in that anterior 

 part of the mid-gut in which the symbionts occur. The function of 

 these symbiotic organisms is to be sought elsewhere (p. 70). 



Absorption 



In the cockroach, in which the food is digested largely in the crop, a 

 small amount of absorption, notably of fats, may also occur in that 

 segment of the fore-gut ; but the greater part of absorption undoubt- 

 edly takes place in the mid-gut. The cells seem never to take up solid 

 particles; the dissolved foodstuffs, like the digestive enzymes, diffuse 

 through the peritrophic membrane, when such is present, and are 

 absorbed by the epithelial cells. Absorption in the mid-gut of the 

 cockroach or the locust seems to be largely a matter of 'facilitated 

 diffusion'; for example, as soon as glucose or other sugars enter the 

 circulating blood they are converted into trehalose. Thus a steep 

 gradient of concentration of the sugar in question as between the gut 

 contents and the haemolymph is maintained and thus facilitates up- 

 take by diffusion. 



In the blood-sucking insects, such as the tsetse-fly or the mosquito, 

 almost nothing is passed on to the hind-gut, save a little haematin. 

 In these forms the hind-gut clearly plays no part in absorbing 



