30 THE BIOLOGY OF INSECTS 



they are replaced by basal cells which grow towards the 

 surface and regenerate the lining layer. In the stomach 

 cavity of bees a large number of small spherical clear, cell- 

 like bodies can be distinguished, as Snodgrass (1910) and 

 other observers have described ; these have a similar origin 

 and function. The peritrophic membrane is beheved to 

 be formed in many insects by the envelopes of such cells, 

 thrown off by the epithelium ; but in other cases it appears 

 to be due to a secretion (Fig. 11,^) formed in successive 

 layers by the cells of the stomach, as described by R. E. 

 Snodgrass (1925) for the Honey Bee. 



Brought thus into close touch with the central food- 

 mass, the digestive juice mixes with the nutrient substances 

 and the proteoclastic ferment acts on the proteins, breaking 

 them up into their constituent amino-acids. In many 

 plant-eating insects it has been shown that diastatic ferments 

 in the juice act on the starch of the food converting it finally 

 into a sugar (monosaccharide) with a relatively small 

 molecule. In the stomach also it appears that fatty con- 

 stituents of the food of many insects are emulsified, at least 

 in part, and hydrolysed by the action of special (lipoclastic) 

 enzymes, giving rise to glycerol and fatt}^ acids. Thus the 

 various nutrient substances are reduced as regards the 

 complexity of their chemical composition and prepared for 

 absorption in the soluble state by the living cells that build 

 up the insect's organs and tissues. This, as has already 

 been emphasised (p. 16), must be regarded as the true 

 feeding process. Absorption of the digested food-substances 

 is carried on through the wall of the stomach. H. Jordan 

 (19 1 3) proved that various insects, with whose food iron- 

 compounds had been mixed, showed the presence of iron 

 in certain cells of the stomach epithelium. The com- 

 parative thinness of the stomach-wall would suggest that 

 there absorption must be actively carried on. But there 

 seems no doubt that in the terminal region of the food- 

 canal — the hind-gut as it is called — absorption also takes 

 place. 



The hind-gut or intestine of an insect has, like the fore- 



