CHAPTER 6 



Nutrition and Metabolism 



Since the alimentary canal of insects contains digestive enzymes 

 competent to hydrolyse those constituents commonly present in 

 their food (p. 48), it is probable that this food is absorbed in the 

 form of simple products of hydrolysis : carbohydrates as monosac- 

 charides, fats as glycerol and fatty acids, protein as amino acids. But 

 at present there is no proof of this, and it is possible that there 

 may be some absorption of more complex bodies. In certain blood- 

 sucking insects, such as Rhodnius, although the greater part of 

 the haemoglobin is broken down to haematin in the lumen of the 

 gut, and the haematin residue eliminated unchanged, a small 

 amount of undigested haemoglobin is absorbed into the blood 

 and is subsequently broken down to bile pigments and other 

 products (p. 55). 



The simple products of digestion provide the raw material for 

 growth and energy production. The sites of the intermediary stages 

 of metabolism and synthesis are not altogether known. It is certain 

 that the fat body plays a major part in the synthesis of trehalose, 

 the formation and storage of glycogen and fat, the deamination and 

 transamination of amino acids, the synthesis of protein and the 

 production of uric acid and other waste products (p. 36). But it is 

 unlikely that the fat body cells have a monopoly of these activities. 

 The epithelium of the gut wall can be the site of massive storage of 

 fats, glycogen, and protein. The haemocytes (p. 35) and pericardial 

 cells (p. 36) may well play a role in intermediary metabolism. And 

 such tissues as the epidermis and the growing muscles doubtless take 

 up the raw materials they require from the haemolymph and accom- 

 plish a large part of the syntheses needed for their own activities; 

 as already pointed out, they may well play a part also in deamination 

 and the production of nitrogenous wastes (p. 57). Certainly the 



