NUTRITION AND METABOLISM 69 



epidermis is concerned in the many complex synthetic activities 

 necessary for cuticle formation. 



Food requirements 



We have seen how exceedingly varied are the apparent food mat- 

 erials of insects (p. 39). But it must be remembered that insects are 

 not always feeding upon what they eat : they may devour rotten wood 

 but feed only upon the fungi which it harbours; or eat large quanti- 

 ties of cellulose and yet assimilate only the associated substances. 

 The principles of insect nutrition are therefore not easy to state. 



For growth and reproduction insects must have an adequate 

 supply of carbon and nitrogen, phosphorus and sulphur, inorganic 

 salts, and all the other elements that go to make up the living sys- 

 tem. The simplest substances which will satisfy these needs may vary 

 to some extent from one insect species to another; but in general the 

 requirements are not very different from those of vertebrates. Dro- 

 sophila has been reared with ammonium salts, and cockroaches with 

 glycine as the sole sources of nitrogen; but these insects have in the 

 intestine a rich flora of yeasts and bacteria which were doubtless 

 responsible for essential syntheses. 



When reared under axenic conditions in the absence of micro- 

 organisms, insects are found almost always to require the same series 

 of ten 'indispensable' amino acids as do vertebrates: leucine, iso- 

 leucine, histidine, arginine, lysine, tryptophane, threonine, phenyl- 

 alanine, methionine, and valine. Nucleic acids they can synthesize. 

 The range of hexose monosaccharides- which they can metabolise 

 varies somewhat in different species and the utilization of disaccha- 

 rides and polysaccharides may be limited by the digestive enzymes 

 present (p. 47). Some insects, such as the mealmoth Ephestia, must 

 be provided with a supply of the more highly unsaturated fatty acids 

 such as linoleic and linolenic acids, but for most species this is not 

 necessary. On the other hand, most insects are unable to synthesize 

 sterols and must have cholesterol or sitosterol in the diet. 



Vitamin requirements 



A heterogeneous series of substances which the animal organism is 

 unable to synthesize and which it requires in varying amounts are, 



