NUTRITION AND METABOLISM 71 



have no intracellular symbionts; but they regularly have the gut 

 contents heavily infected with an Actinomyces which provides a 

 source of B vitamins. When reared on blood in the absence of this 

 micro-organism they cannot grow or reproduce. 



It is clear, therefore, that when insects feed upon a sterile or rest 

 tricted diet, the supply of vitamins may become a problem of some 

 urgency. To provide an endogenous source of vitamins may well be 

 one of the functions of symbiotic micro-organisms in other groups 

 besides those feeding on blood. This has been proved experimentally 

 to be the case in such beetles as Lasioderma and Sitodrepa, which do 

 not normally need vitamins of the B group in their food. If these 

 beetles are deprived of their symbionts (by sterilizing the surface of 

 the egg), their vitamin requirements become the same as those of 

 Tribolium or Ptinus, which always lack symbionts. 



It is possible that the association of sterile food (as in the majority 

 of plant-sucking Hemiptera) with the presence of symbionts may 

 again be traced to this need or to the need for other syntheses. The 

 intracellular symbionts in the fat body of cockroaches, Periplaneta, 

 and Blattella, can be eliminated by feeding the parent insects with 

 aureomycin for a long period, or by exposure to a high temperature. 

 Such insects are unable to grow on diets that are adequate for nor- 

 mal insects. These micro-organisms seem to be particularly impor- 

 tant in amino acid and ascorbic acid synthesis. It is not always easy 

 to see why related forms (Cerambycid larvae, for example), with and 

 without symbionts, should grow with equal rapidity on the same 

 diet. But the insect and its symbionts show such wonderful mutual 

 adaptations, that it is equally difficult to dismiss the whole phenome- 

 non as one of harmless parasitism. 



Water metabolism 



Another fundamental requirement in insect nutrition, and a very 

 important one, is water. Even such an insect as the weevil Calandra, 

 which is accustomed to feed in dry materials and is highly adapted 

 to conserve the water in its body, cannot survive unless its food con- 

 tains some 10 per cent, of water. In other words, the water produced 

 from the oxidation of foodstuffs, although of great importance to 

 the insect, is not, alone, sufficient for its needs. If the mealworm is 



