70 INSECT PHYSIOLOGY 



for historical reasons, commonly termed 'vitamins'. So far as the 

 water-soluble or B group of vitamins is concerned, the requirements 

 of insects are strikingly similar to those of mammals. Although 

 there are some specific variations, thiamin (B 1 ), riboflavin (B 2 ), nico- 

 tinic acid, pyridoxin (B 6 ) and pantothenic acid, are indispensable. 

 Choline and biotin are important; inositol and /?-amino -benzoic acid 

 often of no importance. Folic acid is often required. 



Fat-soluble accessory substances of the vitamin A type are not 

 required by insects. Vitamin C (ascorbic acid) which is present in 

 quantity in their tissues, they can readily synthesize themselves. Vita- 

 min D (calciferol) is not necessary, although as we have already seen 

 they must have supplies of cholesterol or some related sterol. 



Micro-organisms and symbionts as sources of vitamins 



Reliable information on the vitamin requirements of insects can be 

 obtained only if they are reared in the absence of micro-organisms. 

 Larvae of the blowfly Lucilia can be grown from sterile eggs on the 

 sterile brain of mammals; they will not grow on certain types of 

 sterile muscle. But if the muscle is infected with suitable micro- 

 organisms, or if a sterile extract of yeast is added to it, normal growth 

 is obtained. The bacteria and the yeast provide vitamins (belonging 

 to the B group of vitamins) that are present in brain but lacking in 

 muscle. It is interesting to note that sterile blood, also, is an insuffi- 

 cient diet for these larvae; it can be rendered adequate by the ad- 

 dition of vitamin B 1 . Now many insects (the bed-bug (Cimex), the 

 sucking lice, the tsetse-fly (Glossina), the Pupipara) take no food 

 throughout their life except sterile blood. But all these insects con- 

 tain within their bodies special groups of cells (mycetomes) stuffed 

 with supposedly symbiotic micro-organisms, which are absent from 

 such insects as mosquitoes, fleas, &c, that take blood during part 

 only of their life cycle. This naturally suggests that these symbionts 

 may constitute an endogenous source of vitamin, and thus enable 

 these insects to grow on sterile blood; indeed, it has been shown ex- 

 perimentally that if the louse Pediculus is deprived of its mycetome 

 and symbionts, growth and reproduction are greatly impaired, and 

 that this impairment can be made good by a single dose of pure B 

 vitamins. The blood-sucking Reduviidae Rhodnius, Triatoma, &c, 



