21 



Bacteria as a source of vitamins and growth-accessory 

 substances . Portier (1919) va s one of the first to suggest 

 that the source of vitamins for the individual insect is 

 the intracellular organisms it possesses. "Wollman (1926) 

 probably overlooked this possibility when he claimed that 

 cockroaches ( Blattella germanica ) may dispense with vita- 

 mins and generalized that perhaps vitamins are not essen- 

 tial to insects. Though others (Frost, Herms, and Hoskins, 

 1936; Bowers and McCay, 19*10) have shown that mosquitoes, 

 cockroaches, and other insects can apparently do without 

 certain vitamins, it has "been definitely demonstrated that 

 by and large insects need the essential growth substances 

 as do higher animals. Some writers (imms, 1937) have 

 speculated that the chief functions of bacteria in insects 

 are to supply growth promoting substances and to liquefy 

 the food. 



Hobson (1933) found that the larvae of the blow fly, 

 Lucilia sericata, were unable to develop aseptically on 

 sterile blood owing to the lack of growth factors of the 

 vitamin B type. The presence of "bacteria improved growth, 

 and yeast autolysate allowed the larvae to grow at a 

 normal rate. Later (1935) > he reported that the natural 

 flora must supply the necessary vitamins and that larvae 

 grow readily on blood inoculated with pure cultures of 

 various bacilli isolated from the intestine and from blown 

 meat. Escherichia coli proved equally effective. in these 

 experiments. Observations of Wig'glesworth (1936) on 

 Rhodnius prolixus support the view that symbiotic organisms 

 in exclusively blood-sucking insects provide an endogenous 

 source of vitamins. 



At this point we may conclude that insect larvae can 

 be reared on sterile media if they are supplied with all 

 the necessary food factors. As stated by Wigglesworth 

 (1939), "If these are deficient, infection with micro- 

 organisms (in the case of Drosophila , particularly the 

 introduction of yeasts ) improves the rate of growth. 

 Sterile Lucilia larvae will grow on beef muscle; they 

 fail to grow on guinea pig muscle; but if this is infected 

 with Bacillus coli or if a yeast extract is added to it, 

 normal growth takes place. *-**In these cases there is 

 little doubt that the microorganisms are synthesizing the 

 necessary vitamins of the 'B' group." 



In connection with a discussion of growth accessory 

 substances might he mentioned the interesting discovery 

 by Tatum (1939) that certain bacteria synthesize a 



