A NUTRITIONAL STUDY OF INSECTS 13 



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was inoculated with living yeast. This caused a rapid increase 

 in size and ended in pupation six days later. 



The acceleration which takes place on infecting a sterile me- 

 dium with living yeasts indicates that the alcoholic treatment 

 in sterilizing the pupae does not cause the decrease in the rate of 

 growth of the sterile larvae. Other cases of acceleration which 

 occurred due to accidental contamination of a sterile medium 

 quite often bore out this conclusion. 



Therefore, it is certain that sterile larvae grow more slowly 

 than non-sterile larvae on sterile food, and that the rate of 

 growth can be increased by infecting the medium with the living 

 yeast. 



b. Is fruit the food for Drosophila larvae or merely the sub- 

 stratum for yeast cells? As sterile larvae grow so slowly and 

 do not pupate in sterile fruit, but develop normally if it is infected 

 with living yeasts, the question arises as to the true position of 

 the fruit in the ecology the insect. By using a medium con- 

 taining the inorganic salts and the sugars and ammonium tar- 

 trate necessary for yeast growth, the starch, oils, fats, proteins, 

 and other substances of the fruit were eliminated from the 

 experiment. 



The composition of the medium was as follows: 



Agar-agar 4.0 grams K2HPO4 0.165 grams 



Grape-sugar 16.5 grams MgS04 0. 165 grams 



Cane-sugar 16.5 grams H2O 200 cc. 



Ammonium tartrate 3.3 grams 



Sterile larvae lived only five days on this sterile medium and 

 showed no increase in size; but if the medium was infected with 

 living yeasts, the larvae grew at a normal rate, reaching their 

 maximum size in ten days, and pupated normally. The adults 

 which emerged from these pupae were sexually fertile and of 

 large size. Thus, in the presence of living yeast, Drosophila 

 larvae grow normally in a synthetic nutrient medium for yeast 

 with ammonium tartrate as the only supply of nitrogen. There- 

 fore the simplest nutrient medium for yeast if infected with liv- 

 ing yeasts is equavalent to fermenting fruit in the ecology of Dro- 

 sophila larvae. 



