No. 57G] VARIATION IN DROSOPHILA 749 



to the effect of moisture, but corresponds to the results 

 obtained in the study of moisture effects. 



D. The Effects of Acidity and Alkalinity of the Food 



Normally the reaction of food at the time of putting it 

 in the bottles is acid, the degree of acidity depending upon 

 the length of time it has been fermenting. This sourness 

 usually passes gradually away as the larvae grow older, 

 and by the time a brood begins to hatch the reaction is 

 frequently quite alkaline, unless fresh food has been put 

 recently into the bottle. 



On the other hand, if the acidity of the food is neutral- 

 ized at the beginning with sodium hydrate or carbonate, 

 or if the reaction is made alkaline while yet there remains 

 a good deal of unfermented banana, the acidity will re- 

 turn for a time if not carefully guarded against. There- 

 fore to keep the reaction acid or alkaline is a difficult 

 matter, and requires occasional stirring of the food to 

 make the reaction uniform; this operation is likely to 

 prove disastrous for the developing pupa?. 



In the tests here recorded I used food that had been 

 fermenting for one month, so that it had a very acid 

 reaction that lasted till hatching time. For studies of the 

 effect of alkalinity I used food that had been fermenting 

 about one day and mixed with it sodium carbonate, 

 sodium hydrate or ammonia. The results were unsatis- 

 factory and the reaction did not remain constant in spite 

 of my efforts, though on the whole it remained alkaline, 

 and became strongly alkaline, and also slimy towards the 

 end of the experiment, and not a great many flies hatched. 



I also used food that had not been allowed to ferment 

 at all, and although I do not know its reaction, it was 

 certainly not so alkaline as the last mentioned, nor so 

 acid as the first. It was soon attacked by mold (Bread 

 mold). I refer to it here as fresh food. 



The results are given in Table XXXI, but may be more 

 briefly summarized here. 



