4 J. PERCY BAUMBERGER 



eggs, which are readily deposited by the female, are prominent 

 objects on the agar. 



Bacterial and fungous growths occur over the surface, but I no- 

 ticed that unless these become too luxuriant before the larvae 

 hatch, they are destroyed by the insect. 



The agar method has the advantage of permitting observation 

 of the date of egg deposition and hatch and the details of larval 

 habits. It also furnishes a method of making nutritional studies 

 of various synthetic media. 



b. Preliminary observations on the food of Drosophila. In May, 

 1916, while rearing Drosophila melanogaster on banana agar, I 

 noticed that molds and bacteria often completely covered the sur- 

 face of the medium and killed the larvae. This was confined to 

 cultures which had only ten or twenty instead of the usual fifty 

 or a hundred larvae. The larvae congregated at the points 

 where fungus was most abundant and caused the plants to dis- 

 appear, apparently by feeding upon them.- An examination 

 of the flora showed that Saccharomycetes were invariably present 

 and often occured in pure cultures.^ 



This observation suggested an internal symbiosis between Dro- 

 sophila and yeast. I found, nevertheless, that by washing the 

 surface of the pupae with alcohol, the insect could be freed from 

 all microorganisms. The larvae of such sterile insects were not 

 able to mature on banana agar nor could they mature on a syn- 

 thetic medium of salts and sugars with ammonium tartrate as the 

 source of nitrogen, as had been maintained by Loeb ('15^), but 

 were able to develop on either medium in the presence of yeast 

 cells. 



c. Habits of adults and latvae. The Drosophila were intro- 

 duced as pupae, usually three being placed on the side of the test- 

 tube. The adults emerge after five to eight days, the time depend- 

 ing on the temperature, and readily feed on the banana medium, 



- This interpretation was first suggested to me by Mrs. J. Jackson. 



^ In 373 transfers of pupae, all descendants of adult Drosophila, taken from a 

 stock bottle of fermenting banana, all tubes were infected with yeast cells carried 

 on the bodies of the insects. 



^ Loeb has since corrected this view ('16). Loeb and Northrop ('16 b). 



