A NUTRITIONAL STUDY OF INSECTS / 



bacillus, the Drosophila larvae grow rapidly, the fungous 

 growth soon diminishes and is visible at only a few points on the 

 surface. If the flora contained molds, the whole surface of the 

 medium is soon covered and the Drosophila eggs are killed, or 

 more often hatched and the young larvae die. If the mold does 

 not completely cover the surface, many larvae survive, and upon 

 increasing in size, are able to destroy the mass of mold hyphae 

 and form a fairly clean surface. The larvae are able to do this 

 only when they are in large numbers and have reached a size of 3.5 

 to 4.5 mm. before being covered by the molds. It would seem, 

 therefore, that the destructive action of the molds is mechanical 

 rather than toxic. It was also observed that molds seldom gain 

 a foothold on media in which large numbers of larvae are feeding. 

 This observation explains why 'strong' cultures of Drosophila (as 

 usually reared on fermenting banana) remain 'sweet' and seldom 

 go bad. Banana-agar culture tubes in which the Drosophila pu- 

 pae have been placed on the glass, rather than on the medium it- 

 self, often remain sterile till the adults emerge. The latter 

 spread the spores over the surface of the agar at the same time 

 that they deposit their eggs. Thus the molds and bacteria have 

 little time to grow before the larvae are at work. 



The development of molds and bacteria is not apparent in the 

 presence of large numbers of larvae and a strong culture of yeast 

 in the proper nutrient medium. 



e. Media for genetical work. In selecting the best medium in 

 which to rear Drosophila the most important considerations are 

 abundant food for the yeast cells and a moist jellylike consistency 

 of substratum to which the larvae are adapted. Transparency 

 and sohdity of media will add to the convenience of the 

 investigator. 



I have obtained the best results by using Saccharomyces ellip- 

 soideus, in the stock bottle of banana, as the fragrant odors of 

 fermentation produced by this yeast stimulate oviposition by the 

 fly. The two following media have proved most satisfactory: 

 1. Fermented banana agar. Ferment one dozen mashed ba- 

 nanas for 48 hours, strain through cheese-cloth, add agar, steril- 

 ize and slant. 2. Pasteur's culture fluid agar. 



