32 J. PERCY BAUMBERGER 



Drosophila from decaying fruit, vegetables and fungi, and from 

 feces. Henneberg ('02) suggested that the larvae probably live 

 on the microorganisms present in these media. It is apparent 

 that microorganisms are usually present in great abundance in the 

 larval environment, and from my experiments it appears that they 

 normally serve as food for the insect, since the adults are at- 

 tracted by the odors of fermentation ])roducts (p. 68). Micro- 

 organisms, known to occur on the exterior of fruits, probably 

 usually contaminate the substratum before the flies oviposit. 

 Flies assist in establishing a suitable flora by accidentally carry- 

 ing in the digestive tract or on the minute hairs of the body many 

 yeast cells. This can be seen in cross-sections of the body (figs. 

 10 and 11) and has previously been shown to be the case on page 

 8. The yeast growth takes place more rapidly in the presence 

 of larvae, because the latter spread the cells throughout the 

 medium. At first sight, this would appear to throw doubt on 

 the ability of the larvae to digest the yeast cells, but serial sec- 

 tions show clearly the process of disintegration. Figures 11, 12, 

 13, and 14, are microphotographs of successive sections through 

 the stomadaeum, mesenteron, and proctodaeum showing the proc- 

 ess of disintegration which takes place mainly in the middle 

 digestive portion of the tract. However, as in the case of birds 

 which feed upon insect eggs and seeds, many living cells pass 

 through the alimentary canal (page 8). 



In nature Drosophila larvae are usually found in a substratum 

 suitable as a nutritive medium for microorganisms and abound- 

 ing especially in yeasts. In this environment the insect has 

 available as food both the substratum, usually fruit, and the micro- 

 organism. But why have the larvae become dependent on the 

 latter? Three reasons immediately come to mind, viz. : 



1. As all fruits which are soft enough for Drosophila larvae to 

 live in are always infected with yeast from the air, the larvae 

 w^ould unavoidably ingest fungous cells with the fruit. The nu- 

 tritive value of the food would soon affect the life-cycle of the 

 insect and bring about a close adaptation to a yeast diet. 



2. Larvae feed upon microorganisms and by their constant 

 movements carry the spores throughout the substratum. In 



