A NUTRITIONAL STUDY OF INSECTS 37 



towards the place of lowest concentration, and thus the yeast 

 finally adsorbs and builds up into its own protein all the amino- 

 acids of the substratum. The yeast grows at the surface or 

 just below it where it is carried by the larvae and therefore brings 

 .within reach of the larvae nitrogen that had been distributed 

 throughout the medium, many parts of which could not be 

 reached. In the experunent with banana agar the j^east not 

 only concentrated the amino-acids of the substratum, but prob- 

 ably sjTithesized them into more complex molecules; in the 

 second example, the living yeast merely concentrated all amino- 

 acids at the surface of the medium without increasing their com- 

 plexity. In a synthetic medium of sugars and salts, yeasts would 

 concentrate and synthesize into protein, the ammonia of the 

 substratum. 



It has already been shown that concentrated banana permits 

 larvae to pupate, but the rate of growth is not normal. There- 

 fore it is apparent that while the banana is not entirely lacking in 

 the substances necessary for complete growth, it is not as ade- 

 quate to these demands as yeast or yeast nucleoprotein. There- 

 fore we may conclude that the function of yeast in the ecology of 

 Drosophila larvae is to concentrate at the surface and synthesize 

 the ammonia^^ and aminoacids of the substratum into nucleopro- 

 tein, which, fills the protein requirements of the larvae. 



e. Literature on the food of Drosophila. Valuable contribu- 

 tions to our knowledge of the food relations of microorganisms to 

 insects have been made by Delcourt and Guyenot. These au- 

 thors reported in 1910 experiments with Drosophila in which they 

 showed that the larvae could be reared on a potato medium free 

 from all microorganisms except yeast or a complex of yeast and 

 acetic-acid bacilli. Microscopical examination showed the yeast 

 cells in the digestive tract in all stages of digestion. This paper 

 was followed by a second in 1913 (a) in which the authors deter- 

 mined whether the insect fed on the products of the yeast's 

 chemical activities or upon the yeast cell itself. In order to ob- 

 tain this information, it was necessary to operate with larvae 



18 Yeast can also synthesize protein from a urate source of nitrogen. 



