A NUTRITIONAL STUDY OF INSECTS 67 



organisms on a substratum. If we assume that the insect larvae 

 feed upon these microorganisms, the chain of circumstantial evi- 

 dence is complete. Our knowledge of the odorous substances 

 attractive to flies has been advanced greatly by the work of Bar- 

 rows ('07) and Richardson ('16a, b; '17), but these authors have 

 never given the above interpretation to the response. Barrows 

 studied the odors to which the adults Drosophila ampelophila 

 (melanogaster) responds and found that the most attractive odors 

 are those of ethyl and amyl alcohol, acetic acid, lactic acid, and 

 acetic ether. A small amount of acetic ether, isobutyl acetate, 

 methyl acetate, acetic or butyric acid added to ethyl alcohol 

 greatly increased its attractiveness. ''Alcohol and acetic acid 

 are commonly found in cider vinegar, fermented cider and Cali- 

 fornia sherry in per cents that are close to those which call forth 

 the largest number of reactions in Drosophila." This odor is 

 identical to that produced by the wine yeast Saccharomycetes 

 ellipsoideus which I have found to cause females to deposit the 

 largest number of eggs. 



In 1916 (a) Richardson reported that he had carried on a series 

 of experiments with odorous substances as baits for houseflies. 

 The baits tested were placed under wire-gauze traps and were as 

 following: Ammonium carbonate, ammonium sulphide solu- 

 tion, ammonium hydroxide, ethyl alcohol solution of skatol and 

 indol, ethyl alcohol, acetic, formic, butyric and valerianic acids, 

 hydrogen sulphide solution and carbon dioxide. ''Negative re- 

 sults were obtained in all but the ammonium hydroxide and am- 

 monium carbonate experiments." The ammonia was the attrac- 

 tive substance especially to females, which were found in the 

 percentage of 89.2 to 7.5 of the males, although the actual per- 

 centage of sexes in the vicinity was 54 to 45.9 respectively. 

 Valerianic and butyric acid augmented oviposition; the females, 

 however, showed some discrimination between nutritious and 

 non-nutritious material. In 1916 (b) a second paper gave a list 

 of insects attracted to the ammonia, all of which spend at .least 

 part of their life in some form of animal excrement.-^ 



^'' A third paper ('17) showed that aqueous solutions of carbohydrates are far 

 less attractive than alcoholic or acetic acid solutions of such substances. 



