EGG-LAYING REACTIONS IN POMACE FLY 339 
while succinic and tartaric acids are common in grapes. Su- 
erose and glucose, which were found stimulating in the present 
experiments, are known to occur in bananas (Bailey, ’06). 
Recent studies on the life-cycle of Drosophila have revealed 
the fact that these flies depend in some measure upon the pres- 
ence of living yeast for their well-being. Guyénot (’07) was the 
first to recognize this, and only by using dead and autolyzed 
yeast has he succeeded in rearing Drosophila on a sterile medium. 
Baumberger (19) has reviewed the literature upon the role of 
yeast, and considers that its relation to Drosophila is a loose 
symbiosis. The influence of this relation upon egg-laying was 
not studied in these experiments, since the importance of yeast 
in the life of this fly was not recognized at the time of my experi- 
ments. It is evident from the work of Loeb (15) that yeast 
induces egg-laying, for he found that the flies laid their eggs 
readily on filter paper saturated with a solution of sugar and 
inorganic salts. 
Egg-laying responses of other flies 
Because of its economic importance, the house fly has received 
some attention from experimental workers. The older casual 
observations have been reviewed by Hewitt (08), the common 
conclusion being that the flies prefer horse manure over all 
things for egg-laying, but will lay upon the excrement of other 
animals, and upon meat, milk, and other animal and vegetable 
materials which are undergoing decay. ‘This decay is of course 
always characterized by the presence of microérganisms. 
Richardson (’16) found that the odor of ammonia was the 
best stimulus for egg-laying in the house fly, but that the re- 
sponse was not complete unless a mass, such as horse manure, 
was present. Acidified manure, moistened chaff, sawdust, cot- 
ton; or filter-paper were not nearly so conducive to egg-laying 
even with the odor of ammonia near by. <A few substances, 
such as butyric acid and valerianic acid, slightly augmented the 
efficacy of the ammonia odor. 
