Reactions of the Pomace Fly to Odorous Substances 537 



7 Drosophila, when stimulated by weak food odor, first shows 

 random movements, t. e., it attempts to find the food by the method 

 of trial and error, but as the fly passes into an area of greater stimu- 

 lation, these movements give way to a direct orientation. This 

 orientation is a well defined "tropism" response. These reactions 

 of Drosophila are paralleled by those of Perichaeta to strong and 

 weak light and possibly also by the food reactions of planarians. 



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