Rcactiotis of the Pomace Flv to Odorous Siihstafiees 523 



the apparatus and their reactions recorded. These records added 

 to those of the preceding fifty constituted the records of one experi- 

 ment. When the different strengths of the same substance were 

 tested on different days, the last strength used on the previous 

 day was first tested in order to be sure that the hungry flies in 

 stock had remained uniform in their response to this stimulus. 

 Having ascertained this, the experiments were carried forward 

 as though they formed a continuous series. The results of these 

 experiments are given in Tables II to VI. 



TABLE III 



The numbers of fiies which reacted positively to glacial acetic acid and to di^erent strengths of this acid in 

 water. In each experiment the number of flies used was one hundred 



From Table II it can be seen that the greatest number of positive 

 reactions to alcohol were obtained at 20 per cent concentra- 

 tion, while strengths above or below this grade show a decrease 

 in the number of positive reactions. 



It will be seen from Fig. 2, that a fly in passing from the near 

 end to the far end of the trough enters the area of stimulation 

 obliquely. Consequently one side of the animal must be stimu- 

 lated before the other. Many of the flies entering the odor in this 

 way give a positive reaction by turning toward the stimulated side. 



