Reactions of the Poninee Fly to Odorous Substances 525 



It is probable that the dissolved ether evaporates rapidly forming 

 an almost saturated atmosphere inside the trap, and this is known 

 to kill the flies in less than three minutes. Hence the excessive 

 stimulation probably causes them to back out of the entrance to the 

 trap into which they had been enticed by the less concentrated 

 vapor. Acetic ether is never so abundant in decaying fruit as in 

 the weakest solutions (2 per cent) tested in these experiments. 

 In order to make a mixture which should combine the optimum 

 strengths of alcohol and acetic acid, equal volumes of 40 per cent 

 alcohol and 10 per cent acetic acid were mixed. This mixture 

 then contained 20 per cent of alcohol and 5 per cent of acetic 



TABLE v 



The numbers of flies which reacted positively to each of four difjerent strengths of a mixture of equal parts 

 of 40 per cent alcohol and JO per cent actlic acid diluted with water. In each experiment the num- 

 ber of flies used was one hundred 



Number of the 

 experiment 



Number of flies that 

 entered the 





Number of flies that 

 turned to'ward but 

 did not enter the 



JJ 'g Si Charged Control ' Charged 

 ^ ■ trap trap I trap 



Control 

 trap 



1 100 I 13 



2 ; 50 j 26 



3 ! 25 I 29 



4 i2-'5 i 27 



30 

 28 



31 

 39 



acid. Table V shows the results obtained by testing flies with 

 this mixture either pure or diluted with water. 



A solution of 12^ per cent of this mixture, which is equal to a 

 mixture of 2^ per cent alcohol and f per cent acetic acid, gives a 

 slightly higher number of positive reactions than is given either by 

 5 per cent acetic acid (Table III) or 20 per cent alcohol (Table II). 

 The numbers of the positive reactions are not significantly large, 

 yet it is probable that the mixture is uniformly more stimulating 

 than alcohol or acetic acid alone. 



Table VI shows the results obtained by testing the mixture con- 

 taining 20 per cent alcohol and 5 per cent acetic acid (Table V) to 



