454 W. C. ALLEE AND E. R, STEIN, JR. 



species both stimulation and depression resulted in reversal 

 and in positive members of the Lake Forest Heptageninae stim- 

 ulation of positive nymphs caused reversal. In Leptophlebia 

 thirty-six tests with hydrochloric acid showed no effect on the 

 average resistance to potassium cyanide. The biometer tests 

 with the Heptageninae give the explanation. When first sub- 

 jected to the acid the nymphs are stimulated; later they are de- 

 pressed. If taken during the first period the resistance to cya- 

 nide would offset that of the later period and so yield an average 

 about the same as the control. An examination of the carbon 

 dioxide production records shows that about as many nymphs 

 were stimulated as were depressed by the treatment, so that a 

 general average not considering the time factor would show re- 

 versals with no relation to the rate of carbon dioxide production. 



In the May-fly nymphs studied the results obtained demon- 

 strate that there is a relationship between the rate of metabolic 

 processes and the sign of the phototactic reaction. It is also 

 clear that reversals are accompanied by either a marked stimula- 

 tion or marked depression. The experiments indicated but do 

 not prove that the stimulation or depression is causal. From 

 one point of view it makes little difference whether the metabolic 

 changes are causal or only symptomatic ; the fact that they are 

 correlated at all is important. 



If the change in metabolic conditions is causal, the fact is evi- 

 dent that all changes do not cause reversals. This was shown 

 particularly by the action of the ethyl alcohol upon the L:,ke 

 Forest nymphs. This stimulated and later epressed the 

 nymphs with or without an accompanying reversal. On the 

 assumption that metabolic change is causal the non-action 

 of alcohol in 80 per cent of the cases might be explained by 

 supposing that it did not cause a change quantitatively large 

 enough. This suggestion is supported by the observation with 

 the other species that alcohol accompanied by decrease in tem- 

 perature ws more effective in producing reversals than alcohol 

 alone and by the fact that in general these species were more 

 susceptible. 



