456 W. C. ALLEE AND E. E. STEIN, JR. 



(as in cyanide reversals) , This excursion was often the first one 

 the nymph had made to the positive end and frequently the 

 nymphs remained there until they died. The reason for i he rela- 

 tion of metabolic rate with this type of reversal is not evident, 

 especially when the reversal followed a very strong depression. 



An explanation of reversals sometimes advanced (Ewald, '13) 

 is that animals change in their sensitivity to light and hence 

 reverse their reactions. On this basis the change in sensitivity 

 is causal ; the light reversal and change in carbon dioxide produc- 

 tion, resultants. In May fly nymphs these supposed resultants 

 are correlated in such a way that either stimulation or depression 

 may accompany reversals from negative to positive light reac- 

 tions. This means that either increasing or decreasing sensi- 

 tivity to light will make negative May-fly nymphs positive, or 

 that an increase (or decrease) in sensitivity wil cause now de- 

 pression and now stimulation. Either of these necessary assump- 

 tions expand the sensitivity hypothesis beyond the limits justified 

 by known facts. 



CONCLUSIONS 



The light reactions of the positive May-fly nymph, Epeorus, 

 was reversed by treatment with alcohol, lowered temperature, 

 calcium chloride, and other reagents. Nymphs so reversed had a 

 lower rate of metabolism, as measured by resistance to potas- 

 sium cyanide, than normal untreated nymphs. 



The negative nymph, Leptophlebia, was similarly reversed 

 with accompanying stimulation or depression as measured by 

 resistance to the cyanide. 



A negative nymph belonging to the Heptageninae was reversed 

 in its light reactions with accompanying increase or decrease in 

 carbon dioxide production as measured by Tashiro's biometer. 



The experiments conclusively demonstrate that the photo- 

 tactic reaction of these nymphs is correlated with the metabolic 

 condition and indicate, but do not prove, that certain changes in 

 metabolism cause the reversals in reaction to light. 



All nymphs that reversed their light reactions we e e fher 

 stimulated or depressed, but stimulation or depression did not 

 necessarily involve phototactic reversal. 



