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



latter alone becomes significant if the metabolic differences prove 

 to be causal rather than incidental or resultant, for the difference 

 between a stimulated period followed by depression may well be 

 greater than mere depression. 



Sometimes quantitative reversals were obtained by reducing 

 the temperature. Eleven nymphs that had been thus reversed 

 (average length 6.1 mm.) were killed in cyanide and gave an 

 average resistance of 160 ± 16 minutes. This is fifty-two min- 

 utes longer than that given by the control nymphs which is 2.5 

 times the probable error and is probably significant. 



Taken altogether, the evidence here presented indicates that 

 reversed Epeorus nymphs have a lower rate of metabolic activity 

 than do positive animals. One other observation confirms this 

 idea. Epeorus nymphs collected in October were kept in a large 

 aquarium that also contained some fresh-water mussels. In 

 December and January the nymphs were fdund to be dying in 

 large numbers. When tested all were negative to light, although 

 when first collected they had given almost quantitatively posi- 

 tive reactions. Obviously the metabolic process of the njnmphs 

 was strongly retarded and this is correlated with their reversal 

 to light. 



2. Leptophlebia 



Leptophlebia nymphs were usually negative in their reaction 

 to light giving a ratio of twelve negative to five positive animals. 

 The average survival time of sixty-one untreated nymphs 

 (average length 7.9 mm.) that gave the usual negative light 

 reaction was 131 minutes. Forty-two positive njrmphs (aver- 

 age length 7.8 mm.) under conditions similar in every way re- 

 sisted the same strength of cyanide 130 minutes. Thus there 

 was no difference in the metabolic condition of these two groups 

 that could be measured by the cyanide resistance method. 



Hydrochloric acid was very effective in causing reversals in 

 Leptophlebia. The survival time of thirty-six nymphs so 

 reversed (average length 7.7 mm.) was 127 minutes. Twenty 

 nymphs similarly treated that remained negative (average length 

 7.8 mm.) gave a mean survival time of 120 minutes. This 



