402 W. C. ALLEE 



alone. Moreover, the effect of temperature upon metabolism 

 will adequately explain the difference found to exist. Therefore 

 the results of measuring the survival-time of isopods at low and at 

 high temperatures, while not a complete demonstration, strongly 

 indicate that the resistance to potassium cyanide is a measure of 

 metabolic rate in isopods. 



In the case of stimulation of the isopods by shaking them while 

 exposed to the cyanide, there is a marked decrease in the survival- 

 time. This is what would be expected if the resistance to potas- 

 sium cyanide is a measure of metabolic activity. In these experi- 

 ments the isopods were shaken every few minutes as long as this 

 stimulated them to activity. If objection be made that shorten- 

 ing of the survival-time under these conditions is a fatigue response, 

 it is only necessary to point out that fatigued animals would have 

 a lower rate of metabolism and hence a longer survival time than 

 unstimulated animals. Consequently, if fatigue enters as a 

 factor, it tends to lengthen, not shorten, the survival-time. But 

 it is improbable that fatigue is a factor in the survival-time of 

 these stimulated isopods since they were usually lying on their 

 backs unable to respond to stimulation before either the pond or 

 stream mores would have shown a change in their rheotactic 

 reaction under continuous stimulation. 



The figures shown in table 1 regarding the effect of size upon the 

 survival show that the larger the animal the longer is the survival- 

 time in potassium cyanide. These results are clear-cut in all 

 respects save one. The rheotaxis both in percentage of responses 

 and in efficiency of the reaction is practically identical, while the 

 size differences and the survival-time differences are marked. 

 But almost all the larger ones are male and most of the smaller 

 ones are female and most of the records were taken during the 

 breeding season. The females of the same age are usually smaller 

 than the males, but not so much smaller as is here indicated. 

 From general appearance these females were younger than the 

 larger ones that were apparently the same age as the males. 



An examination of the records of 58 males and 28 females from 

 Stock III, mainly out of the breeding period, in which the size 

 and rheotactic responses averaged approximately the same, 



