RHEOTAXIS, RESISTANCE TO POTASSIUM CYANIDE 403 



showed no definite difference in the survival-time of the two sexes. 

 If anything, the females had the longer survival- time. This 

 would tend to rule out the question of sex in the cases above noted 

 hut leaves that of the effect of the breeding season unanswered. 

 The results may fairly be interpreted as showing a difference in 

 survival-time due mainly to difference in age and perhaps partially 

 to a differential action of breeding responses on males and females. 



Very small isopods (1.5-1.75 mm. long) have a much reduced 

 survival-time. Ten of this size lived only three hours and forty 

 minutes in 0.0002 molecular potassium cyanide solution, while 

 the mean for 72 adults averaging 10.7 mm. long, was five hours 

 and fifty-nine minutes. This marked difference may have been 

 partially due to the more tender external covering of the smaller 

 isopods which allowed more direct action of the cyanide, but 

 since the facts now at hand indicate that recently molted isopods 

 live a longer, not a shorter, time than their mates, this does not 

 seem sufficient ground to account for so great difference. 



Finally, the most crucial evidence that resistance to cyanide 

 does measure metabolism in isopods, is to be found in a compari- 

 son between the carbon dioxide output and the survival-time. 

 A number of determinations of this character were made jointly 

 with Dr. Shiro Tashiro at Woods Hole during the past summer 

 (cf. Tashiro '13 p. 141). The results of these experiments will 

 be published in detail elsewhere (Allee and Tashiro '14). Suffice 

 it to say that in the twelve trials made, with one easily-explained 

 exception, the resistance to potassium cyanide was inversely pro- 

 portional to the rate of carbon dioxide production. As the ex- 

 periments were run, this is the most delicate and accurate test 

 and it is the more interesting in that the results with this method 

 were quantitative. 



In summing up the inquiry as to whether the resistance to 

 relatively strong solutions of cyanide does measure the metabolic 

 conditions of isopods, it is found that the survival-time is decreased 

 by raising the temperature and by stimulation by shaking; that it 

 is lengthened by decreasing the temperature; that larger (older) 

 isopods have a longer survival- time than small (young) ones ; and 

 that the carbon dioxide production is inversely proportional to 



