RHEOTAXIS, RESISTANCE TO POTASSIUM CYANIDE 409 



ten minutes. These pond isopods had given an average rheo- 

 tactic response of 51 per cent positive, 37 per cent negative, 10 

 per cent indefinite and 2 per cent no reaction. Their average 

 efficiency in the current was 2.2. Another lot of 55 pond isopods 

 which gave a rheotactic average similar to the above, were killed 

 off from normal conditions in 0.001 molecular potassium cyanide; 

 these gave a mean resistance of five hours and fourteen minutes, 

 just forty-five minutes less than that given by the stream isopods 

 to a cyanide solution one-fifth as strong. 



All this means that the stream isopods, the ones that as a 

 rule are more highly positive to and more efficient in their reac- 

 tions in a water current than pond isopods, have a shorter survival- 

 time in equimolecular solutions of potassium cyanide and therefore 

 have a higher rate of metabolism than do the pond isopods. 



If only the highly positive isopods of the two mores are chosen 

 for a comparison of their resistance to potassium cyanide, another 

 situation is revealed. The individuals of the two groups selected 

 on the basis of the similarity of their rheotactic response show a 

 great difference in their survival-time in equimolecular solutions 

 of the cyanide and hence in their metabolic rate. In table 2 

 (p. 404) the stream isopods with a rheotactic reaction 60 per cent 

 or more positive averaged 88 per cent positive and gave a mean 

 survival- time in 0.0002 molecular potassium cyanide of five hours 

 and twenty-four minutes. Isopods from the pond mores selected 

 on the same basis averaged 83 per cent positive and gave a sur- 

 vival-time of twenty-one hours and sixteen minutes, in the same 

 strength of cyanide solution. Here is a great difference in the 

 metabolic activity of the two groups of isopods but a close simi- 

 larity in their rheotactic reaction. It can also be seen in table 

 2 that the pond isopods that give a low positive response have a 

 longer survival- time in potassium cyanide than those that are 

 more highly positive. This must mean that the rate of metab- 

 olism which is accompanied by a high degree of positiveness is 

 relatively high when compared with the mean metabolic rate 

 given under the conditions to which the isopods are acclimated, 

 but that it may not be an absolutely high rate when compared 



THE JOURNAL OF EXPERIMENTAL ZOOLOOT, VOL. 16, NO. 3 



