RHEOTACTIC REACTION OF STREAM ISOPODS. 



61 



of stream isopods, directly by decreasing the clinging ability and 

 indirectly by decreasing tendency to orient positively in the 

 current which in turn affects the clinging ability of the isopod. 



The rheotactic reaction has been shown to vary with the 

 changing metabolic state of the isopod (Allee and Tashiro, 

 '14). The clinging pow r er of positively oriented isopods also 

 varies to a limited extent, with the degree of positiveness of the 

 rheotactic reaction. This is shown by the data exhibited in 

 Tables II. and III. In Table II, the average rheotactic response 



TABLE II. 



Showing the relation between low and high clinging ability of positively oriented 

 isopods and the rheotactic reaction. For details of methods see pp. 57 and 61. 



of isopods that resisted a current of 3,500 c.c. per minute or 

 more is compared with that given by isopods that were swept off 

 their feet by a current of 2,000 c.c. per minute or less. The 

 isopods with the higher clinging power averaged 78 per cent, 

 positive in their rheotactic reactions and they did not fail to give 

 the rheotactic response when stimulated. On the other hand 

 the isopods with the poorer clinging power averaged 57 per cent. 



3 In order to have a method for measuring and recording the efficiency of move- 

 ment of isopods in the current during rheotactic tests the following arbitrary 

 standard was adopted, which represents numerically the distance covered in a 

 minute's reaction period. The following values were applied to the numbers: 



0, no reaction. 



1, slight movement. 



2, any response between i and 3. 



3, progress one third around the pan positively (approximately 27 cm.) or 



two thirds, negative (54 cm.). 



4, progress two thirds around the pan positive, or one and one third negative. 



5, progress once around the pan positive, or twice around negative. 



6, any distance over 5. 



Experiments showed that isopods oriented negatively covered twice the distance 

 per leg movement as did those oriented positively. 



