56 ANIMAL AGGREGATIONS 



contact with the current and be swept off their feet, regaining a foot- 

 hold only when the current slackens or when they reach a solid foot- 

 ing. 



3. The males may possess less clinging power than the females. 



4. The females may be carried downstream as well as the males, 

 but may escape from the bunches to the lower swamp. 



5. The aggregations may be formed from isopods that start up 

 from the lower swamp and are unable to make progress when the 

 swifter current is encountered. If this is a factor, it would imply that 

 the males are more strongly positive in their rheotropic reaction than 

 are the females. 



The last four possibilities would account for the formation of the 

 aggregations through the operation of tropistic reactions of the iso- 

 pods as individuals, the so-called "primary reactions" of Szymanski 

 (1913); while the first would bring in a secondary or group reaction. 

 The different possibilities may be considered in reverse order. 



The rheotropic reactions of both sexes were tested according to 

 methods developed earHer (Allee, 191 2). These tests indicated that, 

 in the breeding season at least, the males are somewhat more strong- 

 ly positive in their rheotropic reaction than are the females, and 

 that they respond positively to stronger currents. In so far as the 

 aggregations form as the trapping of positive isopods moving up- 

 stream from the lower swamp, this helps to account for the great 

 discrepancy in the sex ratio. However, this is not the whole story. 



There is little evidence for the assumption that the females may 

 be carried down from the upper swamp in the same numbers as the 

 males but escape from the aggregation to the lower swamp. There 

 were very few females found among the many isopods collected 

 while being carried downstream. 



The supposition that the males have less clinging power than the 

 females, at least in the breeding season, was subjected to direct ex- 

 perimentation, using the method described by myself in 1914. The 

 results indicated that there is little, if any, difference in the cHnging 

 ability of the males and females under the conditions of this test, 

 with whatever advantage that may exist favoring the males. Such 

 results are to be expected from a consideration of the mechanical 



