58 ANIMAL AGGREGATIONS 



rent at all. I have seen males which were responding definitely to a 

 water current behave as if they perceived another isopod at a dis- 

 tance of some 2-4 cm., discontinue their rheotropic reaction, and 

 move directly to the nearby isopod and cling to it. I have no evi- 

 dence of such reactions at distances greater than 5 cm., so that their 

 effect would be operative in bunch formation only, after the isopods 

 had been brought close together through the operation of some other 

 factors. 



I have no knowledge of such isopod aggregations except in winter 

 and spring, and unfortunately the sex ratios of the winter aggrega- 

 tions were not taken. In this connection it must be remembered that 

 the isopods do not start their breeding season in December in nature. 

 Yet large aggregations were found at that time. The observations 

 show clearly that the ratio of males to females is high in the spring 

 aggregations, and suggest that this is due to the tendency of males to 

 move about during the breeding season, which makes them more 

 likely to be caught in the current and swept down from the upper 

 swamp, and, on the other hand, more Ukely to come into contact 

 with a current sufficiently strong to cause them to react positively, 

 and so move upstream to the place of aggregation from the lower 

 swamp. 



METHOD OP FORMATION OF THE AGGREGATIONS 



This subject obviously overlaps consideration of the preponder- 

 ance of males, and the conclusions reached from much consideration 

 of the problem are the same as those indicated there. As was to be 

 expected, disturbances in the swamp just above the opening of one 

 of the outlets caused a marked increase in the numbers of isopods 

 carried down. These might lodge in slight depressions in the stream 

 bed where the current was less strong; some 50 were observed to 

 collect in a small depression less than 12 cm. in diameter within 5 

 minutes following a disturbance in the upper swamp. 



Others were carried on by the current until they found physical 

 support against rushes or other debris, or against other isopods which 

 were in turn supported by the rushes. Thus, the bunch may be seen 

 to grow on its upstream side, the newcomers using the other isopods 

 as an extension of the support furnished by the lodged debris. 



