FORMATION OF ANIMAL AGGREGATIONS 57 



difficulties of maintenance of position by females carrying a large 

 brood pouch between their anterior thoracic legs. 



Of the tropistic non-social suggestions advanced as possible ex- 

 planations of the greater proportion of males than females in the 

 spring aggregations, one more remains for detailed consideration. 

 This is the suggestion that the males move about more and so come 

 into contact with the current more frequently than the more passive 

 females. Such ditTerential action would result in more males being 

 swept off their feet and carried down from above, and also in more 

 males coming in contact with a current strength which would call 

 forth a positive rheotropic response and so bring them up from the 

 lower swamp. This possibility is supported by the following kinds 

 of evidence. The direction of the current impinging on a large bunch 

 was artificially changed, and the current change resulted in a re- 

 organization of the bunch of isopods in a new position. At a time 

 when the main bunch showed a ratio of males to females of 25:0, 

 25:3, 25:2, 25:2, with a total of 100:7, the reorganized bunch 

 showed ratios of 50: i and 45 : 4, with a total of 95 : 5, which is nearly 

 twice the number of males per female as found in the bunch of 

 longer standing. Again, I pulled from near a large aggregation a tuft 

 of grass heavily covered with isopods. The sex ratio of those that ac- 

 tively crawled from the grass onto my hand proved to be 4 males to 

 each female. The sex ratio of all the isopods on a similar tuft was 

 found to be i male to 3 females. In both cases the males showed a 

 higher degree of activity. It is also true that the vast majority of 

 animals taken while being carried downstream by the current were 

 males, and that the sex ratio of the isopods on the water plants out- 

 side the main current, but above the roadway, showed a higher 

 number of females than males. 



Regarding the possibility that the males may be responding to a 

 stronger internal sexual stimulant than the females, there is evi- 

 dence from earlier work that in the breeding season the males do tend 

 to cling to any passing isopod, and apparently have this tendency 

 more strongly developed than do the females. The tendency to col- 

 lect in bunches is so strong that spring isopods must frequently be 

 tested singly for rheotropism or they will fail to respond to the cur- 



