FORMATION OF ANIMAL AGGREGATIONS 55 



mals died in situ, especially if located at one side where the current 

 became cut off. 



An increased flow following heavy rains in late May produced 

 physical conditions similar to those of late April, but no aggregations 

 were formed, although a few large isopods were carried downstream 

 through the main spillway. 



The favorable localities were well watched the next winter and 

 spring, but no large aggregations were found. In early April a small 

 aggregation occurred below the culvert at the extreme south side, 

 where the last one had formed the spring before. With the passing of 

 time, the main washout had deepened so that a stronger current was 

 running there than when the isopods had aggregated the preceding 

 year. In general, there appeared to be fewer Aselli in the swamp, 

 and one is led to suspect that there may have been an unusually 

 large production of isopods preceding the formation of the monster 

 aggregation observed in 1927. 



SEX RATIO 



In early spring one can usually determine the sex of Asellus by 

 considering the size, shape of thorax, and presence or absence of the 

 brood pouch. In the laboratory, sexes are easily and accurately de- 

 termined. Careful observations showed that during the time of the 

 great spring aggregations the ratio of the collected isopods ran as 

 high as 25 males to i female, and never ranged below 9:1. 



November coflections from the scattered isopods, both above and 

 below the culverts, showed a i : i ratio. In early April of the next 

 year, random collections from the relatively small aggregation at 

 the lower end of a lateral culvert showed a sex ratio of 12 males to 

 each female. At the same time, similar collections both above and 

 below the aggregation showed a ratio of approximately 1:1. 



Five suggestions readily occur to account for the high ratio of 

 males to females in the great bunches: 



1. The aggregation may be due to a mating or other social im- 

 pulse acting more strongly in the males than in the females, which 

 impels them to gather in these large groups. 



2. The males may tend to move about more and to come into 



