54 ANIMAL AGGREGATIONS 



In late April the water level had again been raised by rain, and in 

 the main current stood about 45 cm. deep, in place of the more usual 

 15-18 cm. A new, smaller overflow had been formed near-by. The 

 isopods were all gone from their place of aggregation ; and although 

 they were still plentiful all around the edge of the fan of sand washed 

 down by the recent rains, they were not collected into the great 

 masses found heretofore. In the slacker current just preceding the 

 rains, isopods were no longer being carried downstream across the 

 roadway; and one could not have collected more than 50 such drift- 

 ers by watching all day. Now with the higher water level and the 

 swifter current they were again being swept downstream from the 

 upper swamp in numbers. 



With the higher water level of early April, smaller aggregations 

 had appeared about the lower ends of the central iron culverts 

 piercing the roadway, but now there, too, were dissipated. With the 

 higher water of late April, the culverts situated at the edges of the 

 swamp showed a marked current for the first time — not nearly so 

 strong as that in the center culverts, but corresponding in strength 

 to the latter when aggregation of isopods occurred near them. Now, 

 for the first time, sizeable aggregations were present at the lower 

 ends of these side culverts. At the upper end of one of these there 

 was a log and much plant debris on and about which isopods borne 

 down from the upper swamp might have lodged; but none were 

 there, while they occurred in large numbers at the lower opening, 

 particularly in eddies out of the main current. The current ceased to 

 flow through the north marginal culvert within a few days, and the 

 aggregations there disintegrated. Those at the opposite margin per- 

 sisted for about two weeks. The aggregations below the main over- 

 flow did not re-form, although many individuals could be seen at any 

 time unsuccessfully attempting to make their way up over the shift- 

 ing sandy bottom. 



The final breaking-up of the aggregations was not observed, 

 though at any time small groups or single individuals might be seen 

 becoming detached and borne away by the current. When the water 

 rose, the increased velocity probably carried the whole lot off in a 

 similar manner. At the end of the season some of the aggregated ani- 



