FORMATION OF ANIMAL AGGREGATIONS 59 



But this is not the only method by which the aggregations are 

 formed. ISIention has been made already of the finding of a large 

 aggregation at the lower end of a culvert whose upper opening was 

 well protected by the presence of logs, grass, and other debris, 

 through which the water ran easily, but upon which few isopods 

 collected even at the sides where the current was certainly not of 

 sufficient strength to tear them loose from available support. 



In laboratory experiments with artificial streams some isopods, 

 mostly males, traveled against the current and collected in the more 

 quiet water at the upper end of the trough. Similar behavior was 

 repeatedly seen in nature. After the ice left, isopods from the great 

 spring aggregations could be seen laboriously moving against the 

 current over the sandy bed of the stream ; while those located below 

 the opening of one of the streams into the lower swamp, if not pres- 

 ent in sufficient numbers to form an aggregation of three dimensions, 

 were frequently spread thickly over the bottom, with all individuals 

 headed upstream. 



Of all the isopods moving upstream, those near the margin were 

 most successful. Usually, however, all were swept down sooner or 

 later to the main group below. When a board was placed with one 

 end resting in an aggregation so that it furnished a solid substratum 

 on which the isopods might crawl, they immediately started up- 

 stream as closely as they could stick on the board. On reaching the 

 upper end, many were immediately washed down by the current, 

 while others would continue over the precarious bottom for a short 

 distance before they, too, lost their footing. If dikes were built so 

 that the current impinging on an aggregation was slackened, the 

 isopods started upstream in numbers, only to be swept down again 

 when a stronger current was encountered. 



There is also a fatigue factor which causes the failure of these 

 isopods to continue their journey upstream even in a fairly weak 

 current. The length of time before reversal is roughly correlated 

 with the physical condition of the isopods. In laboratory tests with 

 isopods from these aggregations, reversal in a straight current oc- 

 curred after an exposure of about an hour. 



If the impinging current is cut off completely by the construction 



