FORMATION OF ANIMAL AGGREGATIONS 53 



times these collected into small balls of from 6 to 20 isopods, which 

 rolled along the bottom until they found a lodging against some ob- 

 struction or settled into a deeper pool where the current was less 

 strong. There were many isopods on the sandy bottom of the stream, 

 mostly facing against the current, but making very little progress 

 against it. Chopping through the ice above or below the roadway 

 revealed no comparable collection of isopods, although there was 

 evidence of an increase in numbers as one neared the narrow chan- 

 nels of the washout either from above or below. 



After the break-up of the winter ice, the majority of the isopods 

 disappeared, although traces of the aggregation could still be seen, 

 particularly in the sheltered places just below the opening of the 

 stream into the lower swamp. There the isopods were mainly travel- 

 ing downstream with the current, or were collected in sheltered 

 places in deeper water or about lodged debris. As before, few were 

 found in the open above the roadway. 



After the ice was entirely gone, and with the usual rise in water 

 level, the aggregation re-formed. In early April a few were being 

 carried downstream through the washout. Several more were to be 

 seen along the margins, for the most part headed upstream, where 

 some were able to make their way for a considerable distance. At 

 the lower edge of the roadway, great masses of isopods had collected 

 about willow shrubs, old cat-tails, or in deeper pools, wherever they 

 might find a lodging. The largest of these masses was about 75 cm. 

 across the current, 30 cm. up and down stream, and over 10 cm. 

 deep, a solid writhing mass of isopods. This was loosely joined with 

 other similar units, each formed about some basis of support from 

 the force of the current, the whole making an isopod barrier all along 

 the lower margin of the washout, over 5 meters in length and about i 

 meter wide. The numbers concerned were unbelievable. They were 

 to be measured by liters rather than by individuals. The mass can 

 be imagined by thinking of the full swarms of some twenty or more 

 beehives settling near each other. Conditions remained much the 

 same for the next 3 weeks, with but a slight variation in the position 

 of the largest mass, depending apparently on the strength of the 

 current. 



