52 ANIMAL AGGREGATIONS 



gations, and that thigmotropic reactions are largely concerned. The 

 observations of Essenberg and of Riley on water striders, of Clark on 

 Notonecta, of the Severins on Belostoma, as well as the tremendous 

 general literature on animal behavior (see Loeb, 1918, for a partial 

 bibliography), show that aggregations do form in many cases with- 

 out evidence of a positive social instinct or appetite, although this is 

 not to be taken as proof that in other instances aggregations may 

 not form as a result of social appetite. 



AGGREGATIONS OF ASELLUS IN NATURE 



The analysis of one other case is illuminating. For a number of 

 years I had been seeking a favorable opportunity to apply in the 

 field certain analytical methods worked out in studying aspects of 

 the laboratory ecology of animal aggregations, and accordingly 

 welcomed the information that a great aggregation of the common 

 fresh-water isopod, Asellus communis Say, had been found in mid- 

 winter in the Indiana dune country near-by. 



At the point where this collection occurred, a low sand ridge had 

 been thrown up to serve as a roadway across an extensive cat-tail 

 swamp, here about a quarter of a mile wide. To the east, the swamp 

 stretched as far as could be seen from the low elevation of the road- 

 way. To the west, there was also a very extensive continuation of 

 the cat-tail swamp for at least a half-mile. The whole formed a major 

 part of the headwaters of a small stream. 



The roadway was pierced at several places by culverts, introduced 

 to relieve the water pressure above. These had proved inadequate, 

 and at one place the water had washed away the ridge of sand and 

 flowed over the roadway through an opening about 5 meters wide, 

 with a current there sufficient to prevent complete freezing. When 

 first seen, the ice was about 6 cm. thick and the effective stream was 

 reduced to about 1.5 meters width. 



Here on the under side of the ice were tens of thousands of isopods, 

 oriented to face upstream, and showing by their arrangement the 

 definite lines of force of the current below. Thousands of other iso- 

 pods were resting on the bottom in protected places, and many more 

 were being swept downstream by the rapidly moving current. Some- 



