6o ANIMAL AGGREGATIONS 



of a dam, the aggregated individuals begin a rearrangement which 

 usually results in new aggregations being formed in depressions, or 

 about some quiet individual or a quiet group, just as such aggrega- 

 tions form in the quiet water of a laboratory tank. These groupings 

 are usually less dense than those exposed to the drive of the current. 

 The negative reaction to light is one of the factors conditioning this 

 reaction ; positive thigmotropism is another. If grass or other debris 

 is present in abundance, the isopods usually collect in contact with 

 the inanimate matter rather than piling up in great isopod aggrega- 

 tions. There may be some collections due to positive chemotropism, 

 for these aggregations cause measurable differences in their chemi- 

 cal environment. 



I was much impressed, in all the observations made upon these 

 groups, by the fact that so large a part of the formation of the aggre- 

 gations could readily be explained on the basis of individual tropistic 

 reactions to environmental stimuli largely produced independently 

 of the massed isopods themselves. Relatively few of the causes of 

 aggregation were left to be explained by the reactions due to social 

 appetite. In this respect the situation is wholly similar to that found 

 with land isopods, with Ophioderma, and with Szymanski's cater- 

 pillars. Again the main social trait exhibited appears to be that of 

 tolerance for the presence of many other individuals in a limited 

 space where they have collected, or — one might almost say — where 

 they have been collected. The same idea can be expressed by saying 

 that almost the sole social trait exhibited is immunity to injurious 

 effects resulting from the presence of many others in a limited 

 amount of space. It is interesting to note that there were also leeches, 

 snails, and other animals collected in the same location and, to a 

 large extent, by the same combination of physical forces and tropis- 

 tic reactions that had brought the isopods together. 



GYRINID BEETLES 



The reactions concerned in the formation and maintenance of two 

 more complex, more closely integrated types of aggregations have 

 also been made. In the case of the gyrinid or whirhgig beetles, giant 

 aggregations may occur on the surface of streams or of still water, 



