GENERAL FACTORS CONDITIONING AGGREGATIONS 71 



tion from flooding, and other benefits, may accrue from the cluster 

 formation of hibernating ants. 



In many cases these over-wintering groups are essentially shelter 

 aggregations, apparently due to the small amount of serviceable 

 shelter available. Often, however, all the apparently equally desir- 

 able space is not occupied, so that the aggregation cannot be entirely 

 explained on the basis of unavoidable crowding. In other cases 

 Holmquist has been unable to find any environmental differences to 

 account for the location of the hibernating aggregation. These 

 groupings are partially under temperature control; but, as with other 

 phenomena connected with hibernation, the temperature control is 

 incomplete, and the problem of the exact nature of the causal factors 

 remains open, 



AESTIVATION 



Aestivating aggregations have been less studied. Land isopods 

 will form aestivating groups which may be either homotypic or 

 heterotypic. Dr. C. H. Abbott has informed me personally that they 

 collect in large numbers in protected places, and so pass the long, 

 hot, dry summer of southern California. 



AGGREGATIONS CONTROLLED BY MOISTURE 



The chief controls of the aestivation reaction of these isopods are 

 temperature and moisture. Of the two, laboratory experiments show 

 the latter to be more important (Allee, 1926). When land isopods of 

 various species are placed on air-dry filter paper, they collect in 

 bunches within a few minutes, unless the substratum is too dry, 

 when they will run about actively until at the point of death. If the 

 substratum is moist, the same isopods will remain quietly scattered. 



These relations are shown in Figure 3. In the upper picture there 

 are 25 isopods in a crystallization dish photographed 30 minutes 

 after being introduced into the cUsh, which had the bottom covered 

 with dry filter paper. In the meantime they were in a darkened 

 room and the exposure was by flashlight. The lower photograph 

 shows the effect of adding enough water to make the filter paper 

 thoroughly moist without being sloppily wet. The same animals are 

 shown as in the preceding photograph, but 15 minutes later, and 5 



