46 THE SOCIAL LIFE OF ANIMALS 



such aggregations to the groupings of foreigners in 

 a strange city that result in Little Italy, or the 

 Mexican settlement, or a German quarter; and yet 

 basically some of the factors involved are similar. 

 Perhaps there is a closer connection between such 

 aggregations in the wide expanse of a clean aqua- 

 rium and the schooling tendency found among 

 many fishes of the open sea; perhaps the same phe- 

 nomenon accounts for the flocking tendency of 

 many birds, as well as mammals on the equally 

 monotonous grassy seas of temperate plains. 



A somewhat different expression of a positive 

 social reaction is shown when animals that are 

 usually more or less isolated come together and pass 

 the night grouped as though they were engaged in 

 a slumber party. This type of behavior has been 

 repeatedly described for different insects, even for 

 the wasps that remain separate to such an extent 

 that they are called solitary wasps. In some forms of 

 solitary wasps both males and females are found in 

 the sleeping group. With solitary bees, such as we 

 have near Chicago, the overnight aggregations are 

 composed of males only. A study which was made 

 of the sleeping habits of a Florida butterfly species 

 indicates that these Heliconii (69) come together 

 night after night in the same location, in part at 

 least as a result of place-memory. The assemblages 



