110 ANIMAL LIFE AND SOCIAL GROWTH 



One of the most interesting of the aggregations 

 of animals is that which occurs in the coming 

 together of many species in the evening to form 

 over-night aggregations, the members of which 

 pass the night close together in a condition closely 

 resembling sleep. The formation of such sleeping 

 collections is more common than is generally sup- 

 posed. We are well acquainted with the habit 

 men have of collecting in groups in some sheltered 

 place to pass the night; travelers arrange for shel- 

 ter and in many lands the peasants return to the 

 common village to sleep after their work in the 

 fields. In a somewhat similar way the so-called 

 solitary wasps may collect to the number of some 

 hundreds in one locality. As high as a thousand 

 such have been counted in a restricted locality 

 and marked wasps have been known to return to 

 the same spot to spend successive nights for at 

 least two weeks. Males of certain solitary digger 

 bees have the same habit. 



Swarming locusts of several species are known 

 to spend the night in dense masses both as wing- 

 less nymphs and later when they become fully 

 adult, although the night clusters of the flying 

 adults are not nearly so dense as are those of the 

 nymphs. These aggregations frequently form 

 conspicuous black patches which stand out against 

 the vegetation. Butterflies representing both 



