INTRODUCTION 21 



gather together in enormous flocks that remind one of the 

 gathering of the elans with the migrating birds. This is 

 one of the least understood of insect activities but it has 

 been observed so often and over so long a period of years 

 that there seems to be no questioning the general habit. 



Like everything else in relation to living things there are 

 numerous variations in the prevailing modes of hiberna- 

 tion. In the case of many species one can find combina- 

 tions of two or more stages in which the winter is passed. 

 Probably if we could observe with sufficient care we might 

 be able to find somewhere examples of almost any con- 

 ceivable double combination — as egg with larva or chrys- 

 alis or adult — the insect hibernating in two of these stages. 

 Many examples are known in which both chrysalis and 

 adult of the same species pass the winter and also of those 

 in which young and well-grown larvae pass the winter. 

 As one would expect, the conditions as to such combina- 

 tions are likely to be more variable in southern than in 

 northern regions. 



Notwithstanding all the attention which has been paid 

 to butterfly life-histories there is still some uncertainty in 

 regard to the hibernation of many of our species. One of 

 the most interesting series of observations which a young 

 naturali^* could undertake would be to learn positively 

 how each species of butterfly in his locality passes the 

 winter. 



Aestivation or Summer Lethargy 



In some species of butterflies there is a special adaptation 

 to passing through the hottest part of the summer season 

 in a state of lethargy which is suggestive of the torpor of 



