INTRODUCTION 17 



ably is Nature's way of providing for an extended food 

 supply for the succeeding generations. That it results in 

 the death of the great majority of the migrants is doubtless 

 true, but it must lead to vast experiments in extending 

 the geographic area inhabited by these species. Numer- 

 ous examples of such migrating swarms will be found in the 

 pages of this little book. {See plates, pages 1 7, 48-49, 1 60.) 



The migrations thus considered are only exceptional 

 occurrences. There is, however, a regularly recurring 

 annual migration on the part of some butterflies which is 

 also a phenomenon of extraordinary inte^^est. The most 

 notable example is that of the Monarch which apparently 

 follows the birds southward every autumn and comes north- 

 ward again in spring. There is much evidence to indi- 

 cate that in some slight degree other butterflies have a 

 similar habit, although the present observations are in- 

 adequate to determine to what extent this habit has be- 

 come fixed in most of these species. 



Hibernation or Winter Lethargy 



The ways in which butterflies spend the winter are al- 

 ways of peculiar interest to the naturalist. Here are 

 creatures with four distinct stages of existence, each of 

 which has the possibihty of carrying the species through 

 the season of cold. It is necessary to learn for each in- 

 sect which stage has been chosen for the purpose, and if 

 possible to find the reasons for the choice. 



As a rule the related members of a group are likely to 

 hibernate in a similar stage. Thus most of the Swallow- 

 tails pass the winter as chrysalids while practically all the 

 Angle- wings pass the winter as adults. This rule, how- 



