BUTTERFLIES OF THE DISTRICT OF COLUMBIA 57 



paper imitations — other surplus males would tend to join them until 

 finally a considerable number would be assembled in a loosely gre- 

 garious swarm. The nucleus of such a swarm might come from 

 puddle butterflies, from exiled males in any area, from persecuted 

 males still in the fields, or from all three sources. 



Elimination of surplus males may take place when, so far as we 

 can see, there is no actual evidence of overcrowding in the fields, 

 especially in the case of such large, powerful, and very active species 

 as Phoebis eubule. 



The migrations of the pier ids of the types corresponding to our 

 Phoebis eubule and Eurema Usa seem to be nothing more than the 

 end product of the natural process of the elimination of surplus 

 males. 



Much has been written regarding the southern late-summer and 

 autumnal migrations of Danaus -plexippus^ but little has been said 

 of the northerly migrations of Phoebis eubule that from time to time 

 take place at the same seasons and in the same regions in the coastal 

 area. As a concrete instance of migrations in opposite directions 

 taking place simultaneously, I may cite the fact that in the very field 

 where I observed Danaus plexip'pus going west with the wind I have 

 seen the males of Phoebis eubule flying high and very fast eastward 

 against the wind. 



If the migrations of these butterflies were anything more than 

 relatively simple responses to the meteorological and other physical 

 conditions — that is, if they served any purpose beneficial to the 

 individuals taking part in them — we should scarcely expect to find 

 two primarily tropical butterflies migrating in opposite directions 

 in the same place simultaneously, especially north and south. 



The solution of the problem of butterfly migrations lies, I believe, 

 in an accurate and detailed knowledge of the normal sequence of 

 habits of both sexes of the species involved — in the case of dimorphic 

 or polymorphic species of both sexes of each of the several forms — 

 from the time of emergence from the pupa until death; of the real 

 or apparent changes in the normal habits induced by overcrowding or 

 by the destruction of the food plant ; of the relations of the individ- 

 uals of a species, particularly the males, to one another and to 

 related species in the same region; and of the relations of the indi- 

 viduals of a species to the meteorological and geographical 

 environment. 



SUGGESTIONS FOR STUDYING BUTTERFLIES 



Though in the District of Columbia, as elsewhere, a complete 

 collection of the local butterflies can be assembled only by long- 

 continued systematic search in all the various types of habitat fre- 

 quented by the species which presumably occur, a fairly representa- 



