56 BULLETIN 15 7, U. S. NATIONAL MUSEUM 



The scarcity of this species in spring over most of the United 

 States is probably simply the natural result of the immense mortality 

 suffered during the winter in northern latitudes. In the case of most 

 butterflies there is an enormous mortality in each generation resulting 

 from the activities of parasites and predators among the eggs and 

 younger stages. In this form the winter seems to be the chief factor 

 in reducing its numbers. 



Before leaving the subject of the milkweed butterfly, it may be 

 recalled that the abundance of this insect late in the summer of 1930 

 was preceded up to the very day of its appearance in great numbers 

 by unusual scarcity. The appearance of the milkweed butterfly in 

 great numbers following a season in which it has been exceptionally 

 uncommon has already been noted. Mr. Scudder wrote that Miss 

 Harrington saw it in abundance at Magnolia, Mass., in the autumn 

 of 1888, and that he had found it abundant at Hampton, N. H., not 

 far away, at the same time, but it had been exceptionally scarce in the 

 spring of that year. 



Quite different in their nature from the migrations of the milk- 

 weed butterfly are the migrations of the pierids, in which the migrat- 

 ing flocks are composed wholly, or almost wholly, of males. 



It has already been suggested (pp. 45-47) that the little groups of 

 male pierids found about puddles late in summer are evidence of 

 overcrowding in the home territory. Puddle butterflies, at least in 

 the pierids, are young males freshly emerged and in need of food, 

 or at least of water. They are unable to feed quietly in the fields 

 because of the constant persecution by the older males, so they escape 

 to areas where the food plant and females are absent, whenever 

 possible gathering on mud from which they can obtain in peace the 

 water that they need. 



Many, perhaps most, of the young males are only temporary exiles, 

 returning to the fields when they have sufficiently matured and as 

 the older males die off. But it is quite probable that many of them 

 would be unable to find a place for themselves in the fields and 

 would therefore be permanently exiled. Such males would be very 

 likely to wander — the males wander in any case — and in their 

 wandering they would presumably fly in a more or less straight line 

 against the wind, which is the usual habit of the individuals males 

 of our common species when simply traveling. Late in summer the 

 conspicuous males of Phoehu eubule are occasionally noticed in the 

 Smithsonian Institution and Department of Agriculture grounds 

 flying eastward against the wind. 



It is a reasonable assumption that surplus males of our two local 

 pierids most given to migratory flights — Phoehis eubule and Eurema 

 lisa — would normally wander away, flying primarily against the 

 wind. Being gregarious and very readily decoyed — even by crude 



