54 BULLETIN 15 7, U. S. NATIONAL MUSEUM 



should have witnessed, instead of the departure of a small number 

 of individuals, the origin and exodus of a migrating swarm. 



On the other hand, we were unable to detect in the butterflies as 

 we observed them after their emergence at Cabin John the slightest 

 departure from the normal habits of individual butterflies as they 

 occur throughout the summer. As in the case of many other species, 

 the first few days after emergence are spent by the milkweed butter- 

 fly in intensive feeding, while the later adult life is spent mostly in 

 wandering and in reproduction. Whenever this butterfly is common 

 it becomes sociable, and throughout the summer individuals are 

 occasionally seen high in the air, and also in trees. 



If the multitudes at Cabin John were developing just as the same 

 number of individuals scattered throughout the summer would have 

 developed, then it becomes evident that the so-called migi'ations of 

 this species are really nothing more than the collective or mass 

 expression of the normal habits of each of the individuals taking 

 part in the migration. A very large number emerging from the 

 pupa at the same time engorge themselves together and reach the 

 traveling and tree-perching phase of their adult life simultaneously. 

 So we find in the air large numbers together instead of isolated 

 individuals. The appearance of this insect in swarms means, I 

 believe, not that they have intentionally gathered for the purpose 

 of traveling, but simply that something has occurred in their place 

 of origin to cause great numbers to emerge simultaneously. 



Two migrations have been reported in this vicinity (see pp. 

 121, 122). In one the butterflies were flying south across Chesa- 

 peake Bay in the face of a stiff breeze, and in the other they were 

 going north with the wind. Those that we saw were going west with 

 the wind. 



Most of the migrations of this butterfly that have been reported 

 have been southerly, and this is especially true in the central por- 

 tion of the continent. It is most unlikely that this southerly direc- 

 tion has more than a coincidental relation to geography. It is far 

 more likely that it will be found to be a function of the prevailing 

 wind plus certain geographical features, especially rivers and the 

 seacoast. 



In short, the migrations of the milkweed butterfly appear to be 

 simply a mass expression of the normal habits of each separate indi- 

 vidual in a migrating swarm, and the relation of these flights to the 

 south commonly noticed seems to rest solely upon the mechanics of 

 autumnal meteorology and to have no other relation to the pro- 

 cession of the seasons. 



It IS generally assumed that early in spring there is a northerly 

 migration of this butterfly from the territory in which it is able to 



