THE CANADIAN ENTOMOLOGIST. 135 



Mr. Riley gives an interesting account of the causes which may lead 

 to the migrations of this butterfly in his 3rd Report. He says : " It would 

 be difficult to give any satisfactory reason for this assembling together of 

 such swarms of butterflies. As I have abundantly proved by examination 

 of specimens, the individuals composing the swarms of our Archippus 

 butterfly comprise both sexes ; if anything the females prevail. The 

 flights almost always occur in the autumn, when the Milk-weeds ( Asclepias ), 

 upon which the larva of this butterfly feeds, have perished. The instinct 

 to propagate is, therefore, at the time in abeyance. The butterflies, unable 

 to supply themselves with sweets from flowers, are either attracted in 

 quantities to trees that are covered with honey-secreting plants, or bark 

 lice ; or else they must migrate southward, w^here flowers are still bloom- 

 ing. The Archippus butterfly hibernates within hollow trees and other 

 sheltered situations. Southerly timber regions offer most favorable con- 

 ditions for such hibernation. Under the most favorable conditions a large 

 majority perish. A small portion of the females survive the winter. Such 

 hibernating individuals, upon waking from their winter torpor, make at 

 once for the prairie, where the Milk-weeds most abound. Faded, and 

 often tattered, they may be seen flying swiftly over such prairies. 



" I have no doubt but that they travel thus for many hundred miles, 

 keeping principally to the north, and ere they perish, supplying the Milk- 

 weeds here and there with eggs. A fresh brood is produced in less than 

 a month, and these extend still farther north, until we find the species late 

 in the growing season as far up as the Saskatchewan country, where it can 

 scarcely successfully hibernate, and from whence the butterflies instinct- 

 ively migrate southward. We can thus understand how there are two, 

 three or more broods in southerly regions and only one towards British 

 America. 



" The exceptional flights noticed in the spring, and which, so far as 

 recorded, take place quite early and in the same southerly direction, find 

 a similar explanation. They may be looked upon as continuations of the 

 autumn flights. Hibernating in the temperate belt, they are awakened and 

 aroused upon the advent of spring, to find the Milk-weeds not yet started, 

 and they instinctively pass to more southern regions. There is a south- 

 ward migration late in the growing season in congregated masses, and a 

 northward dispersion early in the season through isolated individuals." 



It will thus be seen that Mr. Riley looks upon the migration of D. 

 archippus as something analogous to the southern movement of the birds 



