cclxxiii 



ON MIGRATORY BUTTERFLIES. 



Many quadrupeds that multiply rapidly often acquire the migratory- 

 propensity. This is especially true of rats and lemmings, of the migrations 

 of vast numbers of which numerous interesting accounts are recorded. 

 Many insects, normally non-migratory, also exceptionally congregate and 

 migrate in vast swarms; and this is especially the case with butterflies, 

 flights of which, and particularly of the Yellows (genera Callydrias and 

 Colias) and the Whites (genus Pieris), have been reported from Equato- 

 rial and South America, and from different parts of Europe. Vast flocks 

 have also been observed at sea. The newspapers in the Southwest and the 

 Signal officers were constantly reporting the passage over Iowa, Kansas, 

 Missouri and Texas of swarms of butterflies during the months of Septem- 

 ber and October last. These consisted, in every case were determinations 

 were made, of the Archippus butterfly {Danais Archippus)., which is the 

 principal species known to thus migrate in North America. 



In an account of the swarming of this butterfly, published in 1870 (3^ 

 Mo. Ent. Rep., p. 151), I wrote as follows : 



"It would be difficult to give any satisfactory reason for this assembling 

 together of such immense swarms of butterflies. * ♦ ♦ There are two 

 significant facts connected with them from which some corollary might be 

 deduced, namely, that only those species which have a very extended range 

 are known to form such flocks, and that they always travel, under these 

 conditions, in a southerly or southwesterly direction. 



"Mr. Bates* gives an interesting account of the uninterrupted proces- 

 sion of butterflies belonging to the genus Callydrias, which passed from 

 morning to night in a southerly direction across the Amazon, and, as 

 far as he could ascertain, these migratory hordes were composed entirely 

 of males." 



As I have abundantly proved by examination of specimens since the 

 above was written, the individuals composing the swarms of our Archip- 

 pus butterfly comprise both sexes ; if anything, the females prevail. No 

 satisfactory explanation of these swarms has been given, but I think they 

 are, for the most part, due to an instinctive tendency to reach a warmer 

 country in which to hibernate, and to a failure of food in the country 

 where they developed. The flights almost always occur in the autumn, 

 when the milk-weeds (genus Asclefias), upon which alone 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 cov- 

 ered with honey-secreting plant-lice or bark-lice, or else they must migrate 

 southward, where flowers are yet blooming. All insects acquire the mi- 

 grating instinct when crowded together through excessive multiplication. 

 The Archippus butterfly hibernates within hollow trees and in other shel- 

 tered situations. Southerly timber regions offer most favorable conditions 



• Naturalist on the River Amazon, vol. i, p. 249. 



