THE AMERICAN ENTOMOLOGIST. 



gregate and migrate in vast bevies, and 

 numerous instances in several different 

 Orders of insects might be cited. It has 

 been especially noticeable in butterflies 

 and moths, however, and European papers 

 have lately very freely recorded the extra- 

 ordinary abundance of Vanessa cardui and 

 Plusia gamjna, the flights and movements 

 of which were a marked feature of the year 

 1879, and phenomenal. Such occasional 

 migratory movements are beyond doubt a 

 result of excessive multiplication, due to 

 unusually favorable conditions for the de- 

 velopment of the species, but the conviction 

 has been of late years forcing itself upon 

 our minds that, with some butterflies, there 

 are regular annual migrations that are more 

 to be likened to those of birds of passage 



[Fig, 



toward the south and southeast in the Fall 

 of the year, and in the very opposite direc- 

 tion the ensuing spring. In the vast plains 

 and prairies lying to the north between the 

 Mississippi and the Rocky Mountains, upon 

 the richer parts of which milk-weeds abound 

 and Danais archippus more particularly 

 flourishes and multiplies, there is a very 

 general want of such protecting forest as 

 will permit of hibernation, even if the but- 

 terflies could withstand the severe winter 

 of the sub-boreal zone, in which they may 

 be seen in such large numbers during the 

 summer. The more densely timbered re- 

 gions to the south and southeast, as well as 

 the milder winters, undoubtedly offer more 

 favorable hibernating conditions, and we 

 believe that there is an instinctive move- 



Danais archipi 



than to the more erratic and irregular in- 

 sect-flights alluded to. 



The same laws which govern the move- 

 ments of the Rocky Mountain Locust and 

 cause it to move southward and southeast- 

 ward during the latter portion of the grow- 

 ing season, and its issue to return in the 

 opposite direction in spring and early 

 summer, seem also to govern some of our 

 more widely distributed butterflies. Num- 

 erous accounts of the flight in swarms of 

 the common Milk-weed butterfly {Danais 

 archippus^ have been published, and, after 

 collecting all accounts that we have been 

 able to during the last ten years, of the 

 movements of this butterfly in the Missis- 

 sippi valley, the fact becomes apparent 

 that there is an instinctive movement 



;s (after Riley). 



ment toward these more favorable regions — 



a movement quite independent of the fact 



that the prevailing winds in late summer 



and autumn aid it. 



This belief is confirmed by the fact that 



during the winter vast swarms of these 



butterflies are seen congregating in the 



Southern States. Mr. R. Thaxter of New- 



tonville, Mass., gives, in a recent number of 



the Canadian Entomologist^ an interesting 



account of their thus congregating in Ap- 



alachicola in Florida, in pine groves. 



He says : 



"The trees were literally festooned with but- 

 terflies within an area of about an acre, and they 

 were clustered so thickly that the trees seemed to 

 be covered with dead leaves; fig. 35 . will ena- 

 ble the reader to form some idea of their appear- 

 ance thus grouped. Upon shaking some of th- 

 trees a cloud of butterflies flew off, and the flape 



