806 THE CAUSES WHICH 



were uninterrupted from an early hour in the morning until 

 sunset. All the individuals which resort to the margin of sandy 

 beaches are of the male sex." 



Sir Emerson Tennant confirms the migratory nature of this 

 genus in his work on Ceylon. '^ Butterflies of large size and 

 gorgeous colouring flutter over the endless expanse of flowers^ 

 and at times the extraordinary sight presents itself of flights of 

 these delicate creatures^ generally of a Avhite or pale yellow hue, 

 apparently miles in breadth, and of such prodigious extension 

 as to occupy hours, and even days, uninterruptedly in their 

 passage.'''' The butterflies seen in these wonderful migrations 

 in Ceylon are mostly two kinds of CaUidryas, with straggling 

 individuals of the genus Eiqdaa. Their passage takes place in 

 April and May, generally in a north-easterly direction. The 

 testy, jealous nature of insects is probably one source of the 

 heterogeneous nature the migratory flocks present. This we 

 may see in the Painted Lady, who, when smming, arises to buffet 

 and commingle with everything that approaches, and darts even 

 after an obnoxious bird. Darwin encountered one very motley 

 migrating swarm at sea off the north of Patagonia. " One even- 

 ing,^^ ^ he says, " when we were about ten miles from the Bay 

 of San Bias, vast numbers of butterflies, in bands or flocks of 

 countless myriads, extended as far as the eye could range. Even 

 by the aid of a glass it was not possible to see a space free from 

 butterflies. The seamen cried out ' it was snowing butterflies,' 

 and such in fact was the appearance. More species than one 

 were present, but the main part belonged to a kind very similar 

 to, but not identical with, the common English Clouded Yellow. 

 Some moths and bees accompanied the butterflies, and a fine 

 Calosoma flew on board. The day had been fine and calm, and 

 the one previous to it equally so, with light and variable airs. 

 Before sunset a strong breeze sprung up from the north.'' 



As the migration of butterfly flocks appears to originate when 

 the air is light, and the direction to be ruled by the coming 

 breeze, and since Lepidoptera that do not thus band from their 

 large wing-expanse are also influenced by the prevailing winds, 



■* Darwin, " Voyages of the Adventure and Beagle,'' p. 185. See also 

 Zoologist, 1846. In Ceylon the migrations start from the east coast on the 

 setting in of the north-east monsoon, and travel north to Calcutta. 



