232 [September, 1914. 



The " Means of Dispersal " of Danaida plexippus. 



We now come to the consideration of the agencies by means of 

 which our butterfly has been enabled to accomplish its voyages across 

 the ocean, and in so many cases to found successful and nourishing 

 colonies in regions far remote from its present head-quarters. This 

 subject was discussed by Mr. W. L. Distant in a very able and 

 intei-esting paper entitled " The Geographical Distribution of Danais 

 Archippus" (Trans. Ent. Soc. Lond., 1877, pp. 93-104), not long 

 after the wanderings of the insect had begun to attract the attention 

 of entomologists. 



Like myself, Mr. Distant is strongly disposed to believe that it is 

 quite within the power of Danaida plexipptis, aided by favourable 

 circumstances of wind and weather, to cross even the wider parts of 

 the great oceans. He writes (I.e., p. 95) : " That the winds alone are 

 a great agency of dispersal with the Lepidoptera cannot be doubted 

 with the number of authentic records we have of butterflies and moths 

 being found far at sea and great distances from the nearest land ; 

 and it will readily be understood that butterflies of high and strong 

 flight, especially when at certain seasons they become gregarious, must 

 have a greater chance of being borne on the wings. of a strong wind or 

 carried away by a sudden gale than those insects whose flight is low, 

 or whose habits are more solitary and secluded. Now we know that 

 D. Arcliippus has a powerful flight, often sailing with wings expanded 

 high in the air ... . We also learn from Mr. Riley that the butter- 

 fly appears in large bevies or flocks almost every year in some part or 

 other of the west .... Mr. Riley further remarks, 'this assembling' 

 in large flocks at a considerable height would always be a source of 

 danger to them, when overtaken by a gale of wind, when there would 

 be a great probability of their being carried out to sea ' . . . . Winds 

 alone can scarcely be considered the only cause of the migration of 

 butterflies over large areas of sea, though of the highest efficacy for 

 the purpose. When met with far away from land they seem glad to 

 alight on the vessel for rest, and no doubt vast numbers must succumb 

 to the waves ; for apart from the vigour required to sustain such a 

 long flight, the spray in rough weather would sufficiently damp their 

 wings to prevent any onward progress, and very rarely would one be 

 able to survive the long flight across the breadth of the ocean." 



Whatever might be the fate of the butterfly attempting to alight 

 on the surface of the sea when at all rough, the following observation 

 by Dr. Seitz (Macro-Lep., Palsearctic Region, I, p. 77), "I very often 



