BUTTERFLIES OF THE DISTRICT OF COLUMBIA 127 



In size also the two sexes are very nearly equal, the males averag- 

 ing very slightly larger. Dwarfs are frequent among females, but 

 rare among males. In length of fore wing 64 males taken on Sep- 

 tember 18-20, 1930, measured from 48 mm. to 58 mm., averaging 52.8 

 mm., and 54 females measured from 40 mm. to 57 mm., averaging 

 51.7 mm. With four dwarfs omitted, the females averaged 52.3 mm., 

 or only 0.5 mm. less than the males. The close correspondence in 

 the size of the two sexes is shown by the fact that 12 of each had the 

 fore wing 50-51 mm. long, 12 of each had it 52 mm. long, and 9 of 

 each had it 54 mm. long; three males and 2 females had the fore 

 wing 57-58 mm. long. 



Range. — Originally this was an American butterfly, occurring from 

 Nova Scotia, Hudson Bay, Saskatchewan, and British Columbia — in 

 all these districts being of casual appearance late in summer — south- 

 ward throughout North and South America, including most of the 

 West India Islands, to Patagonia. 



Shortly after the introduction of the milkweed into the Hawaiian 

 Islands this butterfly appeared, about 1845 or 1850, and in 1857 it 

 was found on Ponape in the Carolines, not long after the first intro- 

 duction of the milkweed into that island. About 1860 it was first 

 seen on the Marquesas, in 1863 it appeared in the Tonga Islands, in 

 1867 on Tutuila in the Samoan group, and in 1869 on Upolu and 

 Savaii in the same group, and also on Roratonga. It reached Lord 

 Howe Island in 1870, Clarence River on the opposite coast of Aus- 

 tralia in 1871, Melbourne, Australia, in 1872, and the Society Islands 

 between 1870 and 1872. It soon became abundant over the greater 

 part of the Pacific Islands and westward as far as the Andamans, 

 extending southward to northern New Zealand and Tasmania, and 

 northward to Formosa (Taiwan), where it is one of the commonest 

 butterflies. Eastward across the Atlantic it has spread to Ascension 

 Island and the Canaries, has been found in the Azores, and has been 

 rather frequently reported from the coascal region of western 

 Europe — Gibraltar, Portugal, France, and southern England. 



It was probably first carried to these places by ships on which it 

 happened to alight when more or less exhausted from a long flight 

 over the water. 



The typical northern form of this butterfly, which is the one which 

 has extended its range to the Old World, occurs southward into Cen- 

 tral America. In Central America, the West Indies, and northern 

 South America it is represented chiefly by the form or subspecies 

 nigrippus^ in which the large buff spots near the apex of the fore 

 wings are replaced by white and the two innermost are reduced in 

 size or absent. Over most of South America it is replaced by the 

 subspecies erippus, which lacks the dark inner (or lower) border of 

 the fore wings and in which the veins on the underside of the hind 



