BUTTERFLIES OF THE DISTRICT OF COLUMBIA 45 



The facts that seem significant are that Pieris 'protodice, Papilio 

 polyxenes, Pyra?neis atalanta, and P. virginiensis were exceedingly 

 common, the dwarf spring form of Papilio philenor and the spring 

 forms of P. glaucus were unusually common, and Golias philodice 

 and Pieris rapae were very scarce, while Golias eurytheTne was not 

 to be found anywhere. 



Later in the season conditions gradually changed, and the rela- 

 tive abundance of the different species became about the same as in 

 other years. After the middle of June the numbers of Pieris proto- 

 dice rapidly diminished, and those of P. rapae rapidly increased, so 

 that at the end of the season the latter was abundant and the former 

 scarce. Golias eurytheme appeared in small numbers at the end of 

 June when G. philodice was common, and at the end of summer had 

 become very abundant, far outnumbering the latter. Pyrameis 

 cardui appeared in the middle of June, but did not become common ; 

 P, virginiensis and P. atalanta remained common throughout the 

 summer. 



The difficulty of collecting a complete representation of the butter- 

 flies in any region is well illustrated by the fact that in the spring 

 and summer of 1931 I secured no less than five species — Incisalia 

 augustimts, I. hemnci, Thanaos hrieo, Premies ocola, and Phoehis 

 eubule — that previously I had not taken. 



PRESSURE OF POPULATION 



The effect of pressure of population among the butterflies is a 

 subject worthy of intensive study. It is well known that increase 

 beyond a certain point in the numbers of very many, if not of most, 

 types of animals induces more or less profound modifications in what 

 are regarded as the normal habits. 



Pressure of population is usually looked upon as resulting either 

 from an increase in the numbers of a given species to a point where 

 the food supply is endangered or from a reduction in the quantity 

 of the available food following a drought or comparable adverse 

 circumstance. 



But among most of our local butterflies the effects of pressure of 

 population are first evident long before there can be any question of 

 real danger to the food plants of the larvae. Furthermore, the effects 

 are observable only in the males, whereas the menace to the food 

 plants comes from the unaffected females. 



In many species of butterflies the males, if they increase beyond a 

 certain number, seem to be unable to live together peacefully in the 

 presence of females, although they are perfectly well able to live 

 together if they are in areas devoid of females. This is well illus- 

 trated by our mud-puddle and roadside butterflies. 



