672 THE BUTTERFLIES OF NEW ENGLAND. 



allied to our sulphurs, constructs a web, first noticed by Hardy, which is 

 nearly as close as parchment. With rare exceptions, all butterfly cater- 

 pillars feed upon the outside of plants ; but there a few which live in the 

 interior, and one of these, an Indian species of Lycaenidae, is known to 

 be social, living in numbers within the fruit of the pomegranate. 



Among our own butterflies, there is nearly every gradation from brief 

 and partial companionship up to a social life which lasts throughout the 

 entire period of larval existence. The weakest form of social life is found 

 in some of the Polygoniae (others being purely solitary), where the eggs 

 being: often laid in columns of from two to nine, or several eg^ffs beings 

 scattered by the mother upon one leaf, the caterpillars in earliest life are 

 naturally found feeding upon one leaf. Rarely are more than four or five 

 found in company, and each takes up its independent position upon the 

 leaf and acts as if the others were not present. As, however, it is their 

 habit to remain upon the leaf until it is almost eaten, they naturally 

 leave it at the same or nearly the same time, and, following a similar 

 instinct, are apt to pass together to the nearest leaf, but scatter more or 

 less, so that by degrees as they approach maturity they are found widely 

 separated from each other. Yet even in this weakest form their numbers 

 are often so great upon a single plant that when they leave it for pupation, 

 the chrysalids hang almost in company, thirty or forty spinning their 

 silken shrouds in such proximity that they may be pulled down together. 

 A somewhat similar or perhaps weaker case may be found in Pieris rapae, 

 which often lays a considerable number of eggs singly upon one plant 

 and the caterpillars, naturally seeking the interior of the cabbage head, 

 may often be found in close proximity. But this e^en more than the 

 preceding is a case of mere accident, from the nature of the food plant 

 upon which they subsist. In all other cases of social life among our 

 caterpillars the eggs are laid by the parent in decided clusters. The 

 slightest of these is probably that of Laertias philenor, the masses being 

 ordinarily confined to a dozen or so. The caterpillars in this case not 

 only feed in company but, in earliest life at least, range in rows along the 

 edge of the leaf they are eating, with their heads toward the eaten portion ; 

 and in this way they live during at least the earlier half of their lives, scat- 

 tering more or less after the third moult upon separate leaves, so that at 

 maturity rarely more than one is found upon a single leaf, though the leaf 

 of their food plant is exceptionally large. 



This alteration of habit from companionship to solitariness is a natural 

 incident due to growth. Up to the end of the third moult the size of the 

 caterpillar has not increased enough to make it a conspicuous object, but 

 by the time the third moult is passed, the caterpillar is half grown, and 

 during this stage and the next its size becomes an important element in 

 its security ; and this alone is suflficient to account for the fact that mature 



