SOCIAL CATERPILLARS 129 



social life is found in some of the Polygoniae 

 (others being purely solitary), where the eggs 

 beino' often laid in columns of from two to nine, or 

 several eggs being scattered by the mother upon 

 one leaf, the caterpillars in earliest life are natur- 

 ally 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 nmubers 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 the Cabbage butterfly 

 (Pieris rapae), which often lays a considerable 

 number of eggs singly upon one plant, and the 

 caterpillars, naturally seeking the interior of the 



