288 GENERAL BIOLOGY 



caterpillars of various moths and butterflies. Eggs 

 are laid within the body of the living caterpillar, 

 which is frequently packed full of the resulting larvae. 

 These go through a rapid development and, working 

 their way through the outer skin, emerge from the 

 caterpillar and spin a cocoon on the outside. Others 

 wait until the caterpillar has become a chrysalis, 

 and pupate within the pupal case of the host. There 

 emerge in due time, not a butterfly, but a number of 

 parasitic wasps. Not infrequently the parasites 

 themselves are parasitized, and even these secondary 

 parasites by tertiary ones. In some cases it has 

 been discovered that the original parasitic wasp may 

 lay but one egg in the larval host, but that this egg 

 fragments into scores of others (polyembryony) , 

 each of which develops a perfect parasite. In these 

 forms the parasitic condition is passed during the 

 developmental stages, and the mature wasp, using 

 all its senses, shows no trace of degeneracy. There 

 is nothing degenerative about parasitism per se, but 

 the condition is merely a form of " adaptation," an 

 economy of nature resulting in simplification of 

 structure through the loss of useless parts. 



Sacculina. An extreme case of the degeneration 

 incident to parasitism is found in a crustacean, 

 (Sacculina), parasitic upon several species of crabs. 

 In its mature condition this creature has the form of 

 a bag, which is found attached to the abdomen 

 of its host, the crab. The bag is full of eggs which 

 hatch into typical crustacean larvae. It has been 



