208 SCIENCE REMAKING THE WORLD 



habit of eating any conveniently accessible pupae of 

 another species is widespread among ants. 



Finally there is another widespread type of social re- 

 lationship in the life of insects, and that is the relation- 

 ship of parasite to host, or parasitism. Thousands of 

 insect kinds live exclusively as parasites on other in- 

 sects, and in many cases a very high degree of specializa- 

 tion in this relation has been developed. One of the 

 most familiar forms of this relation is shown by the 

 many various ichneumon flies which lay their eggs on 

 or in the bodies of the caterpillars (larvae) of various 

 moths and butterflies. When the ichneumon grubs 

 hatch from these eggs they burrow about in the body 

 of the caterpillar, feeding on its tissues but avoiding 

 till the last the tissues and organs especially necessary 

 for the life of the caterpillar. So the caterpillar moves 

 about, feeding on the leaves of its food plant, while the 

 ichneumon grubs grow and develop inside of it. By 

 the time these grubs are full grown the caterpillar dies 

 or may just be able to change into a chrysalid, from 

 which issues, however, not a butterfly or moth but a 

 number of ichneumon flies. 



It is undoubtedly true that the most effective checks 

 against the too terrible increase in numbers of various 

 serious insect pests of our orchard and field crops are 

 their natural insect parasites. A considerable number 

 of our worst insect pests have come to this country by 

 one means or another from other countries, and in many 

 of these cases they have come unaccompanied by their 

 parasites. Under these circumstances the insect pest 

 has been able to increase so rapidly in this country as 



