INTRODUCTION 41 



the whole of them. The parasite larva now changes to a 

 pupa which a httle later changes again to an adult fly like 

 the one that laid the parasite egg in the beginning. Of 

 course the butterfly egg never hatches into a caterpillar. 



One of the most interesting questions in regard to these 

 egg parasites is this: How does the tiny parasitic fly find 

 the newly laid egg.^^ One would think that the proverbial 

 search for a needle in a haymow would be an easy task 

 compared with that of a fly about as large as the head of a 

 pin finding a butterfly egg of similar size upon some part of 

 one of the millions of leaves upon the trees and shrubs in 

 field and forest. Yet the search is successful, as every one 

 who has tried to get caterpillars from eggs found out of 

 doors will testify. On a later page in this book, in con- 

 nection with the story of the life of the Mourning Cloak 

 butterfly, I have recorded some observations upon the little 

 parasite which seemed to have been riding around upon the 

 body of the butterfly waiting for her to lay her eggs. 



For one parasite upon the eggs of butterflies, there 

 probably are dozens that attack the caterpillars. A large 

 proportion of the butterfly larvae brought in from out- 

 doors, especially those which are half-grown or more, will 

 yield not butterflies but parasites. This is the experience 

 of practically every one who attempts to rear these in- 

 sects, and it emphasizes the value of the advice that in 

 order to get fine specimens, it is desirable to rear them 

 from eggs laid by butterflies beneath netting or in cages. 



The life-histories of the parasites that attack cater- 

 pillars vary greatly. The simplest are those of the large 

 Ichneumon flies : The mother fly lays an egg beneath the 

 skin of the caterpillar. The egg hatches into a larva that 

 absorbs the fatty parts of the body of the caterpillar. 



