2s6 THE INFANCY OF ANIMALS 



of nest-building. These are " instinctive " acts. But 

 what is instinct i 



Mention has already been made in this chapter of the 

 caterpillar's most formidable foe, the ichneumon fly ; 

 and it would be well therefore, in conclusion, to give a 

 brief survey of the fearsome r6le this insect plays. 



The adult ichneumon is a four-winged fly — one of the 

 Hymenoptera — -and goes about seeking dihgently for 

 caterpillars. The success or failure of the quest is all- 

 important both to the seeker and the sought. Should 

 success attend its search, it alights gently on its victim, 

 and, with an instrument which is at once stiletto and 

 ovipositor, it proceeds to thrust down into the body. 

 Once the skin is pierced an egg is passed down the stiletto, 

 which is hollowed on purpose, into the body of the cater- 

 pillar. The process may be repeated many times ; the 

 caterpillar, writhe though it may, is powerless. 



But this is only the beginning of the trouble. Speedily 

 the egg, or eggs, according to the species of ichneumon, 

 hatch out, and the larvae begin their diaboUcal work 

 of feeding in the interior of the caterpillar, yet contriving 

 to touch no vital part. Were the daughters of the horse- 

 leech ever so bloodthirsty ? In some cases these tiny 

 fiends complete their course of development before the 

 caterpillar has run its allotted course, and eat their way 

 through the body of their host and spin for themselves 

 cocoons of golden silk outside. In others the caterpillar 

 completes its growth, and assumes the chrysalis, or pupa 

 stage, as the case may be. But it goes no further. The 

 ravening wolves within complete their work and leave 

 but the empty shell of the chrysalis, which forms their 

 shelter for the winter. With the warm days of spring 

 the chrysalis case yields up, not the body that was to 



