But this is only one of the many dangers to which caterpillars are exposed. Throughout every hour of the day the sharp and hungry eyes of the numerous insect-eating birds are searching the leaves for such delicacies to satisfy the wants of themselves and their broods. The lively little lizards, too, during the sunny hours are busily engaged in searching them out among the foliage of heaths and banks.
Very formidable enemies also exist in the form of Ichneumon and other species of flies, which pierce the skins of caterpillars with their sharp ovipositors, and lay their eggs within the bodies of the unfortunate victims. As soon as the young larvæ are hatched from these eggs, they commence feeding on the fatty substance stored beneath the caterpillar's skin. They carefully avoid, at first, attacking the vital organs of their host's body, and in this way secure for themselves a more lasting supply of fresh food.
When the fatty substance is nearly all gone, they eat their way into the more important structures, of course steadily growing all the time; and so, even though the body of the caterpillar is rapidly diminishing, the total bulk shows often no very appreciable decrease in size. When the larvæ of the flies are fully fed, they either change to the pupa within the carcase of their host, or eat their way out of its body and construct for themselves a cocoon in which to undergo the transformation.
Fig. 17.—An Ichneumon Fly (Cryptus Migrator).
Fig. 18.—Another Ichneumon Fly (Pimpla Instigator).
As for the caterpillar itself, it sometimes dies before the time for its metamorphosis has arrived; but it often changes to the chrysalis before its fate is sealed. In this latter case, a number of flies, having undergone their final transformation within the chrysalis shell (there being but little else than shell remaining of the victim's body), break forth from the remains of the carcase somewhere about the time at which the butterfly or moth should have appeared.