“In the body of every animal are certain organs more indispensable than others for the maintenance of life, and if they are injured, even though slightly, death follows. Such are the heart and brain in the higher animals. In a caterpillar’s body there are, it is true, no heart or brain like those of animals higher in the scale; but there are analogous organs just as necessary to the ongoing of the vital functions. If the ichneumon’s larvæ, in eating their victim’s flesh, were to injure these vital organs, the caterpillar would quickly die, and the larvæ would perish too, for they must have fresh meat, not decayed flesh. It is a question of life or death to them whether the grubs avoid biting in the wrong place or not. The caterpillar must live in order that they may live; it must prolong its miserable existence until they are ready for their metamorphosis. So the little parasites scrupulously respect any organ indispensable to the maintenance of life and feed on the rest; guided by instinct, they distinguish admirably between what they may attack and what they must let alone. A time comes, however, when because of their approaching transformation they do not need to exercise further self-restraint, and they accordingly devour the parts until then left intact. Thereupon the caterpillar dies, being reduced to an empty skin which the larvæ promptly abandon in order to spin their cocoons and turn into nymphs and finally into ichneumons. “Sometimes the caterpillar is spared until it has shut itself up in a case and turned into a chrysalis, with the result that the larvæ inhabiting the caterpillar find themselves, without any labor on their part, provided with snug quarters for the winter. Out of every such wormy chrysalis, consumed to the skin, there emerges in the spring, not a butterfly, but a swarm of ichneumons.” “Last autumn,” said Jules, “I found in the garden a large brown cocoon, and I hoped to see a beautiful butterfly come out of it; but this spring, to my great astonishment, out came a swarm of little flies.” “What you took for flies was a brood of ichneumons. Yet there are flies, real flies, that lay their eggs in the bodies of caterpillars, just as the hymenopterous butterflies do with their terebra.” “With their strange way of living on other creatures,” observed Louis, “ichneumons must destroy lots of caterpillars.” “They destroy so many that often, if you take a hundred caterpillars haphazard from cabbages or other growing vegetables, you will find not more than two or three that are sound and in a condition to undergo metamorphosis.” “Can those that have eggs in them be told from the rest?” asked Louis.