Ichneumon Fly
a, female fly; b, enlarged abdomen of female; c, enlarged abdomen of male; d, enlarged portion of wing.
“No, they were doing something better than that. The larvæ of ichneumons live in the bodies of other larvæ, which they devour little by little without killing them until the very last. They are carnivorous larvæ: they must have fresh meat that is renewed about as fast as they eat it. The ichneumons Louis speaks of were engaged in depositing their eggs in the bodies of plump worms that live in the wood of the tree and turn into beetles.”
“But these beetle-worms,” Jules pointed out, “were not in sight. They were under the bark and perhaps in the wood itself.” [[351]]
“The ichneumon does not need to see them to know where they are.”
“Does it hear them then?”
“No more than it sees them. The worm stays quietly in its little tunnel, being careful not to make any noise that will attract the attention of its enemy.”
“Then the ichneumon must at least smell them.”
“That is very doubtful. A live larva has no odor. And, besides, the most difficult part is not to find out whether or not a suitably plump larva is there under the bark, at such and such a depth; it is also necessary to ascertain whether another ichneumon egg has not already been laid in the body of the coveted worm, as one larva would be insufficient for two nurslings. An egg deposited in some fat worm is not seen, heard, or felt; that is very evident. Nevertheless the ichneumon never plunges its terebra through the bark into a larva already occupied. What guides it? I do not know; nobody knows. Instinct has ways and means unsuspected by our wisdom. An ichneumon alights on the trunk of a tree. The perfectly healthy bark betrays to the sharpest eyes no sign of what the hymenopter is after. Never mind; the insect soon satisfies itself as to whether or not the place is a good one for its purpose. It makes a careful examination, tapping with its antennæ and keeping up a continual vibratory movement. A spot is chosen. The insect plants itself firmly on its legs, draws up its abdomen, and, holding its terebra in a vertical position, [[352]]plunges the point of it into an imperceptible fissure in the bark. The little auger pierces the bark, though not without effort and occasional pauses, for there are difficulties to be overcome; nevertheless it goes as far as its length permits. The end is attained; the point of the instrument penetrates the flesh of the worm hidden under the bark. The egg being deposited in the incision thus made, the insect withdraws its blade carefully, so as not to break it, and proceeds to lay eggs in the bodies of other larvæ.”