Leaving the vegetable world, we may find some very curious examples of parasitism among Insects. Every one who has paid the slightest attention to this class of animals is aware that there are slender flies called _Ichneumons_, whose grubs are hatched and reared in the bodies of other insects. Many of these have the ovipositor greatly lengthened, and projecting like a very slender needle from the extremity of the abdomen. In some species, this needle-like organ is three or four times the entire length of the body; and this great longitude is intended to reach the pupæ of wasps and similar insects which inhabit deep holes. The needle itself is well worthy of study. It is not simple, but composed of two pieces forming a sheath, which open and reveal a central finer filament, furnished at its tip (in _Pimpla manifestator_, for example) with saw-like teeth. With this instrument, which possesses great elasticity and flexibility, the insect works, as a carpenter with his brad-awl, boring through the clay, with which the wasp has closed up the hole that contains her grub, until the tip of the ovipositor reaches the soft body of the insect. Into this it pierces, and deposits an egg, and is withdrawn. The slight puncture is scarcely felt by the grub, which continues to eat and grow; the inserted egg, however, presently hatches, and produces the ichneumon-grub, which begins to feed on the fat of the wasp-grub, instinctively avoiding the vital parts, until the latter has attained nearly its full size, and is ready to pass into the pupa state; when, its vigour being gone, it fails to accomplish the metamorphosis, the insidious intruder, now also full grown, taking its place, and by and by issuing from the hole a perfect Ichneumon. How often has the enthusiastic young entomologist been subjected to sore disappointment by the parasitic habits of these _Ichneumonidæ_! He has obtained some fine caterpillar, a great rarity, and by dint of much searching of his Westwood or his Stainton, feels quite certain that it is the larva of some much-prized butterfly. He ascertains its leaf-food; which it eats promisingly; all goes on encouragingly. Surely it cannot be far from the pupa state now! When some morning he is horrified to behold, instead of the chrysalis, a host of filthy little grubs eating their way out of the skin of his beautiful caterpillar, or covering its remains with their tiny yellow cocoons. Some of these parasites are so minute that their young are hatched and reared in the _eggs_ of other insects. Bonnet found that the egg of a butterfly, itself no bigger than the head of a minikin pin, was inhabited by several of the stranger grubs; for out of twenty such eggs, he says, "a prodigious quantity" of the grubs were evolved.