Ichneumons are therefore decidedly parasitical at this first period of their life. As they approach maturity, the time of which varies 165 more or less according to the species, each ichneumon takes his departure, seeks for booty on his own account, and passes through the last stages of his existence at full liberty in the open air. Nothing is more beautiful than this insect in the plenitude of its life. The species of the ichneumon are very numerous. Mons. Wesmael has devoted a part of his life to the study of these insects. We often ask ourselves what can be the use of these little creatures--what good purpose can be effected by vermin which annoy everybody? Michelet replied to this question when he wrote "The Insect." "Birds," says the brilliant historian, "prefer to destroy those insects which are the most injurious." We may say the same of those which we are now considering. The most common caterpillar, and that which is the most dreaded on account of its great fecundity, is precisely that which is more eagerly sought by the greater number of ichneumons. No less than thirty-five kinds of these little assassins fall on certain species, to make them serve as a quarry to be given to their young ones. The _Bombyx pini_ is one of the most dangerous and destructive insects in our woods. The ichneumons would seem to take into consideration the too great fecundity of this moth, and instead of one species, as is often the case, thirty-five different species direct their attacks upon it. It would be indeed difficult for the mother to withdraw her young ones from the ovipositors of so many enemies, but there will be always enough of them remaining to keep up the balance in this little world; the greatness of the danger with respect to plants will be counterbalanced by the number of ichneumons which arrest the propagation of the caterpillars. These insects 166 contribute more effectually to the destruction of caterpillars than all the means employed by man. To arrest the Pyralis of the vine, its cultivators encourage the little Chalcis (_Chalcis minuta_); and it has lately been recommended to introduce the acarus which attacks the _Phylloxera_, in order to lessen the number of this new pest. Do not aphides also prevent the too rapid development of certain plants? and the black species which lives on Windsor beans has doubtless suggested to the gardener that he ought to cut off the head of the plant when the flowers appear.