There are around us, even in the midst of our cities, insects known 168 under the name of Scolyti, which attracted much attention a few years ago. The trees by the side of the high roads, and even those of our boulevards, were attacked by them, and it was feared for a time that it would not be possible to arrest this new plague, which appeared simultaneously with the oidium of the vine and the parasite of the potato. The boulevards of Brussels were planted with fine elms, and these trees were disappearing one after another. The seeds of this plague were also sown in France, in the environs of Paris. Mons. Eug. Robert had paid attention to it, and had announced to the Académie des Sciences a remedy to arrest the evil. The regency of Brussels invited Mons. Eug. Robert to come and put in practice the means which he had recommended to destroy the scolyti; but, if I remember rightly, the death of the trees quickly followed that of the scolyti. Nature, instead of employing pitch to arrest this plague, has simpler and more expeditious means; these are, to bring forward an insect equally small, which multiplies sufficiently to keep the terrible Scolytus under. Such is the part which has devolved on the _Bracon iniator_. It simply lays its eggs in the bodies of the larvæ of the scolyti, and destroys them. Wesmael has related a curious fact of this kind, concerning this enemy of our plantations. These little people can be well trusted to manage their own affairs. Each of these hymenoptera ascertains with an admirable instinct the place where the larvæ of the scolyti are to be found, and with its long flexible ovipositor darts an egg into the body of its victim. It is not only caterpillars which are assailed by mortal enemies; the 169 eggs themselves are watched by some hymenoptera, which pierce the shell, and lay within it their own eggs. When the larvæ are hatched, the yolk and the young tissues of the legitimate proprietor serve as rations for the usurper. In this manner, the _Ophioneuri_ live, in their larva state, in the egg of the _Pieris brassica_, the cabbage butterfly so abundant in our gardens; without this police establishment they would multiply immoderately, and our kitchen gardens would suffer still more from the ravages of these caterpillars. It is in vain for insects to lay their eggs in the middle of fruits, or in the substance of a leaf or a branch; there will be always some hymenopterous insect which, guided by its marvellous instinct, will pierce them with its ovipositor, and reach them without their even perceiving it.