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 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.
In the substance of those beautiful leaves of the water-lily which cover our ponds in summer, we often see a charming insect, known by the name of Agrion virgo, or damsel dragon-fly, a name given to it on account of its graceful attitudes and its elegant appearance. We observe this insect deposit its eggs with great prudence, fully persuaded that they are safe in the midst of the water; but the poor neuroptera reckons without its host. An hymenopterous insect, named Polynema, is there, watching every movement of the Agrion; and as soon as the latter has laid an egg, the Polynema darts down like a bird of prey on its victim, pierces it, and deposits its own egg in the interior. The egg of the wounded agrion will hatch a polynema. The
cuckoo acts with less cruelty, since she is contented to lay her eggs by the side of those which occupy the nest.
Remarkable examples of the refinement of cruelty and of gluttony are to be found in this little animal world. It is not enough that some among them feed on the entrails of their young neighbours; there are wasps which, in order to make the agony last longer, place by the side of the eggs which they lay, chloroformed flies, which wait patiently for the time when they can yield themselves up, still palpitating, to these young tyrants. The days, the hours, perhaps even the minutes, are scrupulously reckoned for the preparation of this living morsel. As the process of hatching proceeds, the repast acquires properties more and more adapted to the age of the young wasps.
The Sphex is not less cruel. Some of the insects which are found in South America attack, not the young ones, but those which are grown up, and snatch spiders from their webs as slave-hunters carry off negroes from the wood; they garotte them, and cram them into narrow cells, after having chloroformed them to preserve them more effectually. These spiders, retaining enough life not to lose their nutritious qualities, become the easy prey of the larvæ of the Sphex. The mother of these hymenoptera takes care to deposit her eggs, as well as the living booty, in such a manner that the larvæ, at the moment of being hatched, live in abundance. These young larvæ, white and without feet, are dainty enough to reject any other kind of food. This is an act of cruelty which resembles that of the ichneumon, to which it may well be compared.
The Platygasters, another kind of hymenopterous
insects, show their cruelty in a different manner; they live in the bodies of the larvæ of Cecidomyæ which are lodged in the rolled leaves of the Salix, and suck the blood of their victims.