We often see a torrent of muddy water, full of clay in suspension, covered with great streaks and masses of foam. On this fundamental foam, so to call it, which is soiled with earthy matters, we see here and there masses of a beautiful white foam, in which the bubbles are much smaller. A process of selection results from variations in density, and here and there we see foam white as snow resting on the dirty foam from which it is produced. Something of the kind occurs when the Mantis builds her nest. The two appendices whip the viscous secretion of the glands into foam. The lightest portion, whose bubbles are of the greatest tenuity, which is white on account of its finer porosity, rises to the surface, where the caudal filaments sweep it up and gather it into the snowy ribbon which runs along the summit of the nest.
So far, with a little patience, observation is possible and yields a satisfactory result. It becomes impossible in the matter of the complex central zone, where the exits for the larvæ are contrived through the double series of overlapping leaves. The little I have been able to learn amounts to this: The end of the abdomen, deeply cleft in a horizontal direction, forms a kind of fork, of which the upper extremity remains almost motionless, while the lower continuously oscillates, producing the foam and depositing the eggs. The creation of the central zone is certainly the work of the upper extremity.
It is always to be seen in the continuation of this central zone, in the midst of the fine white foam gathered up by the caudal filaments. The latter delimit the zone, one working on either side, feeling the edges of the belt, and apparently testing it and judging its progress. These two filaments are like two long fingers of exquisite sensitiveness, which direct the difficult operation.
But how are the two series of scales obtained, and the fissures, the gates of exit which they shelter? I do not know; I cannot even imagine. I leave the end of the problem to others.
What a wonderful mechanism is this, that has the power to emit and to form, so quickly and methodically, the horny medium of the central kernel, the foam which forms the protective walls, the white creamy foam of the ribbon which runs along the central zone, the eggs, and the fecundating liquid, while at the same time it constructs the overlapping leaves, the imbricated scales, and the alternating series of open fissures! We are lost in the face of such a wonder. Yet how easily the work is performed! Clinging to the wire gauze, forming, so to speak, the axis of her nest, the Mantis barely moves. She bestows not a glance on the marvel which is growing behind her; her limbs are used only for support; they take no part in the building of the nest. The nest is built, if we may say so, automatically. It is not the result of industry and the cunning of instinct; it is a purely mechanical task, which is conditioned by the implements, by the organisation of the insect. The nest, complex though it is in structure, results solely from the functioning of the organs, as in our human industries a host of objects are mechanically fashioned whose perfection puts the dexterity of the fingers to shame.