Social Life in the Insect World

by Jean-Henri Fabre

The end of the abdomen, continually palpitating, rapidly closing and opening its valves, oscillates right and left like a pendulum. From each of these oscillations results a layer of eggs in the interior, and a transversal crevice on the exterior. As it advances in the arc described, suddenly, and at frequent intervals, it plunges deeper into the foam, as though burying something at the bottom of the frothy mass. Each time it does so an egg is doubtless deposited; but the operation is so rapid, and takes place under conditions so unfavourable for observation, that I have never once been enabled to see the oviduct at work. I can only judge of the advent of the eggs by the movements of the end of the abdomen, which is immersed more deeply with a sudden plunging movement.

At the same time the viscous composition is emitted in intermittent waves, and is beaten into a foam by the terminal valves. The foam thus obtained spreads itself over the sides and at the base of the layer of eggs, and projects through the meshes of the wire gauze as a result of the pressure of the abdomen. Thus the spongy envelope is progressively created as the ovaries are gradually emptied.

I imagine, although I cannot speak as the result of direct observation, that for the central core, where the eggs are surrounded by a material more homogeneous than that of the outer shell, the Mantis must employ her secretion as it emerges, without beating it into a foam. The layer of eggs once deposited, the two valves would produce the foam required to envelop the eggs. It is extremely difficult, however, to guess what occurs beneath the veil of foam-like secretion.

In a recent nest the zone of issue is surrounded by a layer of finely porous matter, of a pure matt, almost chalky white, which contrasts distinctly with the remainder of the nest, which is of a dirty white. It resembles the icing composition made by confectioners with whipped white of egg, sugar, and starch, for the ornamentation of cakes.

This snowy border is easily crumbled and easily detached. When it disappears the zone of issue is clearly defined, with its double series of leaves with free edges. Exposure to the weather, wind, and rain result in its disappearance, fragment by fragment, so that old nests preserve no trace of it.

At first sight one is tempted to regard this snowy substance as of a different material to the rest of the nest. But does the Mantis really employ two secretions? No. Anatomy, in the first place, assures us of the unity of the materials of the nest. The organ which secretes the substance of the nest consists of cylindrical tubes, having a curious tangled appearance, which are arranged in two groups of twenty each. They are all filled with a colourless, viscous fluid, which is precisely similar in appearance in all parts of the organ. There is no indication of any organ or secretion which could produce a chalky coloration.

Moreover, the method by which the snowy band is formed rejects the idea of a different material. We see the two caudal appendices of the Mantis sweeping the surface of the foamy mass, and skimming, so to speak, the cream of the cream, gathering it together, and retaining it along the hump of the nest in such a way as to form a band like a ribbon of icing. What remains after this scouring process, or what oozes from the band before it has set, spreads over the sides of the nest in a thin layer of bubbles so fine that they cannot be distinguished without the aid of a lens.

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.