Social Life in the Insect World

by Jean-Henri Fabre

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.

From another point of view the nest of the Mantis is even more remarkable. It forms an excellent application of one of the most valuable lessons of physical science in the matter of the conservation of heat. The Mantis has outstripped humanity in her knowledge of thermic nonconductors or insulators.

The famous physicist Rumford was responsible for a very pretty experiment designed to demonstrate the low conductivity of air where heat other than radiant heat is concerned. The famous scientist surrounded a frozen cheese by a mass of foam consisting of well-beaten eggs. The whole was exposed to the heat of an oven. In a few minutes a light omelette was obtained, piping hot, but the cheese in the centre was as cold as at the outset. The air imprisoned in the bubbles of the surrounding froth accounts for the phenomenon. Extremely refractory to heat, it had absorbed the heat of the oven and had prevented it from reaching the frozen substance in the centre of the omelette.

Now, what does the Mantis do? Precisely what Rumford did; she whips her albumen to obtain a soufflée, a froth composed of myriads of tiny air-bubbles, which will protect the germs of life contained in the central core. It is true that her aim is reversed; the coagulated foam of the nest is a safeguard against cold, not against heat, but what will afford protection from the one will afford protection from the other; so that Rumford, had he wished, might equally well have maintained a hot body at a high temperature in a refrigerator.

Rumford understood the athermic properties of a blanket of air-cells, thanks to the accumulated knowledge of his predecessors and his own studies and experiments. How is it that the Mantis, for who knows how many ages, has been able to outstrip our physicists in this problem in calorics? How did she learn to surround her eggs with this mass of solidifying froth, so that it was able, although fixed to a bough or a stone without other shelter, to brave with impunity the rigours of winter?

The other Mantes found in my neighbourhood, which are the only species of which I can speak with full knowledge, employ or omit the envelope of solidifying froth accordingly as the eggs are or are not intended to survive the winter. The little Grey Mantis (Ameles decolor), which differs so widely from the Praying Mantis in that the wings of the female are almost completely absent, builds a nest hardly as large as a cherry-stone, and covers it skilfully with a porous rind. Why this cellular envelope? Because the nest of the Ameles, like that of the Praying Mantis, has to endure through the winter, fixed to a stone or a twig, and is thus exposed to the full severity of the dangerous season.

The Empusa pauperata, on the other hand (one of the strangest of European insects), builds a nest as small as that of the Ameles, although the insect itself is as large as the Praying Mantis. This nest is quite a small structure, composed of a small number of cells, arranged side by side in three or four series, sloping together at the neck. Here there is a complete absence of the porous envelope, although the nest is exposed to the weather, like the previous examples, affixed to some twig or fragment of rock. The lack of the insulating rind is a sign of different climatic conditions. The eggs of the Empusa hatch shortly after they are laid, in warm and sunny weather. Not being exposed to the asperities of the winter, they need no protection other than the thin egg-cases themselves.

Are these nice and reasonable precautions, which rival the experiment of Rumford, a fortuitous result?—one of the innumerable combinations which fall from the urn of chance? If so, let us not recoil before the absurd: let us allow that the blindness of chance is gifted with marvellous foresight.