Social Life in the Insect World

by Jean-Henri Fabre

The burrow of the Cigale descends about fifteen inches. It is cylindrical, slightly twisted, according to the exigencies of the soil, and always approaches the vertical, or the direction of the shortest passage. It is perfectly free along its entire length. We shall search in vain for the rubbish which such an excavation must apparently produce; we shall find nothing of the sort. The burrow terminates in a cul-de-sac, in a fairly roomy chamber with unbroken walls, which shows not the least vestige of communication with any other burrow or prolongation of the shaft.

Taking its length and diameter into account, we find the excavation has a total volume of about twelve cubic inches. What becomes of the earth which is removed?

Sunk in a very dry, crumbling soil, we should expect the shaft and the chamber at the bottom to have soft, powdery walls, subject to petty landslips, if no work were done but that of excavation. On the contrary, the walls are neatly daubed, plastered with a sort of clay-like mortar. They are not precisely smooth, indeed they are distinctly rough; but their irregularities are covered with a layer of plaster, and the crumbling material, soaked in some glutinous liquid and dried, is held firmly in place.

The larva can climb up and down, ascend nearly to the surface, and go down into its chamber of refuge, without bringing down, with his claws, the continual falls of material which would block the burrow, make ascent a matter of difficulty, and retreat impossible. The miner shores up his galleries with uprights and cross-timbers; the builder of underground railways supports the sides and roofs of his tunnels with a lining of brick or masonry or segments of iron tube; the larva of the Cigale, no less prudent an engineer, plasters the walls of its burrow with cement, so that the passage is always free and ready for use.

If I surprise the creature just as it is emerging from the soil in order to gain a neighbouring bough and there undergo transformation, I see it immediately make a prudent retreat, descending to the bottom of its burrow without the slightest difficulty—a proof that even when about to be abandoned for ever the refuge is not encumbered with rubbish.

The ascending shaft is not a hurried piece of work, scamped by a creature impatient to reach the sunlight. It is a true dwelling, in which the larva may make a long stay. The plastered walls betray as much. Such precautions would be useless in the case of a simple exit abandoned as soon as made. We cannot doubt that the burrow is a kind of meteorological observatory, and that its inhabitant takes note of the weather without. Buried underground at a depth of twelve or fifteen inches, the larva, when ripe for escape, could hardly judge whether the meteorological conditions were favourable. The subterranean climate varies too little, changes too slowly, and would not afford it the precise information required for the most important action of its life—the escape into the sunshine at the time of metamorphosis.

Patiently, for weeks, perhaps for months, it digs, clears, and strengthens a vertical shaft, leaving only a layer of earth a finger's breadth in thickness to isolate it from the outer world. At the bottom it prepares a carefully built recess. This is its refuge, its place of waiting, where it reposes in peace if its observations decide it to postpone its final departure. At the least sign of fine weather it climbs to the top of its burrow, sounds the outer world through the thin layer of earth which covers the shaft, and informs itself of the temperature and humidity of the outer air.

If things are not going well—if there are threats of a flood or the dreaded bise—events of mortal gravity when the delicate insect issues from its cerements—the prudent creature re-descends to the bottom of its burrow for a longer wait. If, on the contrary, the state of the atmosphere is favourable, the roof is broken through by a few strokes of its claws, and the larva emerges from its tunnel.

Everything seems to prove that the burrow of the Cigale is a waiting-room, a meteorological station, in which the larva makes a prolonged stay; sometimes hoisting itself to the neighbourhood of the surface in order to ascertain the external climate; sometimes retiring to the depths the better to shelter itself. This explains the chamber at the base of the shaft, and the necessity of a cement to hold the walls together, for otherwise the creature's continual comings and goings would result in a landslip.

A matter less easy of explanation is the complete disappearance of the material which originally filled the excavated space. Where are the twelve cubic inches of earth that represent the average volume of the original contents of the shaft? There is not a trace of this material outside, nor inside either. And how, in a soil as dry as a cinder, is the plaster made with which the walls are covered?