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?
Larvæ which burrow in wood, such as those of Capricornis and Buprestes, will apparently answer our first question. They make their way through the substance of a tree-trunk, boring their galleries by the simple method of eating the material in front of them. Detached by their mandibles, fragment by fragment, the material is digested. It passes from end to end through the body of the pioneer, yields during its passage its meagre nutritive principles, and accumulates behind it, obstructing the passage, by which the larva will never return. The work of extreme division, effected partly by the mandibles and partly by the stomach, makes the digested material more compact than the intact wood, from which it follows that there is always a little free space at the head of the gallery, in which the caterpillar works and lives; it is not of any great length, but just suffices for the movements of the prisoner.