A garden alley, converted into a little Arabia Petræa by reflection from a wall facing the south, abounds in such holes. During the last days of June I have made an examination of these recently abandoned pits. The soil is so compact that I needed a pick to tackle it.
The orifices are round, and close upon an inch in diameter. There is absolutely no debris round them; no earth thrown up from within. This is always the case; the holes of the Cigales are never surrounded by dumping-heaps, as are the burrows of the Geotrupes, another notable excavator. The way in which the work is done is responsible for this difference. The dung-beetle works from without inwards; she begins to dig at the mouth of the burrow, and afterwards re-ascends and accumulates the excavated material on the surface. The larva of the Cigale, on the contrary, works outward from within, upward from below; it opens the door of exit at the last moment, so that it is not free for the discharge of excavated material until the work is done. The first enters and raises a little rubbish-heap at the threshold of her burrow; the second emerges, and cannot, while working, pile up its rubbish on a threshold which as yet has no existence.
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.