Must not the larva of the Cigale bore its passage in some such fashion? I do not mean that the results of excavation pass through its body--for earth, even the softest mould, could form no possible part of its diet. But is not the material detached simply thrust back behind the excavator as the work progresses?
The Cigale passes four years under ground. This long life is not spent, of course, at the bottom of the well I have just described; that is merely a resting-place preparatory to its appearance on the face of the earth. The larva comes from elsewhere; doubtless from a considerable distance. It is a vagabond, roaming from one root to another and implanting its rostrum. When it moves, either to flee from the upper layers of the soil, which in winter become too cold, or to install itself upon a more juicy root, it makes a road by rejecting behind it the material broken up by the teeth of its picks. That this is its method is incontestable.
As with the larvæ of Capricornis and Buprestes, it is enough for the traveller to have around it the small amount of free space necessitated by its movements. Moist, soft, and easily compressible soil is to the larva of the Cigale what digested wood-pulp is to the others. It is compressed without difficulty, and so leaves a vacant space.
The difficulty is that sometimes the burrow of exit from the waiting-place is driven through a very arid soil, which is extremely refractory to compression so long as it retains its aridity. That the larva, when commencing the excavation of its burrow, has already thrust part of the detached material into a previously made gallery, now filled up and disappeared, is probable enough, although nothing in the actual condition of things goes to support the theory; but if we consider the capacity of the shaft and the extreme difficulty of making room for such a volume of debris, we feel dubious once more; for to hide such a quantity of earth a considerable empty space would be necessary, which could only be obtained by the disposal of more debris. Thus we are caught in a vicious circle. The mere packing of the powdered earth rejected behind the excavator would not account for so large a void. The Cigale must have a special method of disposing of the waste earth. Let us see if we can discover the secret.