Thus the larva works in the midst of a coating of mud, which is the cause of its dirtiness, so astonishing when we see it issue from an excessively dry soil. The perfect insect, although henceforth liberated from the work of a sapper and miner, does not entirely abandon the use of urine as a weapon, employing it as a means of defence. Too closely observed it throws a jet of liquid upon the importunate enemy and flies away. In both its forms the Cigale, in spite of its dry temperament, is a famous irrigator.
Dropsical as it is, the larva cannot contain sufficient liquid to moisten and convert into easily compressible mud the long column of earth which must be removed from the burrow. The reservoir becomes exhausted, and the provision must be renewed. Where, and how? I think I can answer the question.
The few burrows uncovered along their entirety, with the meticulous care such a task demands, have revealed at the bottom, encrusted in the wall of the terminal chamber, a living root, sometimes of the thickness of a pencil, sometimes no bigger than a straw. The visible portion of this root is only a fraction of an inch in length; the rest is hidden by the surrounding earth. Is the presence of this source of sap fortuitous? Or is it the result of deliberate choice on the part of the larva? I incline towards the second alternative, so repeatedly was the presence of a root verified, at least when my search was skilfully conducted.
Yes, the Cigale, digging its chamber, the nucleus of the future shaft, seeks out the immediate neighbourhood of a small living root; it lays bare a certain portion, which forms part of the wall, without projecting. This living spot in the wall is the fountain where the supply of moisture is renewed. When its reservoir is exhausted by the conversion of dry dust into mud the miner descends to its chamber, thrusts its proboscis into the root, and drinks deep from the vat built into the wall. Its organs well filled, it re-ascends. It resumes work, damping the hard soil the better to remove it with its talons, reducing the debris to mud, in order to pack it tightly around it and obtain a free passage. In this manner the shaft is driven upwards; logic and the facts of the case, in the absence of direct observation, justify the assertion.
If the root were to fail, and the reservoir of the intestine were exhausted, what would happen? The following experiment will inform us: a larva is caught as it leaves the earth. I place it at the bottom of a test-tube, and cover it with a column of dry earth, which is rather lightly packed. This column is about six inches in height. The larva has just left an excavation three times as deep, made in soil of the same kind, but offering a far greater resistance. Buried under this short column of powdery earth, will it be able to gain the surface? If its strength hold out the issue should be certain; having but lately made its way through the hard earth, this obstacle should be easily removed.
But I am not so sure. In removing the stopper which divided it from the outside world, the larva has expended its final store of liquid. The cistern is dry, and in default of a living root there is no means of replenishing it. My suspicions are well founded. For three days the prisoner struggles desperately, but cannot ascend by so much as an inch. It is impossible to fix the material removed in the absence of moisture; as soon as it is thrust aside it slips back again. The labour has no visible result; it is a labour of Sisyphus, always to be commenced anew. On the fourth day the creature succumbs.
With the intestines full the result is very different.
I make the same experiment with an insect which is only beginning its work of liberation. It is swollen with fluid, which oozes from it and moistens the whole body. Its task is easy; the overlying earth offers little resistance. A small quantity of liquid from the intestines converts it into mud; forms a sticky paste which can be thrust aside with the assurance that it will remain where it is placed. The shaft is gradually opened; very unevenly, to be sure, and it is almost choked up behind the insect as it climbs upwards. It seems as though the creature recognises the impossibility of renewing its store of liquid, and so economises the little it possesses, using only just so much as is necessary in order to escape as quickly as possible from surroundings which are strange to its inherited instincts. This parsimony is so well judged that the insect gains the surface at the end of twelve days.
The gate of issue is opened and left gaping, like a hole made with an augur. For some little time the larva wanders about the neighbourhood of its burrow, seeking an eyrie on some low-growing bush or tuft of thyme, on a stem of grass or grain, or the twig of a shrub. Once found, it climbs and firmly clasps its support, the head upwards, while the talons of the fore feet close with an unyielding grip. The other claws, if the direction of the twig is convenient, assist in supporting it; otherwise the claws of the two fore legs will suffice. There follows a moment of repose, while the supporting limbs stiffen in an unbreakable hold. Then the thorax splits along the back, and through the fissure the insect slowly emerges. The whole process lasts perhaps half an hour.
There is the adult insect, freed of its mask, and how different from what it was but how! The wings are heavy, moist, transparent, with nervures of a tender green. The thorax is barely clouded with brown. All the rest of the body is a pale green, whitish in places. Heat and a prolonged air-bath are necessary to harden and colour the fragile creature. Some two hours pass without any perceptible change. Hanging to its deserted shell by the two fore limbs, the Cigale sways to the least breath of air, still feeble and still green. Finally, the brown colour appears and rapidly covers the whole body; the change of colour is completed in half an hour. Fastening upon its chosen twig at nine o'clock in the morning, the Cigale flies away under my eyes at half-past twelve.