The grub is much paler in colour than those which I catch as they emerge. Its big eyes in particular are whitish, cloudy, squinting and apparently of little use for seeing. What good is sight underground? The eyes of the larvæ issuing from the earth are, on the contrary, black and shining and indicate ability to see. When it makes its appearance in the sunshine, the future Cicada has to seek, occasionally at some distance from the exit-hole, the hanging [[36]]branch on which the metamorphosis will be performed; and here sight will manifestly be useful. This maturity of vision attained during the preparation for the release is enough to show us that the larva, far from hastily improvising its ascending-shaft, works at it for a long time.
Moreover, the pale and blind larva is bulkier than it is in the state of maturity. It is swollen with liquid and looks dropsical. If you take it in your fingers, a limpid humour oozes from the hinder part and moistens the whole body. Is this fluid, expelled from the intestines, a urinary product? Is it just the residue of a stomach fed solely on sap? I will not decide the question and will content myself with calling it urine, merely for convenience.
Well, this fountain of urine is the key to the mystery. The larva, as it goes on and digs, sprinkles the dusty materials and makes them into paste, which is forthwith applied to the walls by abdominal pressure. The original dryness is succeeded by plasticity. The mud obtained penetrates the interstices of a rough soil; the more liquid part of it trickles in front; the remainder is compressed and packed and occupies the empty [[37]]spaces in between. Thus is an unblocked tunnel obtained, without any refuse, because the dust and rubbish are used on the spot in the form of a mortar which is more compact and more homogeneous than the soil traversed.
The larva therefore works in the midst of clayey mire; and this is the cause of the stains that astonish us so much when we see it issuing from excessively dry soil. The perfect insect, though relieved henceforth from all mining labour, does not utterly abandon the use of its bladder; a few drains of urine are preserved as a weapon of defence. When too closely observed, it discharges a spray at the intruder and quickly flies away. In either form, the Cicada, his dry constitution notwithstanding, proves himself a skilled irrigator.
Dropsical though it be, the larva cannot carry sufficient liquid to moisten and turn into compressible mud the long column of earth which has to be tunnelled. The reservoir becomes exhausted and the supply has to be renewed. How is this done and when? I think I see.
The few wells which I have laid bare throughout their length, with the painstaking [[38]]care which this sort of digging demands, show me at the bottom, encrusted in the wall of the terminal chamber, a live root, sometimes as big as a lead-pencil, sometimes no thicker than a straw. The visible part of this root is quite small, barely a fraction of an inch. The rest is contained in the surrounding earth. Is the discovery of this sort of sap fortuitous? Or is it the result of a special search on the larva’s part? The presence of a rootlet is so frequent, at least when my digging is skilfully conducted, that I rather favour the latter alternative.
Yes, the Cicada-grub, when hollowing out its cell, the starting-point of the future chimney, seeks the immediate neighbourhood of a small live root; it lays bare a certain portion, which continues the side wall without projecting. This live spot in the wall is, I think, the fount from which the contents of the urinary bladder are renewed as the need arises. When its reserves are exhausted by the conversion of dry dust into mud, the miner goes down to his chamber, drives in his sucker and takes a deep draught from the cask built into the wall. With his jug well filled, he goes up again. He resumes his work, wetting the hard earth the [[39]]better to flatten it with his claws and reducing the dusty rubbish to mud which can be heaped up around him and leave a clear thoroughfare. That is how things must happen. So logic and the circumstances of the case tell us, in the absence of direct observation, which is not feasible here.
If this root-cask fail, if moreover the reservoir of the intestine be exhausted, what will happen then? We shall learn from the following experiment. I catch a grub as it is leaving the ground. I put it at the bottom of a test-tube and cover it with a column of dry earth, not too closely packed. The column is nearly six inches high. The larva has just quitted an excavation thrice as deep, in soil of the same nature, but offering a much greater resistance. Now that it is buried under my short, sandy column, will it be capable of climbing to the surface? If it were a mere matter of strength, the issue would be certain. What can an obstacle without cohesion be to one that has just bored a hole through the hard ground?
And yet I am assailed by doubts. To break down the screen that still separated it from the outer air, the larva has expended its last reserves of fluid. The flask is dry; [[40]]and there is no way of replenishing it in the absence of a live root. My suspicion of failure is well-founded. For three days I see the entombed one wasting itself in efforts without succeeding in rising an inch higher. The materials removed refuse to stay in position for lack of anything to bind them; they are no sooner pushed aside than they slip down again under the insect’s legs. The labour has no perceptible result and has always to be done all over again. On the fourth day, the creature dies.
With the water-can full, the result is quite different. I subject to the same experiment an insect whose work of self-deliverance is just beginning. It is all swollen with urinary humours which ooze out and moisten its whole body. This one’s task is easy. The materials offer hardly any resistance. A little moisture, supplied by the miner’s flask, converts them into mud, sticks them together and keeps them out of the way. The passage is opened, very irregular in shape, it is true, and almost filled up at the back as the ascent proceeds. It is as though the larva, recognizing the impossibility of renewing its store of fluid, were saving up the little which it possesses and spending no [[41]]more than is strictly necessary to enable it to escape as quickly as possible from its unfamiliar surroundings. This economy is so well arranged that the insect reaches the surface at the end of ten days. [[42]]