Social Life in the Insect World

by Jean-Henri Fabre

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.

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.

Let us examine a larva at the moment of emerging from the soil. It is almost always more or less smeared with mud, sometimes dried, sometimes moist. The implements of excavation, the claws of the fore-feet, have their points covered by little globules of mortar; the others bear leggings of mud; the back is spotted with clay. One is reminded of a scavenger who has been scooping up mud all day. This condition is the more striking in that the insect comes from an absolutely dry soil. We should expect to see it dusty; we find it muddy.

One more step, and the problem of the well is solved. I exhume a larva which is working at its gallery of exit. Chance postpones this piece of luck, which I cannot expect to achieve at once, since nothing on the surface guides my search. But at last I am rewarded, and the larva is just beginning its excavation. An inch of tunnel, free of all waste or rubbish, and at the bottom the chamber, the place of rest; so far has the work proceeded. And the worker—in what condition is it? Let us see.

The larva is much paler in colour than those which I have caught as they emerged. The large eyes in particular are whitish, cloudy, blurred, and apparently blind. What would be the use of sight underground? The eyes of the larvæ leaving their burrows are black and shining, and evidently capable of sight. When it issues into the sunlight the future Cigale must find, often at some distance from its burrow, a suitable twig from which to hang during its metamorphosis, so that sight is obviously of the greatest utility. The maturity of the eyes, attained during the time of preparation before deliverance, proves that the larva, far from boring its tunnel in haste, has spent a long time labouring at it.

What else do we notice? The blind, pale larva is far more voluminous than in the mature state; it is swollen with liquid as though it had dropsy. Taken in the fingers, a limpid serum oozes from the hinder part of the body, which moistens the whole surface. Is this fluid, evacuated by the intestine, a product of urinary secretion—simply the contents of a stomach nourished entirely upon sap? I will not attempt to decide, but for convenience will content myself with calling it urine.

Well, this fountain of urine is the key to the enigma. As it digs and advances the larva waters the powdery debris and converts it into a paste, which is immediately applied to the walls by the pressure of the abdomen. Aridity is followed by plasticity. The mud thus obtained penetrates the interstices of the rough soil; the more liquid portion enters the substance of the soil by infiltration; the remainder becomes tightly packed and fills up the inequalities of the walls. Thus the insect obtains an empty tunnel, with no loose waste, as all the loosened soil is utilised on the spot, converted into a mortar which is more compact and homogeneous than the soil through which the shaft is driven.