AN UNKNOWN SENSE

The Hunting Wasps by Jean-Henri Fabre, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. AN UNKNOWN SENSE

Chapter XIX. AN UNKNOWN SENSE

I have described the Ammophila’s hunting tactics in detail. The facts which I ascertained seem to me so rich in results that, even if the harmas laboratory supplied me with nothing more, I should think myself indemnified by this one observation. The surgical methods adopted by the Wasp with the object of paralysing the Grey Worm are the highest manifestation in the realm of instinct that I have hitherto met. This inborn science is eminently calculated to give us food for thought. What a subtle logician, what an unerring operator is that unconscious physiologist, the Ammophila!

He who would witness these marvels for himself can hardly count on what a country walk may happen to show him; besides, if the lucky opportunity did present itself, he would not have time to profit by it. An observation, which I kept up for five hours on end, without even then managing to complete the experiment and obtain the proofs which I anticipated, is one that, to be properly conducted, should be made at leisure in one’s own garden. I owe my success, therefore, to my rustic laboratory. I make a present of the secret to whosoever would continue those magnificent studies: the harvest is inexhaustible; there will be sheaves for all.

When we follow the Ammophila’s hunting in the due sequence of her actions, the first question that suggests itself is this: how does the Wasp go to work to recognize the spot beneath which the Grey Worm lies?

There is nothing outside, nothing, at least, perceptible to the eye, to indicate the caterpillar’s hiding-place. The soil that conceals the quarry may be grassy or bare, flinty or earthy, smooth or seamed with little cracks. These varieties of appearance are matters of indifference to the huntress, who prospects every spot without showing preference for one more than another. At no place where the Wasp stops and digs with some persistency do I see anything particular, in spite of all my attention; and yet there must be a Grey Worm there, as I have but now convinced myself, five times in succession, by lending a helping hand to the insect, which was at first discouraged by a task out of proportion to its strength. Sight, therefore, is certainly out of the question here.

What sense, then? That of touch? Let us inquire. Everything tells us that the organs of search are the antennæ. With their tips, bent like a bow and quivering with a continual vibration, the insect tests the ground, giving a number of little taps. When some crack shows, the restless threads enter and sound it; when some grass-tuft spreads its tangled root-stock along the ground, the quivering of the antennæ redoubles as they grope among its knots and angles. Their tips are applied for an instant to the spot explored, moulding themselves, so to speak, upon it. They suggest two tactile filaments, two long fingers of incomparable mobility, which gather information by feeling. But the sense of touch can play no part in revealing what is underground: the thing to be felt is the Grey Worm; and the worm is lying snug in its burrow, at a depth of some inches below the surface.

We thereupon turn our thoughts to the faculty of scent. Insects, there is no denying, possess the sense of smell, often very highly developed. The Necrophori,1 the Silphæ,2 the Histers,3 the Dermestes4 hasten from every side to the spot where lies a little corpse of which the ground is to be purged. Guided by scent, these grave-diggers hurry towards the dead Mole.

But, while the presence of the olfactory sense in insects is indisputable, we still ask ourselves where it is seated. Many declare that the seat is in the antennæ. Let us admit this, though it is difficult to understand how a rod consisting of horny segments, jointed end to end, can fulfil the office of a nostril which is so very differently constructed. The organization of one apparatus having naught in common with the other, can the impressions received by both be of the same nature? When tools are dissimilar, do their functions remain alike?

Besides, there are grave objections in the case of our Wasp. Smell is a passive rather than an active sense; it does not, like touch, anticipate the impression: it receives it; it does not inquire after the scented effluvium: it accepts it when it comes. Now the Ammophila’s antennæ are always moving: they investigate, they anticipate the impression. The impression of what? If it were really an impression of smell, repose would serve them better than a perpetual quivering.

But there is more to be said: the olfactory sense goes for nothing when there is no smell. Now I have tested the Grey Worm for myself; I have given it to young nostrils to sniff, nostrils much more sensitive than mine: not one of us has perceived the faintest trace of smell in the caterpillar. When the Dog, famed for his scent, becomes aware of the truffle underground, he is guided by the tuber’s savour, which is highly appreciable by ourselves, even through the thickness of the soil. I admit that the Dog has a more subtle sense of smell than we have: it is exercised at greater distances, it receives more vivid and lasting impressions; nevertheless, it is impressed by odorous effluvia which becomes perceptible to our own nostrils under the proper conditions of proximity.

I will allow the Ammophila, if you like, a scent as delicate as that of the Dog, more delicate even; but still a smell is needed; and I ask myself how that which is inodorous at the very entrance to our nostrils can be odoriferous to an insect through the intervening obstacle of the ground. The senses, if they have the same functions, have the same excitants, from man to the Infusoria. No animal, so far as I know, can see clearly in what to us is absolute darkness. True, it may be said that, in the zoological progression, perception, always fundamentally the same, has varying degrees of power: this species is capable of more and that species of less; what is perceptible to one is imperceptible to another. This is perfectly right; and yet the insect, generally considered, does not appear to possess exceptional keenness of scent: the effluvia that attract it are perceived without a sense of smell of unusual delicacy. When Dermestes, Silphæ and Histers pour into the chalice of a carrion-scented arum lily, never to come out again; when swarms of Flies buzz around a dead Dog’s blue and swollen belly, the whole neighbourhood reeks with the stench. It hardly requires a scent of exquisite accuracy on the insect’s part to discover putrid meat and rotten cheese. Wherever we see its hordes gather, with scent for their undoubted guide, we ourselves are cognizant of a smell.

There remains hearing. This is another sense about which entomologists are not adequately informed. Where is its seat? In the antennæ, we are told. Those fine, quivering stalks would seem fairly well suited to be put in motion under the impulse of sound. In that case the Ammophila, exploring the region with her antennæ, would be warned of the presence of the Grey Worm by a slight noise coming up from the ground, the noise of the mandibles nibbling a root, the noise of the caterpillar wriggling its hind-quarters. What a faint sound and how difficult to transmit through the spongy cushion of the earth!

It is less than faint, it is non-existent. The Grey Worm is nocturnal in its habits. By day it skulks in its lair and does not stir. It does not nibble either; at least, the Grey Worms which I unearthed upon the Wasp’s indications were nibbling nothing, for the very simple reason that they had nothing to nibble. They were completely motionless and therefore silent in a layer of earth devoid of roots. The sense of hearing must be rejected with that of smell.

The question recurs, more abstruse than ever. How does the Ammophila go to work to recognize the spot beneath which the Grey Worm lies? The antennæ are, beyond a doubt, the organs that guide her. They do not, in this case, act as olfactory instruments, unless we admit that their dry and tough surface, which has none of the delicate structure required for the ordinary sense of smell, is nevertheless capable of perceiving scents that are non-existent to us. This would be equivalent to admitting that coarse tools tend to perfection of work. Nor do they act as instruments of hearing, for there is no sound to be discerned. What then is their function? I do not know and I despair of ever knowing.

Inclined as we are—and it could not well be otherwise—to judge all things by our standard, the only one in any way known to us, we attribute to animals our own means of perception and do not dream that they might easily possess others of which it is impossible for us to have an exact idea because there is nothing like them in ourselves. Are we quite certain that they are not equipped, in very varying degrees, for the purpose of sensations as foreign to ourselves as the sensation of colours would be if we were blind? Has matter no secrets left for us? Are we so very sure that it is revealed to the living being only by light, sound, taste, smell and touch? Physics and chemistry, young though they be, already declare to us that the dark unknown contains an enormous harvest, in comparison with which our scientific sheaf is the merest penury. A new sense, perhaps that which dwells in the grotesquely exaggerated nose of the Rhinolophus,5 perhaps that which dwells in the antennæ of the Ammophila, would open to our search a world which our physical structure no doubt condemns us to leave for ever unexplored. Cannot certain properties of matter, which have no perceptible action upon us, find a receptive echo in animals, which are differently equipped?

When Spallanzani,6 after blinding some Bats, released them in a room converted into a maze by means of cords stretched in every direction and of heaped-up brambles, how were those animals able to find their way about, to fly quickly, to move to and fro, from end to end of the room, without hitting the interposed articles? What sense analogous to any of ours guided them? Would some one tell me and, above all, make me understand? I should also like to understand how the Ammophila infallibly finds her caterpillar’s burrow with the aid of her antennæ. It is not a case of the sense of smell: we should have to presume it to possess an unparalleled delicacy, while recognizing that it is exercised by an organ in which no provision seems made for the perception of smells.

What a number of other incomprehensible things do we not ascribe to the insect’s sense of smell! We are satisfied with a word: the explanation is ready-found, without laborious search. But, if we care to consider the matter thoroughly, if we compare the requisite array of facts, then the cliff of the unknown rises abruptly, not to be climbed by the path which we insist on following. Let us then change our path and admit that animals may have other means of information than our own. Our senses do not represent the sum total of the methods whereby an animal communicates with that which is not itself: there are others not capable of comparison, however remote, with those which we possess.

If the act of the Ammophila were an isolated fact, I should not have lingered over it as I have done; but I propose to speak of others stranger still, which will carry conviction to the most exacting mind. After relating them, therefore, I shall return to the subject of special senses, irreducible senses, unknown to us.

For the moment, let us go back to the Grey Worm, which it would be as well for us to know in a less casual fashion. I have four of them, dug up with the knife at the spots indicated by the Ammophila. My intention was to substitute them, by turns, for the doomed victim, so as to see the Wasp’s operation repeated. When my plan failed, I placed the worms in a glass jar, with a layer of earth and a lettuce-stalk above them. By day, my captives remained buried in the earth; at night, they came up to the surface, where I caught them gnawing at the salad from below. In August, they dug deep down, not to come up again, and fashioned themselves a cocoon apiece of earth, very rough on the outer surface, oval in shape and the size of a small pigeon’s egg. The moth appeared at the end of the same month. I recognized the Dart or Turnip Moth (Noctua segetum, Hübn.).

The Hairy Ammophila, therefore, feeds her grubs on the caterpillars of Noctuæ; and her choice falls exclusively on the species that live underground. These caterpillars, commonly known as Grey Worms, because of their drab garb, are a most formidable scourge to agricultural crops, as well as to garden produce. Curled in their burrows by day, they climb to the surface at night and gnaw the base or collar of the herbaceous plants. Everything suits them: ornamental plants and edible plants alike. Flower-beds, market-gardens, fields are laid waste without distinction. When a seedling withers without apparent cause, draw it to you gently; and the dying plant will come up, but maimed, severed from its root. The Grey Worm has passed that way in the night; its greedy mandibles have performed the deadly amputation. Its havoc rivals that wrought by the White Worm, the grub of the Cockchafer. When it swarms in a beet-country, the damage amounts to millions. This is the terrible enemy against which the Ammophila comes to our aid.

I point out and urgently recommend to agriculturalists this valuable auxiliary, so zealous in her search of the Grey Worm in spring, so skilful in discovering its hiding-place. An Ammophila in a garden may mean the saving of a lettuce-bed, the snatching of a balsam-border from danger. But there is need here for recommendations. None would dream of destroying the pretty Wasp that goes fluttering nimbly from one path to the other, that visits this corner of the garden, then that, then the next, then the one over there; none dreams either—and none, unfortunately, can dream—of assisting her to multiply.

In the immense majority of cases the insect evades our influence: to exterminate it, if it be harmful, to propagate it, if it be useful, are impracticable undertakings for us. By a singular contrast of strength and weakness, man cuts through the neck of continents to join two seas, he pierces the Alps, he weighs the sun; and yet he cannot prevent a wretched maggot from enjoying his cherries before he himself does, nor an odious Louse from destroying his vines! The Titan is vanquished by the pigmy.

Now we have here, in this insect-world, an auxiliary of high merit, the supreme foe of our grievous foe the Grey Worm. Can we do anything to stock our fields and gardens with it at will? We cannot; for the first condition of multiplying the Ammophila would be to multiply the Grey Worm, the only food of her family of grubs. I do not speak of the insurmountable difficulties which this breeding would present. We have not to do with the Bee, who is faithful to her hive, because of her social habits; still less with the stupid Silkworm, perched on its mulberry-leaf, or its clumsy Moth, who for a moment flutters her wings, pairs, lays her eggs and dies: we have to do with an insect that is capricious in its wanderings, swift of flight and independent in its ways.

Besides, the first condition shatters all our hopes. Would we have the helpful Ammophila? Then we must resign ourselves to accepting the Grey Worm. We move in a vicious circle: to produce good we must invoke the aid of evil. The hostile band brings the friendly troop to our fields; but the second cannot live without the first; and the two show an even balance in numbers. If the Grey Worm abound, the Ammophila finds copious provender for her grubs and her race prospers; if the Grey Worm be rare, the Ammophila’s offspring decrease and disappear. This balance between prosperity and decadence is the immutable law that governs the proportions between devourers and devoured.

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