THE AMMOPHILÆ

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

Chapter XIII. THE AMMOPHILÆ

A slender waist, a slim shape; an abdomen tapering very much at the upper part and fastened to the body as though by a thread; black raiment with a red sash across the belly: there you have a summary description of these burrowers, who are akin to the Sphex in form and colouring, but differ greatly from them in habits. The Sphex hunt Orthoptera—Locusts, Grasshoppers, Crickets—while caterpillars are the quarry of the Ammophilæ. This change of prey in itself suggests new methods in the lethal tactics of instinct.

If the name did not sound so pleasant to the ear, I would willingly quarrel with the term Ammophila, which means ‘sand-lover,’ as being too exclusive and often erroneous. The real lovers of sand, of dry, dusty, streaming sand, are the Bembex, who prey on Flies; but the caterpillar-hunters, whose story I now propose to relate, have no predilection for ordinary shifting sand, and even avoid it as being liable to landslips on the slightest provocation. Their perpendicular shaft, which has to remain open until the cell receives the provisions and an egg, requires a firmer setting if it is not to be prematurely blocked. What they want is a light soil, easily tunnelled, in which the sandy element is cemented with a little clay and lime. Edges of paths, sunny banks where the grass is rather bare: those are the favourite spots. In spring, quite early in April, we see the Hairy Ammophila (A. hirsuta) there; when September and October come, we find the Sandy Ammophila (A. sabulosa), the Silvery Ammophila (A. argentata), and the Silky Ammophila (A. holosericea). I will here condense the information which I have gathered from the four species.

In the case of all four the burrow is a vertical shaft, a sort of well, possessing at most the diameter of a thick goose-quill and a depth of about two inches. At the bottom is the cell, which is always solitary and consists of a mere widening of the entrance-shaft. It is, when all is said, a poor lodging, obtained economically, in one day’s work; the larva will find no protection there against the winter except from the four wrappers of its cocoon, copied from that of the Sphex. The Ammophila digs by herself, quietly, without hurrying, without any joyous enthusiasm. As usual, the fore-tarsi serve as rakes and the mandibles do duty as mining-tools. When some grain of sand offers too much resistance to its removal, you hear rising from the bottom of the well, as though to give voice to the insect’s efforts, a sort of shrill grating sound produced by the quivering of the wings and of the whole body. At frequent intervals the Wasp appears in the open with a load of refuse in her teeth, some bit of gravel which she flies away with and drops at a distance of a few inches, so as not to litter the place. Of the grains extracted some appear to deserve special attention, owing to their shape and size; at least, the Ammophila does not treat them as she does the rest: instead of flying off and dropping them far from the work-yard, she removes them on foot and lays them near the well. These are picked materials, ready-made blocks of stone which will serve presently for closing the dwelling.

This outside work is performed with measured movements and solemn diligence. The insect stands high on its legs, with its abdomen stretched at the end of its long pedicle, and turns round slowly, pivoting its whole body stiffly, with the geometrical rigidity of a line revolving on itself. If it wishes to fling to a distance the rubbish which it thinks will be in the way, it does so in short silent flights, often backwards, as though the Wasp, emerging from her well head last, avoided turning, so as to save time. It is the species carrying their abdomens on the longest stalks, such as the Sandy Ammophila and the Silky Ammophila, which mainly display this automaton-like rigidity in action. That belly swelling into a pear at the end of a thread is in fact a very delicate thing to steer: a sudden movement might warp the fine stalk. So we must walk with a sort of geometrical rigour; if we have to fly, we will do so backwards, to avoid tacking too often. On the other hand, the Hairy Ammophila, who has a short abdominal pedicle, works at her burrow with the heedless, nimble movements which we admire in most of the Digger-wasps. She has more freedom of action, because her belly does not get in her way.

The home is dug. At a later hour in the day, or even merely when the sun has left the place where the burrow has just been bored, the Ammophila invariably visits the little heap of stones placed in reserve during the excavating, with the object of choosing a bit to suit her. If there is nothing that satisfies her needs, she explores the neighbourhood and soon discovers what she wants, a small flat stone slightly larger in diameter than the mouth of her hole. She carries off this slab in her mandibles and lays it, as a temporary door, over the opening of the burrow. To-morrow, when the weather is once more hot and the sun bathes the slopes and encourages hunting, the Wasp will know quite well how to find her home, rendered inviolable by the massive door; she will come back with a paralysed caterpillar, grasped by the skin of its neck and dragged between its captor’s legs; she will lift the slab, which nothing distinguishes from other little stones around and which she alone is able to identify; she will let down the game to the bottom of the well, lay her egg, and close the house for good by sweeping into the perpendicular shaft all the rubbish which she has kept in the vicinity.

Time after time the Sandy Ammophila and the Silvery Ammophila have shown me this temporary closing of the hole when the sun begins to go down and when the lateness of the hour compels the victualling to be put off till the morrow. When the dwelling had been sealed up by the Wasp, I too would postpone my observations till the next day, but only after first making a map of the ground, choosing my lines and landmarks and planting a few stalks as signposts to show me the way to the well when it was filled. If I did not come back very early in the morning, if I left the Wasp time to take advantage of the hours of bright sunshine, I invariably found the burrow finally stocked with provisions and closed.

This faithfulness of memory is striking. The Wasp, delayed in her task, puts off the rest of her work to the next day. She does not spend the evening, she does not spend the night in the home which she has just dug: on the contrary, she leaves the premises altogether and goes away, after concealing the entrance with a little stone. The locality is not familiar to her; she knows it no better than any other spot, for the Ammophilæ behave like the Languedocian Sphex and lodge their families here or there, wherever they happen to roam. The Wasp was there by chance; the soil suited her; she dug her burrow; and she now goes off. Where to? Who can tell? Perhaps to the flowers not far away, where, by the last gleams of daylight, she will sip a drop of sugary liquid at the bottom of the cups, even as our miners, after toiling in their dark galleries, fly for comfort to the bottle in the evening. She goes off, to a less or greater distance, stopping at this bin and that in the flowers’ cellar. The evening, the night, the morning slip by. Still, she must return to the burrow and complete her task, she must return after the marches and countermarches of the morning hunt and the bewildering flight from flower to flower during the libations of the evening before. That the Social Wasp should return to her nest and the Social Bee to her hive does not surprise me at all: the hive and the nest are permanent residences, the way to which becomes known by long practice; but the Ammophila has no acquaintance with the locality which could help her to return to her burrow after such a long absence. Her tunnel is at a spot which she perhaps visited yesterday for the first time and which she must find again to-morrow, when she is quite out of her bearings and moreover hampered with a heavy load of game. Nevertheless, this little feat of topographical memory is performed, sometimes with a precision that left me astounded. The Wasp would walk straight to her burrow as if she had long been using all the little paths in the neighbourhood. At other times she would wander backwards and forwards and renew her search over and over again.

If the quest is greatly prolonged, the prey, which is a troublesome burden when you are in a hurry to find your home, is laid down in some high place, on a cluster of thyme or a tuft of grass, where it will be well in sight presently, when wanted. Thus eased, the Ammophila resumes her active search. I made a pencil-sketch, as she moved about, of the tracks followed. The result was a medley of tangled lines, with sudden bends and turns, branches in and branches out, windings and repeated intersections—in short, a regular labyrinth whose complicated maze was an ocular demonstration of the perplexity of the lost one.

When the well has been found and the slab removed, the Wasp has to come back to the caterpillar, which is not always done without some groping about, in cases where her wanderings to and fro have been very numerous. Though she left her prey easily visible, the Wasp appears to foresee the difficulty of finding it again when the moment comes to drag it home. At least, if the search is unduly prolonged, you see her suddenly interrupt her exploration of the ground and return to her caterpillar, which she feels and nibbles at for a moment, as though to make sure that it is really her own game, her property. Then she hurries back again to the field of search, which she leaves a second time, if need be, and a third, in order to inspect the prey. I am not at all sure that these repeated visits of the Wasp to the caterpillar are not a means of refreshing her memory of the place where she left it.

This is what happens in exceedingly complicated cases; but as a rule the Wasp goes back quite easily to the well dug the day before on the spot to which chance has taken her. The vagabond’s guide is her topographical memory, whose marvellous feats I shall have to tell later. As for me, in order to return next day to the well hidden under the lid of the little flat stone, I dared not trust to my unaided memory: I needed notes, sketches, lines of latitude and longitude, landmarks—in short, all the minutiæ of geometry.

The temporary closing of the burrow with a flat stone, as practised by the Sandy Ammophila and the Silvery Ammophila, is apparently unknown to the other two species. At any rate, I never saw their homes protected by a lid. Besides, this absence of a provisional door seems to be obligatory upon the Hairy Ammophila. In fact, as far as I could see, this species hunts its prey first and then digs its burrow near the place of capture. In this way the storing of the provisions can be done straight away; and there is no need to trouble about a lid. As for the Silky Ammophila, I suspect that she has another reason for not employing a temporary cover. Whereas the three others put only one caterpillar in each burrow, she puts in as many as five, though much smaller ones. Just as we ourselves neglect to shut a door through which we are constantly passing, so perhaps the Silky Ammophila neglects the precaution of placing a stone over a well down which she has to go at least five times in a short space of time.

In the case of all four, the provisions of the larvæ consist of caterpillars of Moths. The Silky Ammophila selects, though not exclusively, those long, thin caterpillars which walk by looping and unlooping their bodies. Their gait suggests a pair of compasses that makes its way by opening and closing in turns. Hence they are known by two expressive names: Loopers and Measuring-worms.1 The same burrow contains provisions varying greatly in colour, a proof that the Ammophila hunts without distinction every species of Loopers, provided that they be small, for the huntress herself is anything but large and her grub cannot get through very much, in spite of the five pieces of game set before her. If Loopers fail, the Wasp falls back on other equally slender caterpillars. Curved into a hoop as the result of the sting that paralysed them, the five pieces are stacked up in the cell: the uppermost carries the egg for which the provisions are made.

The three other Ammophilæ give only one caterpillar to each larva. It is true that here bulk makes up for number: the game selected is big, plump, capable of amply satisfying the grub’s appetite. For instance, I have taken from the mandibles of the Sandy Ammophila a caterpillar weighing fifteen times as much as its captor: fifteen times, an enormous figure when we consider the strength which the huntress must expend in dragging game of this kind by the skin of the neck over the countless obstacles on the road. No other Wasp, tried in the balance with her prey, has shown me a like disproportion between spoiler and booty.

The almost indefinite variety of colouring in the provisions which I unearth from the burrows or see between the legs of the Ammophilæ also proves that the three brigands have no preference and pounce upon the first caterpillar which comes along, provided that it be of a suitable size, neither too large nor too small, and that it belongs to the Moth division. The commonest game consists of those grey-clad caterpillars which penetrate a little way into the ground and devour the plant at the junction of root and stem.

What governs the whole history of the Ammophilæ and more particularly attracted my attention is the manner in which the insect overpowers its prey and reduces it to the condition of helplessness which the safety of the larva requires. The game hunted, the caterpillar, possesses a very different structure from that of the victims which we have seen immolated hitherto: Buprestes, Weevils, Locusts and Ephippigers. The creature is composed of a series of similar rings or segments set end to end. Three of these segments, the first three, carry the real legs, which will become the legs of the future Moth; others have membranous legs, or pro-legs, which are peculiar to the caterpillar and not represented in the Moth; others, lastly, have no limbs at all. Each segment has its nerve-nucleus, or ganglion, the seat of sensibility and movement, so that the nervous system includes twelve distinct centres, separated one from the other, without counting the ganglionic neck-piece placed under the skull and comparable, in a manner of speaking, with the brain.

We are here very far removed from the nerve-centralization of the Weevils and the Buprestes, which lends itself so well to general paralysis by a single prick of the sting; we are also a long way from the thoracic ganglia which the Sphex smites, one after the other, to suppress all movement in her Crickets. Instead of a solitary centralized point or of three nerve-nuclei, the caterpillar has twelve, separated from one another by the distance between one segment and the next and arranged like a string of beads on the ventral surface, along the median line of the body. Moreover, as is the general rule in the lower animals, where the same organ is repeated a great number of times and loses power by its diffusion, these different nerve-centres are largely independent of one another: each of them exercises its influence over its particular segment; and its functions are only very gradually affected by the derangement of the adjoining segments. One of the caterpillar’s rings can lose its power of moving and feeling and the remainder will nevertheless remain capable of both for a considerable time. These facts are enough to show the great interest attaching to the methods of slaughter which the Wasp adopts with her prey.

But, while the interest is great, the difficulty of observation is not small. The solitary habits of the Ammophilæ, their distribution one by one over wide areas, the fact that one almost always comes across them merely by chance: all this makes it hardly possible to carry out premeditated experiments with them, anymore than with the Languedocian Sphex. You have to be on the look-out a long time for an opportunity, to wait for it with untiring patience, and to know how to profit by it at the very moment when at last it presents itself, a moment when you were not thinking of it. I watched for that opportunity for years and years; then one day it suddenly appeared before my eyes, offering a facility of examination and a clearness of detail that compensated me for my long waiting.

At the beginning of my investigations I was twice enabled to witness the murder of the caterpillar, and I saw, as far as the swiftness of the operation permitted, the Wasp’s sting applied once and for all to either the fifth or the sixth segment of the victim. To confirm this result, I thought of ascertaining which ring had been stabbed on caterpillars which I had not seen sacrificed, but which I had taken from their captors while they were being dragged to the burrow. It was no use employing a magnifying-glass, for no magnifying-glass enables one to discover the least trace of a wound upon the victim. The method adopted is the following: when the caterpillar is quite still, I try each segment with the point of a fine needle and thus measure the amount of sensibility by the more or less manifest signs of pain in the insect. When the needle pricks the fifth segment or the sixth, even piercing it right through, the caterpillar does not stir. But if you prick even slightly a second segment, behind or in front of that insensible segment, the caterpillar wriggles and struggles with a violence which increases in proportion to the distance of the point attacked from the original segment. At the hinder end in particular, the least touch provokes wild contortions. There was only one sting, therefore, and it was administered to the fifth or sixth ring.

What peculiarity then do these two segments possess that one or other of them should be the target of the assassin’s weapon? None whatever in their organization; but their position is another matter. Leaving the Silky Ammophila’s Measuring-worms on one side, I find that the prey of the others is organized as follows, the head being counted as the first segment: three pairs of real legs on the second, third and fourth rings; four pairs of membranous legs on the seventh, eighth, ninth and tenth rings; lastly, a final pair of membranous legs on the thirteenth and last ring, making in all eight pairs of legs, of which the first seven form two vigorous groups, one of three, the other of four pairs. These two groups are separated by two legless segments, which are precisely the fifth and sixth.

Now, in order to deprive the caterpillar of its means of escape, to render it motionless, will the Wasp drive her sting into each of the eight rings provided with locomotory organs? Above all, will she take this superfluity of precaution when the prey is quite weak and small? Certainly not: a single stab will be enough; but it will be given at a central point, whence the torpor produced by the tiny drop of poison can spread gradually, with the least possible delay, to the segments furnished with legs. There is no doubt about the segment to be picked out for this single inoculation: it must be the fifth or the sixth, which separate the two groups of locomotory rings. The point indicated by rational inferences is therefore also the point adopted by instinct.

Lastly, let us add that the Ammophila’s egg is invariably laid on the ring that has been rendered insensible. Here and here alone the young larva can bite without provoking dangerous contortions; where a needle-prick has no effect, the grub’s bite will have no effect either. The grub will thus remain motionless until the nurseling has gained strength and can forge ahead without running a risk.

In my later researches, as the number of my observations increased, I began to entertain doubts, not as to the conclusions which I had formed, but as to their general application. That feeble Loopers and other small caterpillars are rendered harmless by a single thrust, especially when the sting strikes the favourable spot described, is a thing quite probable in itself and one which can also be proved either by direct observation or by testing the insect’s sensibility with a needle. But the Sandy Ammophila and especially the Hairy Ammophila capture enormous victims, whose weight, as I have said, is fifteen times that of the kidnapper. Will this giant prey be treated in the same manner as the frail Measuring-worm? Will one dagger-thrust be sufficient to subdue the monster and render it incapable of doing harm? Will the horrid Grey Worm, lashing the walls of the cell with its powerful tail, not endanger either the egg or the little grub? We dare not picture the encounter, in the narrow cell of the burrow, between those two—the feeble, new-hatched creature and that dragony thing still possessing freedom in its movements to twist and untwist its tortuous coils.

My suspicions were confirmed by an examination of the caterpillar from the point of view of sensibility. Whereas the small game of the Silky Ammophila and the Silvery Ammophila struggle violently if the needle touches them elsewhere than in the ring stung by the Wasp, the big caterpillars of the Sandy Ammophila and especially of the Hairy Ammophila remain motionless, no matter which segment we prick. With them there are no contortions, no sudden twists of the hinder parts; the steel point produces no sign of a remnant of sensibility beyond a faint quivering of the skin. The power of moving and feeling is therefore almost wholly abolished, as it needs must be if the grub is to feed in safety on this monstrous prey. Before placing it in the burrow, the Wasp has turned it into an inert though still living mass.

I have been permitted to watch the Ammophila operating with her scalpel on the sturdy caterpillar, and never did the intuitive science of instinct show me anything more exciting. With a friend—soon, alas, to be snatched from me by death!—I was coming back from the plateau of Les Angles to lay snares for the Sacred Beetle and put his skill to the test, when we caught sight of a Hairy Ammophila very busily employed at the foot of a tuft of thyme. We at once lay down on the ground, close to where she was working. Our presence did not frighten the Wasp; in fact, she came and settled on my sleeve for a moment, decided that her two visitors were harmless, since they did not move, and returned to her tuft of thyme. As an old stager, I knew what that daring familiarity meant: the Wasp’s attention was occupied with a serious business. We would wait and see.

The Ammophila scratched the ground at the foot of the plant, at the junction of root and stem, pulled up slender grass rootlets and poked her head under the little clods which she had lifted. She ran hurriedly this way and that around the thyme, inspecting every crevice that could give access to what lay below. She was not digging herself a home but hunting some game hidden underground; this was evident from her behaviour, which resembled that of a Dog trying to dig a Rabbit out of his hole. Presently, excited by what was happening overhead and close-pressed by the Ammophila, a big Grey Worm made up his mind to leave his lair and come up to the light of day. That settled him; the huntress was on the spot at once, gripping him by the skin of his neck and holding tight in spite of his contortions. Perched on the monster’s back, the Wasp bent her abdomen and deliberately, without hurrying, like a surgeon thoroughly acquainted with his patient’s anatomy, drove her lancet into the ventral surface of each of the victim’s segments, from the first to the last. Not a ring was left without receiving a stab; all, whether with legs or without, were dealt with in order, from front to back.

That is what I saw with all the leisure and ease that an observation needs in order to be above reproach. The Wasp acts with a precision that would make science turn green with envy; she knows what man hardly ever knows; she knows her victim’s complex nervous system and reserves her successive dagger-thrusts for the successive ganglia of her caterpillar. I said, she knows; what I should say is, she behaves as though she knew. Her act is simple inspiration. Animals obey their compelling instinct, without realizing what they do. But whence comes that sublime inspiration? Can theories of atavism, of natural selection, of the struggle for life interpret it reasonably? To me and my friend, this was and remained one of the most eloquent revelations of the unutterable logic that rules the world and guides the ignorant by the laws of its inspiration. Stirred to our innermost being by this flash of truth, both of us felt tears of undefinable emotion spring to our eyes.

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