Chapter XII. The Method of The Scoliae
Jean-Henri Fabre More Hunting Wasp
Digger Wasp (Scolia Dubia)
After the Ammophilae, the paralysers who multiply their lancet-thrusts to destroy the influence of the various nerve-centres, excepting those of the head, it seemed advisable to interrogate other insects which also are accustomed to a naked prey, vulnerable at all points save the head, but which deliver only a single thrust of the sting. Of these two conditions the Scoliae fulfilled one, with their regular quarry, the tender Cetonia-, Oryctes-or Anoxia-larva, according to the Scolia's species. Did they fulfil the second? I was convinced beforehand that they did. From the anatomy of the victims, with their concentrated nervous system, I foresaw, when compiling my history of the Scoliae, that the sting would be unsheathed once only; I even mentioned the exact spot into which the weapon would be plunged.
These were assertions dictated by the anatomist's scalpel, without the slightest direct proof derived from observed facts. Manoeuvres executed underground escaped the eye, as it seemed to me that they must always do. How indeed could I hope that a creature whose art is practised in the darkness of a heap of mould would decide to work in broad daylight? I did not reckon upon it all. Nevertheless, to salve my conscience, I tried bringing the Scolia into contact with her prey under the bell-glass. I was well-advised to do so, for my success was in inverse ratio to my hopes. Next to the Philanthus, none of the Hunting Wasps displayed such ardour in attacking under artificial conditions. All the insects experimented upon, some sooner, some later, rewarded me for my patience. Let us watch the Two-banded Scolia (S. bifasciata, VAN DER LIND) operating on her Cetonia grub.
The incarcerated larva strives to escape its terrible neighbour. Lying on its back, it fiercely wends its way round and round the glass circus. Presently the Scolia's attention awakens and is betrayed by a continued tapping with the tips of the antennae upon the table, which now represents the accustomed soil. The Wasp attacks the game, delivering her assault upon the monster's hinder end. She climbs upon the Cetonia-grub, obtaining a purchase with the tip of her abdomen. The quarry merely travels the more quickly on its back, without coiling itself into a defensive posture. The Scolia reaches the fore-part, with tumbles and other accidents which vary greatly with the amount of tolerance displayed by the larva, her improvised steed. With her mandibles she nips a point of the thorax, on the upper surface; she places herself athwart the beast, arches herself and makes every effort to reach with the end of her abdomen the region into which the sting is to be driven. The arch is a little too narrow to embrace almost the whole circumference of her corpulent prey; and she renews her attempts and efforts for a long time. The tip of the belly tries every conceivable expedient, touching here, there and everywhere, but as yet stopping nowhere. This persistent search in itself demonstrates the importance which the paralyser attaches to the point at which her lancet is to penetrate the flesh.
Meanwhile, the larva continues to move along on its back. Suddenly it curls up; with a stroke of its head it hurls the enemy to a distance. Undiscouraged by all her set-backs, the Wasp picks herself up, brushes her wings and resumes her attack upon the colossus, almost always by mounting the larva's hinder end. At last after all these fruitless attempts, the Scolia succeeds in achieving the correct position. She is seated athwart the Cetonia-grub; the mandibles grip a point on the dorsal surface of the thorax; the body, bent into a bow, passes under the larva and with the tip of the belly reaches the region of the neck. The Cetonia-grub, placed in serious peril, writhes, coils and uncoils itself, spinning round upon its axis. The Scolia does not interfere. Holding the victim tightly gripped, she turns with it, allows herself to be dragged upwards, downwards, sidewards, following its contortions. Her obstinacy is such that I can now remove the bell-glass and follow the details of the drama in the open.
Briefly, in spite of the turmoil, the tip of the abdomen feels that the right spot has been found. Then and only then the sting is unsheathed. It plunges in. The thing is done. The larva, at first plump and active, suddenly becomes flaccid and inert. It is paralysed. Henceforth there are no movements save of the antennae and the mouthparts, which will for a long time yet bear witness to a remnant of life. The point wounded has never varied in the series of combats under glass: it occupies the middle of the line of demarcation between the prothorax and the mesothorax, on the ventral surface. Note that the Cerceres, operating on Weevils, whose nervous system is as compact as the Cetonia-grub's, drive in the needle at the same spot. Similarity of nervous organization occasions similarity of method. Note also that the Scolia's sting remains in the wound for some time and roots about with marked persistence. Judging by the movements of the tip of the abdomen, one would certainly say that the weapon is exploring and selecting. Free to shift in one direction or the other, within narrow limits, its point is most probably seeking for the little mass of nerve-tissue which must be pricked, or at least sprinkled with poison, to obtain overwhelming paralysis.
I will not close this report of the duel without relating a few further facts, of minor importance. The Two-banded Scolia is a fierce persecutor of the Cetonia. In one sitting the same mother stabs three larvae, one after the other, in front of my eyes. She refuses the fourth, perhaps owing to fatigue or to exhaustion of the poison-bag. Her refusal is only temporary. Next day, she begins again and paralyses two grubs; the day after that, she does the same, but with a zeal that decreases from day to day.
The other Hunting Wasps that pursue the chase far afield grip, drag, carry their prey, after depriving it of movement, each in her own fashion and, laden with their burden, make prolonged attempts to escape from the bell-glass and to gain the burrow. Discouraged by these futile endeavours, they abandon them at last. The Scolia does not remove her quarry, which lies on its back for an indefinite time on the actual spot of the sacrifice. When she has withdrawn her dagger from the wound, she leaves her victim where it lies and, without taking further notice of it, begins to flutter against the side of the glass. The paralysed carcase is not transported elsewhere, into a special cellar; there where the struggle has occurred it receives, upon its extended abdomen, the egg whence the consumer of the succulent tit-bit will emerge, thus saving the expense of setting up house. It goes without saying that under the bell-glass the laying does not take place: the mother is too cautious to abandon her egg to the perils of the open air.
Why then, recognizing the absence of her underground burrow, does the Scolia uselessly pursue the Cetonia with the frantic ardour of the Philanthus flinging herself upon the Bee? The action of the Philanthus is explained by her passion for honey; hence the murders committed in excess of the needs of her family. The Scolia leaves us perplexed: she takes nothing from the Cetonia-grub, which is left without an egg; she stabs, though well aware of the uselessness of her action: the heap of mould is lacking and it is not her custom to transport her prey. The other prisoners, once the blow is struck, at least seek to escape with their capture between their legs; the Scolia attempts nothing.
After due reflection, I lump together in my suspicions all these surgeons and ask myself whether they possess the slightest foresight, where the egg is concerned. When, exhausted by their burden, they recognize the impossibility of escape, the more expert among them ought not to begin all over again; yet they do so begin a few minutes later. These wonderful anatomists know absolutely nothing about anything, they do not even know what their victims are good for. Admirable artists in killing and paralysis, they kill or paralyse at every favourable opportunity, no matter what the final result as regards the egg. Their talent, which leaves our science speechless, has not a shadow of consciousness of the task accomplished.
A second detail strikes me: the desperate persistence of the Scolia. I have seen the struggle continue for more than a quarter of an hour, with frequent alternations of good luck and bad, before the Wasp achieved the required position and reached with the end of her abdomen the spot where the sting should penetrate. During these assaults, which were resumed as soon as they were repulsed, the aggressor repeatedly applied the tip of her belly to the larva, but without unsheathing, as I could see by the absence of the start which the larva gives when it feels the pain of the sting. The Scolia therefore does not prick the Cetonia anywhere until the weapon covers the requisite spot. If no wounds are inflicted elsewhere, this is not in any way due to the structure of the larva, which is soft and vulnerable all over, except in the head. The point sought by the sting is no more unprotected than any other part of the skin.
In the scuffle, the Scolia, curved into a bow, is sometimes seized by the vice-like grip of the Cetonia-grub, which is violently coiling and uncoiling. Heedless of the powerful grip, the Wasp does not let go for a moment, either with her mandibles or with the tip of her abdomen. At such times the two creatures, locked in a mutual embrace, turn over and over in a mad whirl, each of them now on top, now underneath. When it contrives to rid itself of its enemy, the larva uncoils again, stretches itself out and proceeds to make off upon its back with all possible speed. Its defensive ruses are exhausted. Formerly, before I had seen things for myself, taking probability as my guide I willingly granted to the larva the trick of the Hedgehog, who rolls himself into a ball and sets the Dog at defiance. Coiled upon itself, with an energy which my fingers have some difficulty in overcoming, the larva, I thought, would defy the Scolia, powerless to unroll it and disdaining any point but the one selected. I hoped and believed that it possessed this means of defence, a means both efficacious and extremely simple. I had presumed too much upon its ingenuity. Instead of imitating the Hedgehog and remaining contracted, it flees, belly in air; it foolishly adopts the very posture which allows the Scolia to mount to the assault and to reach the spot for the fatal stroke. The silly beast reminds me of the giddy Bee who comes and flings herself into the clutches of the Philanthus. Yet another who has learnt no lesson from the struggle for life.
Let us proceed to further examples. I have just captured an Interrupted Scolia (Colpa interrupta, LATR.), exploring the sand, doubtless in search of game. It is a matter of making the earliest possible use of her, before her spirit is chilled by the tedium of captivity. I know her prey, the larva of Anoxia australis (1); I know, from my past excavations, the points favoured by the grub: the mounds of sand heaped up by the wind at the foot of the rosemaries on the neighbouring hill-sides. It will be a hard job to find it, for nothing is rarer than the common if one wants it then and there. I appeal for assistance to my father, an old man of ninety, still straight as a capital I. Under a sun hot enough to broil an egg, we set off, shouldering a navvy's shovel and a three-pronged luchet The local pitchfork of southern France. (2). Employing our feeble energies in turns, we dig a trench in the sand where I hope to find the Anoxia. My hopes are not disappointed. After having by the sweat of our brow — never was the expression more justified — removed and sifted two cubic yards at least of sandy soil with our fingers, we find ourselves in possession of two larvae. If I had not wanted any, I should have turned them up by the handful. But my poor and costly harvest is sufficient for the moment. To-morrow I will send more vigorous arms to continue the work of excavation.
And now let us reward ourselves for our trouble by studying the tragedy in the bell-glass. Clumsy, awkward in her movements, the Scolia slowly goes the round of the circus. At the sight of the game, her attention is aroused. The struggle is announced by the same preparations as those displayed by the Two-banded Scolia: the Wasp polishes her wings and taps the table with the tips of her antennae. And view, halloo! The attack begins. Unable to move on a flat surface, because of its short and feeble legs, deprived moreover of the Cetonia-larva's eccentric means of travelling on its back, the portly grub has no thought of fleeing; it coils itself up. The Scolia, with her powerful pincers, grips its skin now here, now elsewhere. Curved into a circle with the two ends almost touching, she strives to thrust the tip of her abdomen into the narrow opening in the coil formed by the larva. The contest is conducted calmly, without violent bouts at each varying accident. It is the determined attempt of a living split ring trying to slip one of its ends into another living split ring, which with equal determination refuses to open. The Scolia holds the victim subdued with her legs and mandibles; she tries one side, then the other, without managing to unroll the circle, which contracts still more as it feels its danger increasing. The actual circumstances make the operation more difficult: the prey slips and rolls about the table when the insect handles it too violently; there are no points of purchase and the sting cannot reach the desired spot; the fruitless efforts are continued for more than an hour, interrupted by periods of rest, during which the two adversaries represent two narrow, interlocked rings.
What ought the powerful Cetonia-grub to do to defy the Two-banded Scolia, who is far less vigorous than her victim? It should imitate the Anoxia-larva and remain rolled up like a Hedgehog until the enemy retires. It tries to escape, unrolls itself and is lost. The other does not stir from its posture of defence and resists successfully. Is this due to acquired caution? No, but to the impossibility of doing otherwise on the slippery surface of a table. Clumsy, obese, weak in the legs, curved into a hook like the common White Worm (3), the Anoxia-larva is unable to move along a smooth surface; it writhes laboriously, lying on its side. It needs the shifting soil in which, using its mandibles as a plough-share, it digs into the ground and buries itself.
Let us try if sand will shorten the struggle, for I see no end to it yet, after more than an hour of waiting. I lightly powder the arena. The attack is resumed with a vengeance. The larva, feeling the sand, its native element, tries to escape. Imprudent creature! Did I not say that its obstinacy in remaining rolled up was due to no acquired prudence but to the necessity of the moment? The sad experience of past adversities has not yet taught it the precious advantage which it might derive from keeping its coils closed so long as danger remains. For that matter, on the unyielding support of my table, they are not one and all so cautious. The larger seem even to have forgotten what they knew so well in their youth: the defensive art of coiling themselves up.
I continue my story with a fine-sized specimen, less likely to slip under the Scolia's onslaught. When attacked, the larva does not curl up, does not shrink into a ring as did the last, which was younger and only half as large. It struggles awkwardly, lying on its side, half-open. For all defence it twists about; it opens, closes and reopens the great hooks of its mandibles. The Scolia grabs it at random, clasps it in her shaggy legs and for nearly a quarter of an hour battles with the luscious tit-bit. At last, after a not very tumultuous struggle, when the favourable position is attained and the propitious moment has come, the sting is implanted in the creature's thorax, in a central point, below the throat, level with the fore-legs. The effect is instantaneous: total inertia, except of the appendages of the head, the antennae and mouth-parts. I achieved the same results, the same prick at a definite, invariable point, with my several operators, renewed from time to time by some lucky cast of the net.
Let us mention, in conclusion, that the attack of the Interrupted Scolia is far less fierce than that of the Two-banded Scolia. The Wasp, a rough sand-digger, has a clumsy gait; her movements are stiff and almost automatic. She does not find it easy to repeat her dagger-thrust. Most of the specimens with which I experimented refused a second victim on the first two days after their exploits. As though somnolent, they did not stir unless excited by my teasing them with a bit of straw. Although more active and more ardent in the chase, the Two-banded Scolia likewise does not draw her weapon every time that I invite her. For all these huntresses there are moments of inaction which the presence of a fresh prey is powerless to disturb.
The Scoliae have taught me nothing further, in the absence of subjects belonging to other species. No matter: the results obtained represent no small triumph for my ideas. Before seeing the Scoliae operate, I said, guided solely by the anatomy of the victims, that the Cetonia-, Anoxia-and Oryctes-larvae must be paralysed by a single thrust of the lancet; I even named the point where the sting must strike, a central point, in the immediate vicinity of the fore-legs. Of the three genera of paralysers, two have allowed me to witness their surgical methods, which the third, I feel certain, will confirm. In both cases, a single thrust of the lancet; in both cases, injection of the venom at a predetermined point. A calculator in an observatory could not compute the position of his planet with greater accuracy. An idea may be taken as proved when it attains to this mathematical forecast of the future, this certain knowledge of the unknown. When will the acclaimers of chance achieve a like success? Order appeals to order; and chance knows no laws.
1. The Anoxia are a genus of Beetles akin to the Cockchafers.
2. The local pitchfork of southern France.
3. The larva of the Cockchafer.