THE SCIENCE OF INSTINCT

Insect life: Souvenirs of a naturalist by Jean-Henri Fabre, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. THE SCIENCE OF INSTINCT

XI. THE SCIENCE OF INSTINCT

I have no doubt that in order to paralyse her prey, Sphex occitanica follows the method of the one that hunts grasshoppers, plunging her sting repeatedly into the breast of the ephippiger in order to reach the thoracic ganglia. She must be familiar with the operation of injuring the nerve centres, and I am assured beforehand of her consummate skill in the learned operation. It is an art familiar to all the predatory Hymenoptera who bear a poisoned dagger, and it is not given them for nothing. But I must own that I have never yet beheld the deadly manœuvre, thanks to the solitary life of this Sphex.

When a number of burrows are made and then provisioned on some common ground, one has only to wait there to see now one insect return from the chase, now another, with her prey, and it is easy to substitute a live victim for the one sacrificed, renewing the experiment at will. Besides, the certainty that the subjects for experiment will not fail when wanted allows everything to be prepared beforehand, while with S. occitanica these conditions of success do not exist. To set out and look for her with one’s [147]preparations made is all but useless, so sparsely are these solitary insects scattered. Moreover, if you do meet with one, it will probably be during her idle hour when nothing is to be learned. I repeat that it is almost always unexpectedly, when you are not thinking about it, that the Sphex appears with her ephippiger. This is the moment—the one propitious moment—to attempt a substitution of prey and to induce her to let you witness those dagger thrusts. Let us hasten; time presses; in a few moments the burrow will have enclosed the provender, and the grand chance will be lost.

Need I speak of my mortification in these promising moments—a mocking lure offered by fortune! Under my eyes is matter for curious observations, and I cannot profit by it! I cannot steal the Sphex’s secret, for I have no equivalent to offer for her prey. Just try, if you like, to go about looking for an ephippiger when there are but a few minutes to find it in! Why, it took me three days of wild search before I could find weevils for my Cerceris! Yet twice did I make that desperate attempt. Ah! if the garde-champêtre had caught me then rushing about the vineyards, what a chance he would have had to believe me guilty of theft, and of reporting me! Vines and grapes—nothing was respected by my hurried steps, fettered by the vine garlands. I must and would have an ephippiger, and have it then and there. And once I did find one during one of these rapid expeditions. I beamed with joy, little foreseeing the bitter disappointment awaiting me.

If only I can come in time! if only the Sphex is [148]still dragging her victim! Thank heaven! all favours me. She is still at some distance from her hole, and is bringing along her prey. With my pincers I gently draw it back. She resists, clutches the antennæ and will not let go. I pull harder, even making her go backwards; it is in vain, she holds on. I had with me a pair of delicate little scissors, part of my entomological outfit, and I rapidly cut the harness, otherwise the long antennæ of the ephippiger. The Sphex still advanced, but soon paused, surprised by the sudden lightening of her load, which now indeed only consisted of the antennæ detached by my malicious artifice. The real burden, the heavy-bodied insect, remained behind, instantly replaced by my living one. The Sphex turned, let go the ropes, which now drew nothing, and retraced her steps. Now she is face to face with the prey substituted for her own. She examines it, walks round it with suspicious caution, stops, wets her foot with saliva and washes her eyes. While thus meditating does she say to herself something of this kind: “Well, am I awake or am I asleep? Do I see clearly or not? This thing is not mine. Of what or whom am I the dupe?” At all events she is in no haste to bite my prey. She holds aloof, and shows not the smallest wish to seize it. To excite her I offered the insect with the tips of my fingers, putting the antennæ almost in her jaws, well aware of her audacious tameness, and that she will take from your fingers prey withdrawn and then offered. What is this? She draws back, disdaining my offers and the prey put within her reach. I put down the ephippiger, which, unconscious of danger, goes [149]straight to its assassin. Now for it. Alas! no; the Sphex continues to draw back, behaves like a veritable coward, and finally takes wing. I never saw her again. Thus ended to my confusion an experiment which had so excited my enthusiasm.

Later, and gradually, as I visited more burrows I came to understand my want of success and the obstinate refusal of the Sphex. I always, without exception, found stored a female ephippiger with an abundant and succulent store of eggs inside her. This, it would seem, is the favourite food of the larvæ. In my rush among the vines I had laid hands on one of the other sex. It was a male which I offered to the Sphex! More clear-sighted than I in the great victualling question, she would have nothing to say to my game. “A male! Is that the kind of dinner for my larvæ? And, pray, for whom do you take them?” How sensitive must be these dainty eaters who appreciate the difference between the tender flesh of the female and the comparatively dry body of the male! What a penetrating glance which can distinguish instantly the one sex from the other, though alike in form and colour! The female has an ovipositor to bury her eggs with, and this is almost the only outward difference between her and the male. This difference never escapes the keen-sighted Sphex, and that is why my experiment made her rub her eyes, immensely puzzled by a prey without an ovipositor, which she was perfectly sure had one when it was caught. At such a transformation what must have passed in her little Sphex brain?

Now let us follow her when, the burrow being ready, she returns to find her victim, deserted not [150]far from the place of capture, and after the operation which paralysed it. The ephippiger is in a state like that of the cricket slain by S. flavipennis—a certain proof that stings have been darted into the ganglia of the thorax. Nevertheless, many movements continue, but disconnected, though endowed with a certain vigour. Unable to stand, the insect lies on one side or on its back, moving its long antennæ and palpi rapidly, opening and closing its mandibles, and biting as hard as in its normal condition. The abdomen pants fast and deeply; the ovipositor is suddenly brought under the stomach, which it almost touches. The feet move, but languidly and irregularly, the middle ones seemingly more benumbed than the others. If touched with a needle, the whole body starts wildly; efforts are made to rise and walk without success. In short, the creature would be full of life but for the impossibility of locomotion and even of getting on its feet. There is then a paralysis altogether local—paralysis of the feet, or rather partial abolition and ataxy of movement in them. Is this very incomplete inertia caused by some special disposition of the victim’s nervous system, or is it that only a single stab is given, instead of wounding each ganglion of the thorax, as does the huntress of grasshoppers? I cannot say.

However, for all its starts, its convulsions, its irregular movements, the victim is none the less unable to harm the larvæ destined to devour it. I have taken from the Sphex’s burrow ephippigers struggling just as much as in the first moments of their semi-paralysis, and yet the feeble grub, born but a few hours earlier, was biting the gigantic victim [151]with entire immunity. This striking result is caused by the mother laying her egg in one particular spot. I have already told how S. flavipennis glues her egg on the cricket’s breast, rather on one side, between the first and second pairs of feet. S. albisecta chooses the same place, and S. occitanica an analogous one, rather further back toward the base of one of the large hind thighs, all three thus evincing admirable knowledge as to where the egg will be safe.

For consider the ephippiger shut in the burrow. It is on its back, absolutely incapable of turning over. Vainly does it struggle; the irregular movements of its feet are useless, the cell being too wide for them to gain support from the walls. What do the victim’s convulsions matter to the larva? It is on a spot where it cannot be reached by tarsi, mandibles, ovipositor, or antennæ—a point absolutely motionless, where there is not even a shudder of the skin. There is entire security unless the ephippiger can move, turn, and get on its feet, and that one condition is admirably guarded against.

But with several, all in the same degree of paralysis, there would be great risk for the larva. Though there would be nothing to fear from the first insect attacked, as the larva is out of its reach, there would be peril from the neighbourhood of the others, which in stretching out their legs hither and thither might strike it and tear it up with their spurs. Perhaps this is why S. flavipennis, which heaps three or four grasshoppers in one cell, almost entirely paralyses them, while S. occitanica, providing each burrow with a single victim, leaves great power of motion to the ephippiger, simply preventing change [152]of place or rising to its feet, thus—though I cannot affirm it—economising dagger thrusts.

If the half-paralysed ephippiger be harmless for the larva established on a point of its body where defence is impossible, things are otherwise for the Sphex itself, which has to get it home. First, the prey clutches bits of grass with its tarsi as it is dragged along, being still able to use them pretty freely, causing considerable difficulty in getting it onward. The Sphex, heavily weighted by her load, is exposed to exhaustion by her efforts to make her prey let go its desperate hold on grassy places. But that is the least of the difficulties; it has full use of its mandibles, which snap and bite with their old vigour. Just in front of these terrible pincers is the slender body of the spoiler, as the latter draws the victim along. The antennæ are grasped not far from their root, so that the ephippiger, lying on its back, has its mouth now opposite the abdomen, and now the thorax of the Sphex, who, standing high on her long legs, watches, I am convinced, in order not to be seized by the mandibles gaping beneath. A moment of forgetfulness, a slip, a mere nothing, might bring her within reach of a pair of strong nippers which would not let slip the chance of a pitiless vengeance. In certain specially difficult cases, if not always, the movement of these redoubtable pincers must be stopped, and the harpoon-like tarsi prevented from adding to the difficulties of transport.

What will the Sphex do to obtain this result? Man, and even a learned man, would hesitate, bewilder himself with vain attempts, and perhaps despair of success. Let him come and take a lesson [153]from the Sphex, who, without having learned, without ever seeing any one else at work, is thoroughly up in her profession of operator. She knows that under her victim’s skull lies a circlet of nerve-knots, somewhat analogous to the brain of higher animals. She knows too that this chief nerve centre directs the action of the mouth-parts, and, moreover, is the seat of will, without whose command no muscle acts; finally, she is aware that if this kind of brain be injured, all resistance will cease, the insect no longer possessing will-power. As for the method of operation, it is the easiest thing possible for her, and when we have studied at her school we may try in our turn. The sting is no longer employed; in her wisdom the Sphex decides compression to be preferable to the poisoned sting. Let us bow to her decision, for we shall presently see how prudent it is to be convinced of our ignorance compared with the animal’s knowledge. Lest by re-writing my account I fail to do justice to the sublime talent of this masterly operation, I transcribe my notes written on the spot directly after witnessing the exciting spectacle.

The Sphex, finding that her prey resists too much, hooking itself here and there to blades of grass, pauses to perform the singular operation about to be described—a kind of coup de grâce. The Hymenopteron, still astride her victim, makes the articulation in the upper part of the neck, at the nape, to open wide. Then she seizes the neck with her mandibles, groping as far forward as possible under the skull, but making no outward wound, grasps and chews repeatedly the nerve-centres of the head. This renders her victim quite motionless, and incapable [154]of the least resistance, whereas previously the feet, though unable to move in the manner necessary for walking, vigorously resisted being dragged along. This is the fact in all its eloquence. While leaving intact the thin, supple membrane of the neck, the insect finds a way into the skull with the point of its mandibles, and bruises the brain. There is neither effusion of blood nor wound, but merely external compression. Of course I kept the paralysed ephippiger under inspection in order to watch the consequences of the operation at my leisure, and equally of course I hastened to repeat on living specimens what the Sphex had taught me. I will now compare my results with hers.

Two ephippigers, whose cervical ganglia I compressed with pincers, fell quickly into a state like that of her victims, only they sounded their harsh cymbals if irritated by the point of a needle, and their feet made some irregular languid movements. The difference in the results obtained doubtless arises from the fact that my victim had not been previously stung in the thoracic ganglia, as those had been which the Sphex had struck in the breast. Allowing for this important point, it will be seen that I made no bad pupil, and imitated my teacher in physiology, the Sphex, not ill. I own that it was not without a certain satisfaction that I found I had done almost as well as the insect does.

As well! What have I just said? Wait a little, and it will be seen that I had to attend the Sphex’s school for many another day. For my two ephippigers speedily died—died outright, and after three or four days I had only decaying bodies under my eyes. [155]But the ephippiger of the Sphex? Need I say that ten days after the operation this was perfectly fresh, as it has to be for the larva whose destined prey it is. Yet more, a few hours after the operation under the skull, there reappeared as if nothing had happened movements of an irregular kind in feet, antennæ, palpi, ovipositor, and mandibles—in short, the creature was again in the same state as before the Sphex bit its brain. And the movements went on, only feebler each day. The Sphex had only benumbed her victim for a period amply sufficient to enable her to get it home without resistance, while I, who thought myself her rival, was but a clumsy, barbarous butcher, and killed mine. She, with her inimitable dexterity, compressed the brain scientifically to cause a lethargy of a few hours; I, brutal through ignorance, perhaps crushed this delicate organ, primal source of life, with my pincers. If anything could prevent my blushing at my defeat, it would be that few if any could rival the Sphex in skill.

Ah! now I comprehend why she did not use her sting to injure the ganglia of the neck. A drop of poison instilled here, at the centre of vital force, would annihilate all nerve power, and death would soon follow. But the Sphex does not at all desire death. Dead food by no means suits the larvæ, and still less a body smelling of decay. All that is needed is lethargy, a passing torpor, hindering resistance while the victim is carted along—resistance difficult to overcome and dangerous to the Sphex. This torpor is obtained by the proceeding known in laboratories of experimental science as compression [156]of the brain. The Sphex acts like a Flourens who, baring an animal’s brain and pressing on the cerebrum, abolishes at once sensibility, will, intelligence, and motion. The pressure ceases and all reappears. So reappear the remains of life in the ephippiger as the lethargic effects of a skilful pressure go off. The ganglia of the skull, squeezed by the mandibles, but without mortal contusions, gradually recover activity, and put an end to the general torpor. It is alarmingly scientific!

THE SPHEX OF LANGUEDOC DRAGGING TO ITS BURROW AN EPHIPPIGER OF THE VINE

Fortune has her entomological caprices; you run after her and do not come up with her; you forget her, and lo, here she is tapping at your door! How many useless excursions, how many fruitless plans, you made to try to see Sphex occitanica sacrifice her victim! Twenty years go by; these pages are already in the printer’s hands, when, in the first days of this month (August 8, 1878), my son Emile darts into my study. “Quick! quick!” he cries, “a Sphex is dragging along her prey under the plane trees, before the door of the court!” Emile, initiated into the affair by what I had told him, and, better still, by like facts seen in our out-of-door life, was quite right. I hurried away, and saw a splendid S. occitanica dragging a paralysed ephippiger by the antennæ. She moved toward the poultry yard, seemingly desirous of scaling the wall, to make her burrow under some roof tile. Some years before I had seen a similar Sphex accomplish the ascent with her game, and choose her domicile under the arch of an ill-joined tile. Perhaps this new one was descended from her whose difficult ascent I have chronicled. A like feat is probably about to be [157]repeated, and this time before numerous witnesses, for all the household working under the shade of the plane trees formed a circle round the Sphex. They wonder at the audacious tameness of the insect, noways disturbed by the gallery of interested spectators. All are struck by her proud and robust bearing, as, with raised head and the victim’s antennæ well grasped by her mandibles, she drags after her the enormous burden. I alone among the spectators feel some regret. “Ah, had I but some live ephippigers!” I could not help saying, without the least hope of seeing my wish realised. “Live ephippigers!” replied Emile; “why, I have some quite fresh, caught this morning.” Four steps at a time he flew upstairs to his little study, where barricades of dictionaries enclosed a park wherein was brought up a fine caterpillar of Sphinx euphorbiæ. He brought back three ephippigers as good as heart could wish—two females and one male. How came these insects at hand just at the right moment for an experiment vainly tried twenty years before? This is another story. A southern shrike had nested on one of the tall plane trees in the avenue. Some days before the Mistral, the rude wind of our parts, had blown so violently that branches bent as well as reeds, and the nest overturned by the undulations of its branch let fall the four nestlings it contained. The next day I found the brood on the ground—three killed by the fall, the fourth still alive. The survivor was entrusted to Emile, who thrice a day went cricket-hunting on the turf in the neighbourhood to feed his charge. But crickets are not very large, while the nestling’s appetite was. Something [158]else was preferred—ephippigers, collected from time to time on the dry stalks and prickly leaves of the Eryngium. The three insects brought me by Emile came from the shrike’s larder. My pity for the fallen nestlings had brought me this unhoped-for good luck.

Having made the circle of spectators draw back and leave free passage for the Sphex, I took away her prey with my pincers, giving her immediately in exchange one of my ephippigers with an ovipositor like that of the one abstracted. Stamping was the only sign of impatience shown by the bereaved Hymenopteron. She ran at the new prey, too corpulent to try to avoid pursuit, seized it with her mandibles by the saddle-shaped corslet, got astride, and curving her abdomen, passed its end under the ephippiger’s thorax. There doubtless the stings are given, but the difficulty of observation prevents me from telling how many. The ephippiger—gentle victim—lets itself be operated on unresistingly, like the dull sheep of our slaughter-houses. The Sphex takes her time and manœuvres her lancet with a deliberation favourable to the observer; but the prey touches the ground with the whole lower part of its body, and what happens there cannot be seen. As for interfering and lifting the ephippiger a little so as to see better, it is not to be thought of; the murderess would sheath her weapon and retire. The next act is easy to observe. After having stabbed the thorax, the end of the abdomen appears under the neck, which she forces widely open by pressing the nape. Here the sting enters with marked persistence, as if more effective than elsewhere. One [159]might suppose that the nerve centre struck was the lower part of the œsophagean collar, but the persistence of movement in the mouthpieces, mandibles, jaws, and palpi, animated by this source of nerve power, shows that this is not so. Through the neck the Sphex simply reaches the thoracic ganglia, or at least the first, more easily attainable through the thin skin of the neck than through the integuments of the chest.

All is over. Without one convulsion or sign of pain the ephippiger is rendered henceforward an inert mass. For the second time I deprived the Sphex of the subject operated on, replacing it by the second female at my disposal. The same manœuvres were followed by the same result. Three times, almost without a pause, the Sphex repeated her skilled surgery, first on her own capture, then on those exchanged by me. Will she do so a fourth time on the male which I still have? It is doubtful, not that she is weary, but because the game does not suit her. I have never seen a Sphex with any but female prey, which, filled as they are with eggs, are the favourite food of the larvæ. My suspicion was well founded. Deprived of her third capture, she obstinately refused the male which I offered her. She ran hither and thither with hurried steps, seeking her lost prey. Three or four times she approached the ephippiger, walked round it, cast a disdainful glance at it, and finally flew away. This was not what her larvæ wanted. Experiment reiterated it after twenty years’ interval.

The three females, two stabbed under my eyes, remained mine. All the feet were quite paralysed, [160]Whether in its natural position or on its back or side, the creature retains whichever is given it. Constant oscillations of the antennæ, and, at intervals, some pulsations of the stomach and movements of the mouthpieces, are the only sign of life. Motion is destroyed but not feeling, for at the least prick where the skin is thin, the whole body shudders faintly. Perhaps one day physiology will discover in these victims a subject for fine studies on the functions of the nervous system. The Hymenopteron’s sting, incomparably skilful in reaching a given point and inflicting a wound to affect it alone, will replace, with immense advantage, the brutal scalpel of the experimenter, which disembowels where it should lightly touch. Meanwhile, here are the results obtained from the three victims, but from another point of view.

Only movement of the feet being destroyed, there being no injury save that to the nerve centres, the source of motion, the creature perishes, not from its wound, but from inanition. The experiment was tried thus:

Two uninjured ephippigers found in the fields were imprisoned without food, one in the dark, the other in the light. In four days the latter died of hunger, in five the former. This difference of a day is easily explained. In the light the creature is more eager to recover liberty, and as every movement of the animal machine causes a corresponding expenditure of energy, greater activity used up sooner the reserves of the organisation. With light, more agitation and shorter life; in darkness, less movement and longer life; both insects fasted [161]equally. One of the three stabbed was kept in the dark and foodless. In this case there was not only darkness and want of food, but the serious wounds inflicted by the Sphex, and yet for seventeen days it perpetually moved its antennæ. As long as this kind of pendulum oscillates, the clock of life has not stopped. On the eighteenth day the creature ceased to wave its antennæ and died. Thus the seriously wounded insect lived in the same conditions as the uninjured one four times as long. What seems as if it should be a cause of death is really the cause of life.

However paradoxical it may at first appear, this result is perfectly simple. Intact, the creature agitates and spends itself; paralysed, it makes only those feeble, internal movements, inseparable from all organised life, and the waste of substance is in proportion to the amount of action employed. In the first case the animal machine works and spends itself; in the second it is at rest and saves itself up. Nourishment no longer repairing loss, the insect in motion spends in four days its food reserves and dies; the motionless one does not spend them, and lives eighteen. Physiology tells us that life is continual destruction, and the Sphex’s victims are a most elegant demonstration of this fact.

One more remark. Fresh food is absolutely necessary to larvæ of the Hymenopteron. If the prey were stored intact, in four or five days it would be a dead body, given up to decay, and the newly hatched grubs would find no food but a corrupted mass. Touched by the sting it can live two or three weeks—a period more than sufficient for the [162]egg to hatch and the grub to develop. The paralysis has thus a double result—immobility, so as not to endanger the life of the delicate larvæ, and long preservation of the flesh to assure wholesome nourishment for them. Even when enlightened by science human logic could find nothing better.

My two other ephippigers, stung by the Sphex, were kept in darkness with food. To feed inert creatures, differing only from dead bodies by the perpetual oscillation of their long antennæ, seems at first an impossibility; however, the play of the mouth organs gave me some hope, and I made the attempt. My success surpassed my expectations. There was no question, of course, of offering them a lettuce leaf or any other green thing on which they might have browsed in their normal condition; they were feeble invalids, to be nourished with a feeding-cup, so to say, and broth. I used sugar and water.

The insect being laid on its back, I put a drop of sugared liquid on its mouth with a straw. Instantly the palpi stirred, mandibles and jaws moved; the drop was consumed with evident satisfaction, especially if the fast had been somewhat prolonged. I renewed the dose till it was refused. The repast took place once or twice a day at irregular intervals, as I could not devote myself very much to a hospital of this kind.

Well, with this meagre diet one of the ephippigers lived twenty-one days. This was little longer than the life of the one which I allowed to die of inanition. It is true that twice the insect had had a bad fall, having dropped from the experiment table to the floor through some awkwardness of mine. [163]The bruises consequent may have hastened its end. As for the other, exempt from accidents, it lived six weeks. As the nourishment offered, sugar and water, could not indefinitely replace the natural food, it is very probable that it would have lived longer still had its customary diet been available. Thus the point which I had in view is demonstrated: victims pierced by the sting of the Hymenopteron die from inanition and not of their wound.

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