Insect life: Souvenirs of a naturalist

by Jean-Henri Fabre

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.