But if the interest be great, the difficulty of observation is not small. The solitary habits of the Ammophila,—their being scattered singly over wide spaces, and their being almost always met with by mere chance,—almost forbid, as in the case of Sphex occitanica, any experiment being prepared beforehand. Long must a chance be watched for and awaited with unalterable patience, and one must know how instantly to profit by it when at last it comes just when least expected. I have waited for such a chance for [[214]]years and years, and then, all at once, I got the opportunity with a facility for observation and clearness of detail which made up for the long waiting.
At the beginning of my observations I succeeded twice in watching the murder of the caterpillar, and saw, as far as the rapidity of the operation allowed, that the sting of the Hymenopteron struck once for all at the fifth or sixth segment of the victim. To confirm this I bethought myself of making sure which ring was stabbed by examining caterpillars which I had not seen sacrificed, but had carried off from their captors while they were being dragged to the burrow; but it was vain to use a microscope,—no microscope can show any trace of such a wound. This was the plan adopted. The caterpillar being quite still, I tried each segment with the point of a fine needle, measuring the amount of sensibility by the greater or less pain given. Should the needle entirely transpierce the fifth segment or the sixth, there is no movement. But prick even slightly one in front or behind, the caterpillar struggles with a violence proportioned to the distance from the poisoned segment. Especially does the least touch on the hinder ones produce frantic contorsions. So there was but one stab, and it was given in the fifth or sixth segment.
What special reason is there that one or other of these two should be the spot chosen by the assassin? None in their organisation, but their position is another thing. Omitting the Loopers of Ammophila holosericea, I find that the prey of the others has the following organisation, counting the head as the first segment:—Three pairs of true feet on rings two, three, and four; four pairs of membranous feet on [[215]]rings seven, eight, nine, and ten, and a last similar pair set on the thirteenth and final ring; in all eight pairs of feet, the seven first making two marked groups—one of three, the other of four pairs. These two groups are divided by two segments without feet, which are the fifth and sixth.
Now, to deprive the caterpillar of means of escape, and to render it motionless, will the Hymenopteron dart its sting into each of the eight rings provided with feet? Especially will it do so when the prey is small and weak? Certainly not: a single stab will suffice if given in a central spot, whence the torpor produced by the venomous droplet can spread gradually with as little delay as possible into the midst of those segments which bear feet. There can be no doubt which to choose for this single inoculation; it must be the fifth or sixth, which separate the two groups of locomotive rings. The point indicated by rational deduction is also the one adopted by instinct. Finally, let us add that the egg of the Ammophila is invariably laid on the paralysed ring. There, and there alone, can the young larva bite without inducing dangerous contorsions; where a needle prick has no effect, the bite of a grub will have none either, and the prey will remain immovable until the nursling has gained strength and can bite farther on without danger.
With further researches doubts assailed me, not as to my deductions, but as to how widely I might extend them. That many feeble Loopers and other small caterpillars are disabled by a single stab, especially when struck at so favourable a point as the one just named, is very probable in itself, and, moreover, is [[216]]shown both by direct observation and by experiments on their sensibility with the point of a needle. But Ammophila sabulosa and hirsuta catch huge prey, whose weight, as already said, is fifteen times that of the captor. Can such giant prey be treated like a poor Looper? Can a single stab subdue the monster and render it incapable of harm? If the fearsome gray worm strike the cell walls with its strong body, will it not endanger the egg or the little larva? One dares not imagine a tête-à-tête in the small cell at the bottom of the burrow between the frail, newly-hatched creature and this kind of dragon:—still able to coil and uncoil its lithe folds.
My suspicions were heightened by examination as to the sensitiveness of the caterpillar. While the small game of Ammophila holosericea and hirsuta struggle violently if pricked elsewhere than in the part stabbed, the large caterpillars of A. sabulosa, and above all of A. hirsuta, remain motionless, no matter which segment be stimulated. They show no contortions or sudden twisting of the body, the steel point only producing as a sign faint shudderings of the skin. As the safety of a larva provided with such huge prey requires, motion and feeling are almost quite destroyed. Before introducing it into the burrow, the Hymenopteron turns it into a mass—inert indeed, yet not dead.
I have been able to watch the Ammophila use her instrument on the robust caterpillar, and never did the infused science of instinct show me anything more striking. With a friend—alas! soon after snatched from me by death—I was returning from the tableland of Les Angles after preparing snares to put [[217]]the cleverness of Scarabæus sacer to the proof, when we caught sight of an Ammophila hirsuta very busy at the foot of a tuft of thyme. We instantly lay down very close by. Our presence noways alarmed the insect, which alighted for a moment on my sleeve, decided that since her visitors did not move they must be harmless, and returned to her tuft of thyme. Well used to the ways of Ammophila, I knew what this audacious tameness meant—she was occupied by some serious affair. We would wait and see. The Ammophila scratched in the ground round the collar of the plant, pulling up thin little grass roots, and poked her head under the tiny clods which she raised up, ran hurriedly, now here, now there, round the thyme, visiting every crack which gave access under it; yet she was not digging a burrow, but hunting something hidden underground, as was shown by manœuvres like those of a dog trying to get a rabbit out of its hole. And presently, disturbed by what was going on overhead and closely tracked by the Ammophila, a big gray worm made up his mind to quit his abode and come up to daylight. It is all over with him; the hunter is instantly on the spot, gripping the nape of his neck and holding on in spite of his contortions. Settled on the monster’s back the Ammophila bends her abdomen, and methodically, deliberately—like a surgeon thoroughly familiar with the anatomy of his subject—plunges a lancet into the ventral surface of every segment, from the first to the last. Not one ring is omitted; with or without feet each is stabbed in due order from the front to the back.
This is what I saw with all the leisure and ease [[218]]required for an irreproachable observation. The Hymenopteron acts with a precision of which science might be jealous; it knows what man but rarely knows; it is acquainted with the complex nervous system of its victim, and keeps repeated stabs for those with numerous ganglia. I said “It knows; is acquainted”: what I ought to say is, “It acts as if it did.” What it does is suggested to it; the creature obeys, impelled by instinct, without reasoning on what it does. But whence comes this sublime instinct? Can theories of atavism, of selection, of the struggle for life, interpret it reasonably? For my friend and myself it was and is one of the most eloquent revelations of the ineffable logic which rules the world and guides the unconscious by the laws which it inspires. Stirred to the heart by this flash of truth, both of us felt a tear of emotion rise to our eyes. [[219]]