The Hunting Wasps

by Jean-Henri Fabre

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 [[240]]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 [[241]]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, [[242]]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 [[243]]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 [[244]]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 [[245]]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 [[246]]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.