Part 12

The shaft found and the stone lifted, she must return to the prey, not without some uncertainty when comings and goings have been too many. Although it was left in a place obvious enough, the Ammophila often seems at a loss when the time comes to drag it home; at least, if there be a very long search for the burrow, one sees her suddenly stop and go back to the caterpillar, feel it and give it a little bite, as if to make sure that it is her very own game and property, hurrying back to seek for the burrow, but returning a second time if needful, or even a third, to visit her prey. I incline to believe that these repeated visits are made to refresh her memory as to where she left it.

This is what happens in very complex cases, but generally the insect returns without difficulty to the spot whither its vagrant life may have led it. For guide it has that local memory whose marvellous feats I shall later have occasion to relate. As for me, in order to return next day to the burrow hidden under the little flat stone, I dared not trust to my memory, but had to use notes, sketches, to take my bearings, and stick in pegs—in short, a whole array of geometry.

The temporary closing of the burrow with a flat stone as practised by A. sabulosa and A. holosericea appears unknown to the two other species; at least I never saw their homes protected by a covering. This is natural in the case of A. hirsuta, for, I believe, this species hunts the prey first and then burrows near the place of capture. As provender can therefore be at once stored it is useless to take any trouble about a cover. As for A. holosericea, I suspect there is another reason for not using any temporary door. While the two others only put one caterpillar in each cell, she puts as many as five, but much smaller ones. Just as we ourselves neglect to shut a door where some one is constantly passing to and fro, perhaps this Ammophila neglects to place a stone on a well which she will go down at least five times within a short space of time. All four lay up caterpillars of moths for their larvæ. A. holosericea chooses, though not exclusively, those slender, long caterpillars known as Loopers. They move as a compass might by opening and closing alternately, whence their expressive French name of Measurers. The same burrow includes provisions of varied colours—a proof that this Ammophila hunts all kinds of Loopers so long as they are small, for she herself is but feeble and the larva cannot eat much, in spite of the five heads of game set before it. If Loopers fail, the Hymenopteron falls back on other caterpillars equally small. Rolled up from the effect of the sting which paralysed them, all five are heaped in the cell; the top one bears the egg for which the provender is destined.

The three other Ammophilæ give but one caterpillar to each cell. True—size makes up for this; the game selected is corpulent, plump, amply sufficing the grub’s appetite. For instance, I have taken out of the mandibles of A. holosericea a caterpillar fifteen times her own weight—fifteen times!—an enormous sum if you consider what an expenditure of strength it implies to drag such game by the nape of its neck over the endless difficulties of the ground. No other Hymenopteron tried in the scales with its prey has shown me a like disproportion between spoiler and capture. The almost endless variety of colouring in the provender exhumed from the burrows or recognised in the grasp of the various species also proves that the three have no preference, but seize the first caterpillar met with, provided it be neither too large nor too small, and belongs to the moths. The commonest prey are those gray caterpillars which infest the plant at the junction of a root and stem just below the soil.

That which governs the whole history of the Ammophila, and more especially attracted my attention, was the way in which the insect masters its prey and plunges it into the harmless state required for the safety of the larva. The prey, a caterpillar, is very differently organised from the victims which we have hitherto seen sacrificed—Buprestids, Weevils, Grasshoppers, and Ephippigers. It is composed of a series of segments or rings set end to end, the three first bearing the true feet which will be those of the future butterfly; others bear membranous or false feet special to the caterpillar and not represented in the butterfly; others again are without limbs. Each ring has its ganglion, the source of feeling and movement, so that the nerve system comprehends twelve distinct centres well separated from each other, without counting the œsophageal ganglion placed under the skull, and which may be compared to the brain.

We are here a long way from the nerve centralisation of the Weevil and Buprestis that lends itself so readily to general paralysis by a single stab; very far too from the thoracic ganglia which the Sphex wounds successively to put a stop to the movements of her crickets. Instead of a single centralised point—instead of three nerve centres—the caterpillar has twelve, separated one from another by the length of a segment and arranged in a ventral chain along the median line of the body. Moreover, as is the rule among lower animals, where the same organ is very often repeated and loses power by diffusion, these various nervous centres are largely independent of each other, each animating its own segment, and are but slightly disturbed by disorder in neighbouring ones. Let one segment lose motion and feeling, yet those uninjured will none the less remain long capable of both. These facts suffice to show the high interest attaching to the murderous proceedings of the Hymenopteron with regard to its prey.

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 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 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 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 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 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.

XVI

THE BEMBEX

Not far from Avignon, on the right bank of the Rhône opposite the mouth of the Durance, is one of my favourite points for the observations about to be recorded. It is the Bois des Issarts. Let no one deceive himself as to the value of the word “bois”—wood, which usually gives the idea of a soil carpeted with fresh moss and the shade of lofty trees, through whose foliage filters a subdued light. Scorching plains, where the cicada grinds out its song under pale olives, know nothing of such delicious retreats full of shade and coolness.

The Bois des Issarts is composed of thin and scattered groups of ilex, which hardly lessen the force of the sun’s rays. When I established myself during the dog days in July and August, I used to settle myself at some spot in the wood favourable for observations. I took refuge under a great umbrella, which later lent me most unexpected aid of another kind, very valuable too, as my story will show in good time. If I had neglected to equip myself with this article, embarrassing enough in a long walk, the only way to avoid sunstroke was to lie at full length behind some heap of sand, and when my temporal arteries beat intolerably, the last resource was to shelter my head at the mouth of a rabbit hole. Such are the means of getting cool in the Bois des Issarts.

The soil, unoccupied by any woody vegetation, is almost bare and composed of a fine, arid, very light sand, heaped by the wind in little hillocks where the stems and roots of the ilex hinder its blowing about. The slope of such hillocks is generally very smooth, from the extreme lightness of the material, which runs down into the least depression, thus restoring the regularity of the surface. It is enough to thrust a finger into the sand, and then to withdraw it in order immediately to cause a downfall, which fills up the cavity and re-establishes the former state of things without leaving any trace. But at a certain depth, varying according to the more or less recent date of the last rains, the sand retains a dampness which keeps it stable, and lends a consistency allowing of slight excavations without roof and walls falling in. A burning sun, a radiant blue sky, sand slopes yielding without the least difficulty to the strokes of the Hymenopteron’s rake, abundant game for the larvæ, a peaceful site rarely troubled by the foot of the passer-by,—all unite here in this paradise of the Bembex. Let us see the industrious insect at work.

If the reader will come under my umbrella, or profit by my rabbit burrow, this is the sight which will meet him towards the end of July. A Bembex (B. rostrata) arrives of a sudden and alights without hesitation or investigation at a spot which, as far as I see, differs in nothing from the rest of the sandy surface. With her front tarsi, which, armed with stiff rows of hairs, suggest at once broom, brush, and rake, she begins to dig a subterranean dwelling, standing on her four hind feet, the two last slightly apart, while the front ones alternately scratch and sweep the loose sand. The precision and rapidity of the action could not be greater were the circular movement of the tarsi worked by a spring. The sand, shot backward under the creature, clears the arch of its hind legs, trickling like a liquid in a continuous thread, describing a parabola and falling some eight inches away. This dusty jet, constantly fed for five or ten minutes, is enough to show with what dizzy rapidity the tools are used. I could quote no second example of equal swiftness, which yet in no way detracts from the elegance and free movements of the insect as it advances and retires, now on one side, now on another, without allowing the parabola of sand to stop.

The soil hollowed is of the lightest kind. As the Hymenopteron excavates, the sand near falls and fills the cavity. In the landslip are mingled little bits of wood, decayed leaf-stalks, and grains of gravel larger than the rest. The Bembex picks these up in her mandibles, and, moving backward, carries them to a distance, returning to sweep again, but always lightly, without attempting to penetrate into the earth. What is the object in this surface labour? It would be impossible to learn from a first glance, but after spending many days with my dear Hymenoptera, and grouping together the scattered results of my observations, I think I divine the motive of these proceedings.

The nest is certainly there—underground, at the depth of a few inches: in a little cell, dug in cool firm sand, is an egg, perhaps a larva, which the mother feeds daily with flies, the invariable food of Bembex larvæ. She must be able at any moment to penetrate to this nest, carrying on the wing, between her feet, the nursling’s daily ration, just as a bird of prey arrives at its eyrie carrying game for its brood in its claw. But while the bird returns to a nest on some inaccessible shelf of rock, without any difficulty beyond the weight of its prey, the Bembex must undertake each time the hard work of mining, opening afresh a gallery blocked and closed by ever-sliding sand in proportion as she proceeds. The only stable part of this underground abode is the spacious cell inhabited by the larva amid the remains of a fortnight’s feast; the narrow vestibule entered by the mother to go down to the cell, or come forth for the chase, gives way each time, at all events at the upper end, built in dry sand, rendered even looser by her constant goings and comings. Thus at each entrance or exit the Hymenopteron must clear out a passage. The exit offers no difficulties, even should the sand have the same consistency as when first stirred; the insect’s movements are free; it is safe under cover, can take its time and use tarsi and mandibles at its leisure. Going in is another matter. The Bembex is embarrassed by her prey, pressed to her body by her feet, so that there is no free use of the mining tools. What is more serious is that impudent parasites—veritable bandits in ambush—are crouching here and there about the burrow watching her difficult entrance to hurriedly drop their egg on the game just as it disappears into the gallery. If they succeed, the son of the house, the Hymenopteron’s nursling, will perish, starved by greedy guests.

The Bembex seems aware of this danger, and arranges so as to enter quickly, without serious obstacles, so that the sand blocking the door should yield to a mere push from her head, aided by a rapid sweep of the forelegs. To this end she, so to say, sifts the materials round her abode. In leisure moments, when the sun shines and the larva has its food, and does not need her care, the mother rakes before her door, and puts on one side all the tiny bits of wood, of over-large gravel or leaves, which might get on her path and bar the passage at the perilous moment of return. The Bembex which we saw so hard at work was busy sifting so as to make access to her abode easier; the materials of the vestibule are examined, minutely sorted, and cleared of every encumbrance. Who can tell whether the rapid labour and joyous activity of the insect do not express in their own way her maternal satisfaction and happiness in caring for the roof of the cell which has received the precious trust of the egg? As the Bembex confines herself to exterior household cares without seeking to penetrate the sand, everything must be in order within, and there is nothing pressing to do. We may wait, but for the time the insect will teach us nothing more. Let us therefore examine the underground dwelling.

By lightly scratching the bank with the blade of a knife just where the Bembex was oftenest seen, one soon discovers the entrance hall, which, blocked as it is for part of its length, is none the less recognisable by the special look of the materials moved about. This passage, a finger’s-breadth in size, rectilinear or winding, longer or shorter, according to the nature of the ground, measures eight to twelve inches. It leads to a single chamber, hollowed in damp sand, with walls undaubed with mortar, which might prevent landslips and lend polish to the rough surface. Enough if the ceiling lasts while the larva is being fed up. Future falling-in matters little when the larva is enclosed in its stout cocoon—a kind of strong box, which we shall see in process of construction. In workmanship the cell is as rustic as possible, being merely a rude excavation with no well-determined form, low roofed, and of a size which might hold two or three nests.

Within lies one head of game—one only—quite small and quite insufficient for the voracious nursling for whom it is destined. It is a golden green-fly, Lucilia Cæsar, a dweller in tainted meat, and is quite motionless. Is it really dead or only paralysed? This will be cleared up later. Just now let us observe the cylindrical egg upon its side, white, slightly curved, and a couple of millimetres in length. It is a Bembex egg. As we have foreseen from the mother’s behaviour, there is no pressing household business; the egg is laid and a first ration provided for the needs of the feeble larva, which ought to hatch in twenty-four hours. For some time the Bembex need not re-enter her hole, confining herself to keeping a good lookout in the neighbourhood, or possibly making new burrows and laying there egg after egg, always in a separate cell.

This peculiarity of beginning to lay in food by a single small piece of game is not peculiar to Bembex rostrata; all the other species do the same. Open any cell after the egg is laid, and you always find it glued to the side of a Dipteron—all the food there is; moreover, this first ration is invariably small, as if the mother had sought some specially tender mouthful for her frail nursling. Another motive, the freshness of the food, may also have guided her choice. Later we will look further into the matter. This first ration—always a moderate one—varies much, according to the frequency of such or such a kind of game in the neighbourhood. It is sometimes a Lucilia Cæsar, sometimes a Stomoxys, or some small Eristalis, or a delicate Bombylius clad in black velvet, but the commonest is a Sphærophoria with a slender abdomen. This fact (and it has no exception) of storing the nest with but a single Dipteron,—a ration far too meagre for a larva with a voracious appetite,—at once puts us on the track of the most remarkable habit of the Bembecidæ. Hymenoptera whose larvæ live on prey heap into each cell the whole number of victims needed by the grub, which is hatched and lives alone,—an egg having been laid on one fly and the dwelling closed up. The larva has before it its whole store of food. But the Bembex is an exception to this rule. First a head of game is brought to the cell and an egg dropped on it. Then the mother leaves the burrow, which closes of its own accord; besides which she takes care to rake the surface smooth, and hide the entrance from every eye but her own.

Two or three days pass: the egg hatches and the small larva eats up its choice ration. Meanwhile, the mother remains near: one may see her licking the sugary exudations on the flower-heads of Eryngium campestre for nourishment, then settling with enjoyment on the burning sand, whence she doubtless surveys the exterior of her dwelling, or she sifts the sand at its entrance, then flies off and vanishes—perhaps to excavate other cells to be stored in a like manner. But however prolonged her absence, she does not forget the young larva so scantily provided for; maternal instinct teaches her the hour when the grub has finished its food and needs new sustenance. Then she comes back to the nest whose invisible entrance she knows right well how to find, and penetrates the hollow—this time laden with a larger prey. This deposited, she goes out again, and awaits outside the time for a second expedition. It soon comes, for the larva shows a devouring appetite. Again the mother arrives with fresh provender.

During almost a fortnight, while the larva is growing, the meals follow each other thus, one by one, as it needs them, and so much the nearer together as the nursling grows stronger. Toward the end of the fortnight the mother requires all her activity to supply the glutton’s appetite as it crawls heavily amid the remains of its repasts—wings, feet, and horny rings of abdomens. Each moment she returns with a new capture or comes forth for the chase. In short, the Bembex brings up her family from hand to mouth without storing provisions, like the bird which brings a beakful of food to the little ones still in the nest. Among the numerous proofs of this method of upbringing—one very singular in a Hymenopteron which feeds its family on prey—I have already mentioned the presence of the egg in a cell where but one little fly is found as provender—always one—never more. Another proof is the following one, which does not require any special moment for its ascertainment.