A PARASITE OF THE BEMBEX. THE COCOON

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Chapter XVI. A PARASITE OF THE BEMBEX. THE COCOON

I have shown the Bembex hovering with her cumbrous prize above the nest and then dropping vertically and very slowly: a hesitating descent accompanied by a sort of plaintive hum. This cautious arrival might suggest that the insect is examining the ground from above in order to find its door and trying to recall the locality before alighting. But another motive is at work, as I propose to demonstrate. Under ordinary conditions, when no sign of danger is apparent, the Wasp comes suddenly, at full tilt, without any hovering, hesitating or whimpering, and settles at once on her threshold or very near it. Her memory is so faithful that she has no need to search about. Let us then look into the cause of that hesitating approach which I described in the last chapter.

The Wasp hovers, descends slowly, ascends again, flies away and returns, because the nest is threatened by a very grave danger. Her plaintive hum denotes anxiety: she never emits it when there is no peril. But who is the enemy? Can it be I, sitting here and watching? Why, no: I am nothing to her, nothing but a shapeless mass unworthy of her attention. The formidable enemy, the fearsome foe that must be avoided at all costs, is there, sitting motionless on the sand, near the house. It is a miserable little Fly, feeble and inoffensive in appearance. This insignificant Gnat is the terror of the Bembex. The scourge of the Fly-tribe, the fierce slayer who so swiftly wrings the necks of colossal Gad-flies sated with blood from an Ox’s back, does not enter her own residence because she sees herself watched by another Fly, a regular pigmy, who would make scarcely a mouthful for her larvæ.

Why does she not pounce upon her and get rid of the little wretch? The Wasp is quick enough on the wing to catch her; and, small though the capture be, the larvæ will not scorn it, since any sort of Fly suits them. But no: the Bembex flees from a foe whom she could cut to bits with a single stroke of her mandibles; it is to me as though I saw my Cat fleeing in terror from a Mouse. The ardent huntress of Flies is hunted by a Fly, and a small one at that. I bow before the facts without hoping ever to understand this inversion of the parts played by each insect. To be able to rid yourself easily of a mortal enemy who is contemplating the ruin of your family and would furnish a nice little meal for it, to be able to do that and not do it when the enemy is there, within reach of you, watching you, defying you: this is the height of animal aberration. But aberration is not the right word; let us rather speak of the harmony of created things, for, since this wretched little Fly has her tiny part to play in the general order, the Bembex must needs respect her and like a craven flee before her, else there would long since have been none of her left in the world.

Let us now tell the history of this parasite. Among the nests of the Bembex, we find very frequently some that are occupied at the same time by the larva of the Wasp and by other larvæ, strangers to the family and gluttonous companions of the first. These strangers are smaller than the Bembex’ nurseling, tear-shaped and of a purplish colour, due to the tint of the baby-food that shows through the transparent body. They vary in number: there are sometimes half-a-dozen of them, sometimes ten or more. They belong to a species of Fly, as is evident from their shape and also confirmed by the pupæ which we find in their place. Home-breeding completes the proof. When reared in boxes, on a layer of sand, with Flies renewed from day to day, they turn into pupæ from which, a year later, there issues a small Fly, a Tachina of the genus known as Miltogramma.

It is the same Fly that caused the Bembex such lively fears by lying in ambush near the burrow. The Wasp’s terror is but too well founded. This is what happens inside the dwelling: around the heap of food which the mother exhausts herself in keeping up to the requisite quantity, seated in company with the lawful offspring, are from six to ten hungry guests, who dip their sharp-pointed mouths into the common dish with no more restraint than if they were at home. Harmony seems to prevail at the table. I have never seen the lawful larva grow indignant at the indiscretion of the alien grubs, nor have I seen these appear to wish to interfere with the other’s repast. All help themselves indiscriminately and eat away peaceably without seeking a quarrel with their neighbours.

So far all would be well, if a serious difficulty did not now arise. However active the mother-nurse may be, she is obviously not equal to such an output. She had to be constantly hunting to feed one larva, her own; how could she possibly manage to provide for a dozen greedy mouths? The result of this enormous increase of family can only be want, or even starvation, not for the Fly’s maggots, which, developing more quickly than the Bembex’ larva, get ahead of it and profit by the days when there is still plenty for everybody, as their host is too young to need much, but certainly for that unfortunate host, who arrives at the transformation period without being able to make up for lost time. Besides, even if the first visitors, in becoming pupæ, leave him the free run of the table, others appear upon the scene, so long as the mother continues to come to the nest, and complete his starvation.

In burrows invaded by numerous parasites, the Bembex’ larva is in point of fact much smaller than one would suppose from the heap of food consumed, the remains of which encumber the cell. Limp, emaciated, reduced to a half or a third of its normal size, it vainly tries to weave a cocoon for which it does not possess the silk; and it perishes in a corner of the house among the pupæ of its more fortunate companions. Its end may be more cruel still. Should the provisions fail, should the mother-nurse delay too long in returning with food, the Flies devour the larva of the Bembex. I verified this black deed by rearing the brood myself. All went well so long as there was plenty to eat; but, if the daily portion was omitted by accident or design, next day or the day after I was sure to find the Fly’s grubs greedily slicing up the larva of the Bembex. So, when the nest is invaded by the parasites, the lawful larva is doomed to perish, either by hunger or by a violent death; and this is what makes the Bembex hate the sight of the Miltogrammæ prowling around her home.

The Bembex are not the only victims of these parasites: all the Digger-wasps without distinction have their burrows plundered by Tachinæ and especially Miltogrammæ. Different observers, notably Lepeletier de Saint-Fargeau, have spoken of the wiles of these bold-faced Flies; but none of them, so far as I know, has remarked this very curious instance of parasitism at the expense of the Bembex. I say very curious, because the conditions are quite different. The nests of the other Digger-wasps are stocked beforehand and the Miltogramma drops her eggs on the pieces of game as they are taken in. When the Wasp has finished her catering and laid her egg, she closes the cell, where henceforth the lawful larva and the alien larvæ hatch and live together without ever being visited in their solitude. The mother therefore is not aware of the parasites’ brigandage, which remains unpunished because it is unknown.

With the Bembex it is quite another matter. The mother is constantly returning indoors during the fortnight which it takes to rear her grubs; she knows that her offspring is living in the company of a number of intruders, who appropriate the best part of the food; each time that she brings provisions to her larva, she touches and feels at the bottom of the cavity those hungry guests who, far from contenting themselves with the remnants, seize upon the pick of the victuals; she must perceive, however limited her arithmetical faculties, that twelve are more than one; besides, the consumption of food, which is out of all proportion to her hunting powers, would tell her; and yet, instead of taking those presumptuous aliens by the skin of the belly and chucking them out of doors, she placidly tolerates them.

Tolerates them, did I say? Why, she feeds them, she brings them provisions, having perhaps for those intruders the same affection as for her own larva! It is a new version of the story of the Cuckoo, but with even more singular circumstances. The theory that the Cuckoo, almost the size of the Sparrow-hawk and wearing the same dress, inspires enough respect to enable her to introduce her egg with impunity into the feeble Warbler’s nest, and that the latter, in her turn, perhaps over-awed by the fearsome appearance of her Toad-faced nurseling, accepts and looks after the stranger: this theory has some plausibility. But what should we say if the Warbler turned parasite and, with superb audacity, went and confided her eggs to the eyrie of the bird of prey, to the nest of the Sparrow-hawk himself, the bloodthirsty devourer of Warblers? What should we say if the rapacious Hawk accepted the trust and fondly reared the brood of little birds? And this is exactly what the Bembex does, that ravisher of Flies who tenderly nurses other Flies, that huntress who provides food for a quarry whose last meal will be made on her own disembowelled larva! I leave it to others, cleverer than myself, to interpret these astonishing relations.

Let us observe the tactics employed by the Tachina for the purpose of confiding her eggs to the Digger’s nest. It is an absolute rule that the Gnat never enters the burrow, even though she should find it open and the owner absent. The sly parasite would think twice about venturing down a passage where, being no longer free to escape, she might pay dear for her brazen effrontery. For her the one and only favourable moment for her designs, a moment awaited with exquisite patience, is that at which the Wasp dives into the gallery, with her prey clasped to her belly. At that instant, however short it may be, when the Bembex or any other Digger has half her body well within the entrance and is about to disappear underground, the Miltogramma dashes up and settles on the piece of game that projects a little way beyond the hinder extremity of the ravisher; and, while the Bembex is delayed by the difficulty of entering, the other, with unparalleled swiftness, lays an egg on the prey, or even two or three in quick succession.

The hesitation of the Wasp hampered by her load lasts but the twinkling of an eye. No matter: this is long enough for the Gnat to accomplish her misdeed without allowing herself to be carried beyond the threshold. How smoothly her organs must work to adapt themselves to this instantaneous laying! The Bembex disappears, herself introducing the enemy to the home; and the Tachina goes and squats in the sun, close to the burrow, to meditate fresh deeds of darkness. If we wish to make sure that the Fly’s eggs have really been laid during this rapid manœuvre, we need only open the burrow and follow the Bembex to the bottom of her dwelling. The prey which we take from her bears at the tip of its abdomen at least one egg, sometimes more, according to the length of the delay at the entrance. These eggs are too small to belong to any but a parasite; besides, if any doubt remained, separate rearing in a box results in Fly-grubs, followed by the pupæ and lastly the Miltogrammæ themselves.

The moment adopted by the Gnat is chosen with great discrimination: it is the only moment when she is able to accomplish her designs without danger, and without useless dodging about. The Wasp, half-trapped in the entrance-hall, cannot see the foe so daringly perched on the hind-quarters of the prey; if she suspects the parasite’s presence, she cannot drive her away, having no liberty of movement in the narrow corridor; lastly, in spite of all the precautions which she takes to facilitate her entrance, she cannot always vanish underground with the necessary speed, the fact being that the bandit is much too quick for her. This indeed is the auspicious moment and the only one, since prudence forbids the Fly to penetrate into the cave where other Flies, far stronger than herself, serve as food for the grub. Outside, in the open air, the difficulty is insurmountable, thanks to the intense vigilance of the Bembex. Let us turn for a minute to the arrival of the mother while her home is being watched by Miltogrammæ.

A number of these Midges, greater or less from time to time but usually three or four, station themselves on the sand and remain perfectly still, all gazing at the burrow, of which they well know the entrance, carefully hidden though it be. Their dull-brown colour, their great blood-red eyes, their indefatigable patience have often suggested to me a picture of brigands, clad in dark frieze, with a red handkerchief round their heads, waiting in ambush for the moment to strike a felon blow. The Wasp arrives carrying her prey. If nothing of an alarming nature troubled her, she would then and there alight at her door. But she hovers at a certain height, comes down slowly and circumspectly, hesitates; and a plaintive whimpering, resulting from a special vibration of her wings, expresses her fears. She has seen the malefactors therefore. They too have seen the Bembex: they follow her with their eyes, as the movement of their red heads shows; every gaze is turned towards the coveted booty. Now come the marches and countermarches of craft striving to outwit prudence.

The Bembex comes straight down, with an imperceptible flight, as though letting herself drop inertly, buoyed up by the parachute of her wings. She is now hovering a hand’s breadth above the ground. This is the moment. The Midges take flight and all make for the rear of the Wasp; they hover in her wake, some nearer, some farther, in a geometrical line. If the Bembex turns to thwart their designs, they also turn, with a precision that keeps them in the rear on the same straight line; if she advances, they advance; if she retreats, they retreat, letting the Wasp set their pace all the time, now flying slowly, now coming to a standstill, according to the behaviour of their leader, the Bembex. They make no attempt to fling themselves on the object of their cupidity; their tactics are confined to keeping ready, in this rearguard position, which will save them any hesitation at the critical moment.

Sometimes, wearying of this obstinate pursuit, the Bembex alights; the others instantly settle on the sand, still in the rear, and do not budge. The Wasp darts off again, with a shriller whimpering, a sign no doubt of increasing indignation; the Midges dart after her. One last method remains of throwing off the persistent Flies: dashing off at full speed, the Bembex flies far away, hoping perhaps to mislead the parasites by rapid evolutions across country. But the wary Gnats are not caught in the trap: they let her go and once more take up their positions on the sand around the burrow. When the Bembex returns, the same pursuit will begin all over again, until at last the parasites’ obstinacy has worn down the mother’s prudence. In that second when her vigilance is relaxed, the Flies are straightway there. One of them, occupying the most favourable spot, swoops upon the disappearing prey and the deed is done: the egg is laid.

There is ample evidence that the Bembex is aware of the danger. The Wasp knows how disastrous the presence of the hateful Gnat may be to the future of the nest; on this point her prolonged attempts to put off the Tachinæ, her hesitations, her flights leave not the shadow of a doubt. Then how is it, I ask myself once more, that the Fly-huntress allows herself to be worried by another of the tribe, by an infinitesimal bandit, incapable of the least resistance, whom she could reach with a sudden rush if she tried? Why not relieve herself of the prey that clogs her movements and swoop down upon those evil-doers? What would be needed to exterminate the ill-omened brood that hangs around the burrow? A battue that would take her a few seconds. But the harmony of the universe, the laws that regulate the preservation of species, will not have it so; and the Bembex will always allow themselves to be harassed without ever learning from the famous ‘struggle for life’ the radical method of extermination. I have seen them sometimes, when too close-pressed by the Midges, drop their prey and fly away in mad haste, but without any hostile demonstration, though the putting down of the burden left them quite free in their movements. The abandoned prey, but now so ardently coveted by the Tachinæ, lay on the ground, for all to do as they pleased with; and not one of them took any notice of it. This game lying in the open air had no value for the Midges, whose larvæ require the shelter of a burrow. It was valueless also to the suspicious Bembex, who, on returning, felt it for a moment and left it with scorn. A momentary break in her vigilance had made her doubtful of it.

We will end this chapter with the story of the larva. Its monotonous life offers nothing remarkable in the fortnight during which it eats and grows. Next comes the construction of the cocoon. The meagre development of the silk-producing organs does not allow the grub a dwelling of pure silk, composed, like those of the Ammophilæ and the Sphex, of several wrappers, one outside the other, which protect the larva and afterwards the nymph against the inroads of damp in a shallow and exposed burrow when the rains of autumn come and the snows of winter. Nevertheless, the Bembex’ burrow is in a worse plight than that of the Sphex, being situated at a depth of a few inches in easily saturated soil. Therefore, in order to construct itself an adequate shelter, the larva makes up by its industry for its small quantity of silk. With grains of sand artistically put together and cemented with the silky material it builds itself an exceedingly solid cocoon, impenetrable to damp.

Three general methods are employed by the Digger-wasps in constructing the sanctum in which the metamorphosis is to take place. Some dig their burrows at great depths, under shelter: their cocoon then consists of a single envelope, so thin as to be transparent. This is the case with the Philanthi and the Cerceres. Others are content with a shallow burrow in open ground; but in that case they sometimes have enough silk to increase the number of wrappers for the cocoon, as we see with the Sphex, the Ammophilæ and the Scoliæ, or sometimes the quantity of silk is insufficient, when they have recourse to gummed sand, this being the method practised by the Bembex, the Stizi and the Palari. A Bembex-cocoon is so compact and strong that it might be taken for the kernel of some seed. The form is cylindrical, with one end rounded and the other pointed. The length is about three-quarters of an inch. On the outside it is slightly wrinkled and rather coarse to look at; but the inner walls are glazed with a fine varnish.

My experiments in indoor breeding have enabled me to observe every detail of the construction of this architectural curiosity, a regular strong-box inside which the inclemencies of the weather can be braved in safety. The larva first pushes away the remains of its food and forces them into a corner of the cell or compartment which I have arranged for it in a box with paper partitions. Having swept the floor, it fixes at the different walls of its dwelling threads of a beautiful white silk, forming a spidery web which keeps off the cumbrous heap of broken victuals and serves as a scaffolding for the next work.

This work consists of a hammock slung far from any dirt, in the centre of the threads stretched from wall to wall. Nothing but silk, magnificently fine, white silk, enters into its composition. Its shape is that of a sack open at one end with a wide circular mouth, closed at the other and ending in a point. An eel-trap would give a very fair picture of it. The edges of the mouth are kept apart and permanently stretched by numerous threads starting from there and fastened to the adjoining walls. Lastly, the texture of this sack is extremely fine and allows us to see all the grub’s proceedings.

Things had been in this condition since the day before, when I heard the larva scratching in the box. I opened it and found my prisoner engaged in scraping the cardboard wall with its mandibles, while its body was half outside the sack. The cardboard had already suffered considerably and a heap of tiny fragments were piled in front of the opening of the hammock, to be used later. For lack of other materials, the grub would doubtless have employed these scrapings for its building. I thought it better to provide something in accordance with its tastes and to give it sand. Never had Bembex-larva built with such sumptuous materials. I poured before the captive sand from my ink-stand: blotting-sand, blue sand sprinkled with little gilt mica spangles.

This supply is placed in front of the mouth of the bag. The bag itself is in a horizontal position, which is convenient for the coming task. The larva, leaning half out of the hammock, picks up its sand almost grain by grain, rummaging in the heap with its mandibles. If any grain is found to be too bulky, the grub takes it and throws it away. When the sand is thus sorted, the larva introduces a certain quantity into the silken edifice by sweeping it with its mouth. This done, it retires into the eel-trap and begins to spread the materials in a uniform layer on the lower surface of the sack; then it gums the different grains and inlays them in the fabric, using silk as cement. The upper surface is built more slowly: the grains are carried up one by one and fixed on with the silken putty.

This first layer of sand as yet embraces only the front half of the cocoon, the half that ends at the mouth of the bag. Before turning round to work at the back half, the grub renews its supply of materials and takes certain precautions so as not to be hindered in its mason’s work. The sand outside, heaped up in front of the entrance, might slip inside and embarrass the builder in so narrow a space. The grub foresees this possibility: it glues a few grains together and makes a rough curtain of sand, which stops up the orifice very imperfectly, but sufficiently to prevent an accident. Having taken these precautions, the larva works at the back half of the cocoon. From time to time it turns round to fetch fresh supplies from outside, tearing a corner of the curtain that protects it against the outer sand and grabbing through this window the materials which it requires.

The cocoon is still incomplete, wide open at the big end; it wants the spherical cap that is to close it. For this final labour the grub takes a plentiful supply of sand, the last supply of all, and then pushes away the heap outside the entrance. At the opening it now weaves a silken cap, which fits the mouth of the primitive eel-trap precisely. Lastly, grains of sand, kept in reserve inside, are laid one by one upon this silken foundation and glued together with silky slime. Having finished this lid, the larva has nothing else to do but give the last finish to the inside of the abode and glaze the walls with varnish to protect its delicate skin against the rough sand.

The hammock of pure silk and the hemisphere that closes it later are, as we see, but a scaffolding intended to support the masonry of sand and give it a regular curve; they might be compared with the wooden moulds which builders set up when constructing an arch, a vault. Once the work is done, the timber frame is taken away and the vault is sustained by virtue of its perfect balance. Even so, when the cocoon is finished, the silken support disappears, partly lost in the masonry, partly destroyed by contact with the coarse earth; and not a trace remains of the ingenious method followed in welding together materials with so little consistency as sand into a building of such perfect regularity.

The round cap closing the mouth of the original eel-trap is a work apart, adjusted to the main body of the cocoon. However well the two parts are fitted and soldered, the solidity is not the same as the larva would obtain if it built its whole dwelling continuously. The circumference of the lid therefore has a circular line of least resistance. But this is not a fault of construction; on the contrary, it is a fresh improvement. The insect would find grave difficulty in issuing later from its strong-box, so stout are the walls. The line of junction, weaker than the others, would seem to save it a good deal of effort, for it is mostly along this line that the cover is removed when the Bembex emerges from the ground in the perfect state.

I have called this cocoon a strong-box. It is indeed a very solid piece of work, both from its shape and from the nature of its materials. Landslips or subsidences cannot alter its outline, for the strongest pressure of one’s fingers does not always succeed in crushing it. Therefore it matters little to the larva if the ceiling of its burrow, dug in loose soil, should fall in sooner or later; it does not care much if a passing foot should press upon it under its thin covering of sand; it has nothing to fear once it is enclosed in its stout bulwark. Nor does damp endanger it. I have kept Bembex-cocoons immersed in water for a fortnight at a time without afterwards discovering the least trace of dampness inside them. Why have we no such waterproofing for our dwellings!

Lastly, thanks to its graceful oval, this cocoon seems rather the product of some elaborate manufacture than that of a grub. To any one unacquainted with the secret, the cocoons which I had built with blotting-sand might have been jewels of some unknown workmanship, great beads studded with golden spots on a lapis-lazuli ground, destined to form the necklace of a Polynesian belle.

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