A PARASITE—THE COCOON

Insect life: Souvenirs of a naturalist by Jean-Henri Fabre, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. A PARASITE—THE COCOON

XVIII. A PARASITE—THE COCOON

I have just described the Bembex hovering, loaded with her prey, above the nest, and descending with a vertical flight—very slow, and accompanied by a plaintive hum. This cautious, hesitating mode of arrival might suggest that the insect was examining from above in order to find her door, and trying to recall the locality before alighting. But I shall show that there is another motive. In ordinary conditions, when nothing alarms her, she comes suddenly, without hovering or plaintive hum or hesitation, and alights at her threshold, or close by. So faithful is her memory that she has no need to search about. Let us find out the cause of the hesitating arrival just described.

The insect hovers, descends slowly, mounts again, flies off and returns, because serious danger threatens. That plaintive hum is a sign of anxiety, and is never produced unless there is peril. But who is the enemy? Is it I, sitting by and watching? Not in the least; I am quite unimportant—a block unworthy of notice. The dreaded enemy—the foe who must be avoided at any price—is on the ground, perfectly [244]still upon the sand, near the nest. It is a small Dipteron—nothing at all to look at—of inoffensive aspect. This petty fly is the terror of the Bembex. That bold assassin of Diptera, who so promptly twists the neck of colossal gadflies, full fed on blood from an ox’s back, dares not enter her home because she sees herself watched by another Dipteron—a mere pigmy, which would scarce make one mouthful for her larva.

Why not pounce on it and get rid of it? The Bembex flies fast enough to overtake it, and, small as it is, the larvæ would not disdain it, since they eat all and every Diptera. Yet the Bembex flies in terror before an enemy which one bite would hew in pieces. I really feel as though I saw a cat wild with terror before a mouse. The ardent pursuer of Diptera is driven away by a Dipteron, and that one of the smallest! I bow before the facts without any hope of ever comprehending this reversal of parts. To be able to get rid easily of a mortal enemy, who is meditating the ruin of your family, and who might make a feast for them—to be able, I say, to do this, and not to do it when the foe is there, within reach, watching you, defying you,—is the height of folly in an animal. Folly, however, is not rightly the word: let us rather talk of the harmony of creatures, for since this wretched little Dipteron has its small part to play in the great whole of things, the Bembex must needs respect it and basely flee before it,—otherwise long ago there would have been no more Dipteron of this species in the world.

Let us trace the history of this parasite. Among Bembex nests there are found, and that frequently, [245]some which are occupied at the same time by the larvæ of the Hymenoptera and by other larvæ—strangers to the family and greedily sharing their food. These strangers are smaller than the nursling of the Bembex—shaped like a tear, and of the colour of wine, from the food paste which can be seen through their transparent bodies. Their number varies from six to ten or more. They belong to a kind of Dipteron, as may be perceived from their form and from the pupæ which one afterwards finds in their place. The demonstration is completed by bringing them up one’s self in a box, where, fed daily with flies, and laid on sand, they turn into pupæ, whence issue the following year little Diptera—Tachinids of the genus Miltogramma.

This is the Dipteron which, when lying in wait near the burrow, awakens such alarm in the Bembex. Her terror is only too well founded. This is what happens in the dwelling. Around the heap of food which the mother wears herself out in providing in sufficient quantity, sit in company with the legitimate nursling from six to ten hungry guests, who put their sharp mouths into the general heap as unceremoniously as if they were at home. Concord seems to reign at table. I have never seen the legitimate larva take offence at the indiscretion of the strangers, nor observed these attempt to trouble its repast. All keep themselves together, and eat peaceably without annoying their neighbours.

So far all would be well, were it not that a grave difficulty arises. However active may be the mother-nurse, it is clear that she cannot meet such a consumption of food. She has to be incessantly on the [246]wing to feed one larva: what must happen if there are a dozen gluttons to provide for? The result of this enormous increase of family can only be want, or even famine, not for the larvæ of the Dipteron (which develop more rapidly than that of the Bembex, profiting by the days when abundance still reigns, their host being yet in early youth), but for the latter, who reaches the moment of metamorphosis without being able to make up for lost time. Besides, when the first guests become pupæ and leave the table free to it, others come, as long as the mother visits the nest, and complete its starvation.

In burrows invaded by numerous parasites the Bembex larva is undoubtedly much smaller than one would expect from the heap of food consumed, the remains of which encumber the cell. Limp, emaciated,—only half or a third of its proper size,—it vainly tries to spin a cocoon, the silk for which it has not got, and it perishes in a corner of the cell, amid the pupæ of guests more fortunate than itself. Or its end may be yet more tragic. Should provender fail, or the mother delay too long in returning with food, the Diptera devour it. I ascertained this black deed by bringing up the brood myself. All went well as long as food was plentiful, but if through neglect, or on purpose, the daily supply failed, next day or the day after I was sure to find the Diptera larvæ greedily rending that of the Bembex. Thus, when the nest is invaded by parasites, the legitimate larva is fated to perish either by hunger or a violent death, and this it is which makes the sight of Miltogramma prowling round the nest so odious to the Bembex.[247]

The Bembex is not the only victim of these parasites: the burrows of one and all of the mining Hymenoptera are invaded by Tachinids, especially by the Miltogramma. Various observers—notably Lepeletier de Saint Fargeau—have spoken of the manœuvres of these impudent Diptera; but as far as I know none have perceived the very curious case of parasitism at the expense of the Bembex—very curious, because the conditions are quite different. Nests of other Fossors are stored beforehand, and the Miltogramma drops an egg on the prey just as it is being carried in. The provender stored and her egg laid, the Hymenopteron closes up the cell where thenceforward live the legitimate larva and the strangers, unvisited in their prison. Thus, the robbery committed by the parasite is unknown to the mother, and must consequently remain unpunished.

With the Bembex it is quite otherwise. The mother constantly returns during the fortnight that she is bringing up the larva; she knows that her offspring is living among numerous intruders, who appropriate the greater part of the food; every time that she brings provender she touches and feels at the bottom of her den these detestable guests, who, far from contenting themselves with remains, seize what is best. She must perceive, however small her powers of arithmetic may be, that twelve are more than one; besides, she would discover this from the disproportion between the consumption of food and her means of hunting, and yet, instead of seizing these bold intruders and bundling them out, she serenely tolerates them. Tolerates! Why, she [248]feeds them and brings them their rations, and perhaps feels as much tenderness for them as for her own larva. It is a new version of the cuckoo story in yet more singular circumstances. The theory that the cuckoo, almost as big as a sparrowhawk and coloured like it, should look imposing enough to introduce an egg unresisted into the nest of the weak hedge-sparrow, and that the latter, overawed perhaps by the alarming look of her toad-faced nursling, should accept and care for the stranger, has something in its favour. But what shall we say of a sparrow which, turning parasite, should go with splendid audacity and intrust her eggs to the eyrie of a bird of prey—the nest of the sparrowhawk itself—the sanguinary devourer of sparrows? What should we say of the bird of prey who should accept the charge and bring up the brood tenderly? It is precisely thus that the Bembex acts,—she, a captor of Diptera who yet brings up other Diptera—a huntress who distributes food to a prey whose last repast will be her own disembowelled offspring! I leave to cleverer people the task of explaining these amazing relations.

Let us observe the tactics employed by the Tachinid, whose object is to confide her egg to the nest of the miner. It is an invariable rule that the fly should never penetrate into the burrow, even if left open and the owner absent. The crafty parasite would take good care not to entangle itself in a passage, where, having no possibility of flight, it might pay dearly for its effrontery. The only moment for its designs—a moment watched for with the greatest patience—is that when the Hymenopteron [249]enters the gallery, clasping her prey. At that instant, brief as it is, when the Bembex or any other miner has half her body within the entrance, and is about to disappear underground, the Miltogramma arrives on the wing, perches on the prey slightly, projecting beyond the hinder end of the Bembex, and while she is delayed by the difficulties of entering, the Miltogramma, with unparalleled promptitude, lays an egg on the prey, or two, or even three eggs, successively. The hesitation of the Bembex, embarrassed by her load, lasts but the twinkling of an eye; but that matters not—it is long enough for the fly to accomplish its misdeed without being dragged beyond the threshold. What must not be the suppleness of organs to achieve this instantaneous laying of the egg! The Bembex disappears, herself introducing the enemy, and the Tachinid goes and crouches in the sun, close to the burrow, and meditates fresh crimes. If one would make sure that the Dipteron’s eggs have really been deposited during this rapid manœuvre, it suffices to open the burrow and follow the Bembex to the bottom of her abode. The prey which one takes from her bears underneath at least one egg—sometimes more, according to the length of the delay at the entrance. These very minute eggs could only belong to a parasite, and if any doubt remained, you can bring up the brood in a box, and the result will be Diptera larvæ—later pupæ, and finally Miltogramma.

The fly shows wonderful sagacity in the moment selected by it—the only one which could permit of her carrying out her purpose with neither peril nor vain efforts. The Bembex, half-way through [250]the entrance, cannot see her enemy audaciously perched on the hind quarters of the prey, or, if she suspects the bandit’s presence, cannot drive it away, having no freedom of movement in the strait passage, and in spite of all precautions to facilitate speedy entrance, cannot always vanish underground with the celerity required, so quick is the parasite. In fact, this is the only propitious moment, since prudence forbids the Dipteron to penetrate into the den, where other Diptera, far stronger than itself, are served up as food for the larvæ. Outside, in the open air, the difficulty is insurmountable, so great is the vigilance of the Bembex. Let us give a moment to the arrival of the mother, when the nest is being watched by the Miltogramma.

Some of these flies—more or fewer, generally three or four—have settled on the sand and are quite motionless, all gazing at the burrow, the entrance of which they know very well, carefully hid though it be. Their dull-brown colour, their large crimson-red eyes, their intense stillness, have often made me think of bandits who, dressed in a dark material, with a red kerchief over their heads, are lying in wait to do some evil deed. The Hymenopteron comes, loaded with prey. Had she no anxieties she would alight straightway at her door. Instead, she hovers at a certain height, descends slowly and circumspectly, hesitates, and vibrates her wings, producing a plaintive hum denoting apprehension. She must have seen the malefactors. They too have seen the Bembex. The movement of their red heads shows that they are following her with their eyes; every gaze is fixed on the coveted [251]booty. Then come marches and counter-marches of cunning versus prudence.

The Bembex drops straight down with an imperceptible flight, as if she let herself sink gently, making a parachute of her wings. Now she is hovering just above the ground; the flies take wing, placing themselves one and all behind her,—some nearer, some farther,—in a geometrical line. If she turns round to disconcert them, they turn too, with a precision which keeps them all in the same straight line; if she advances, so do they; if she draws back, they draw back too, measuring their flight, now slow, now stationary, on that of the Bembex at the head of the file. They do not attempt to fling themselves on the desired object, their tactics being merely to hold themselves in readiness in the position of rearguard, so as to avoid any hesitation when the rapid final manœuvre shall come.

Sometimes, wearied out by their obstinate pursuit, the Bembex alights, and the flies instantly settle on the sand, still behind her, and keep quite still. She rises again, with a sharper hum—the sign no doubt of increasing indignation; the flies follow her. One last means remains to throw the tenacious Diptera off the track; the Bembex flies far away—perhaps hoping to mislead the parasites by rapid evolutions over the fields. But the crafty flies are not taken in; they let her go, and settle down again on the sand round the burrow. When the Bembex returns the same manœuvres begin again until the obstinacy of the parasites has exhausted her prudence. At a moment when her vigilance fails, the flies are [252]instantly there. Whichever is at the most favourable point drops upon the vanishing prey, and the thing is done—the egg is laid.

There is ample evidence that the Bembex is conscious of danger, and knows how disastrous for the future of her nest is the presence of the hated fly; her long efforts to throw the parasites off her track, her hesitation and flights, leave not a doubt on the subject. How is it then, I ask myself once more, that the enemy of Diptera should allow herself to be annoyed by another Dipteron—a tiny robber, incapable of the least resistance, which, if she chose, she could destroy instantly? Why, when once free from the prey which hampers her, does she not pounce on these ill-doers? What is needed to exterminate the evil brood around her burrow? Merely a battle which would take but a few instants. But the harmony of those laws which govern the preservation of species will not have it so, and the Bembex will always allow herself to be harassed without ever learning from the famous “struggle for life” the radical means of extermination. I have seen some which, pressed too closely, let fall their prey and flew off wildly, but without any hostile demonstration, although dropping their game left them full liberty of action. The prey, so ardently desired a moment earlier by the Tachinidæ, lay on the ground at the mercy of them all, and not one cared about it. It had no value for the flies, whose larvæ need the shelter of a burrow. It was valueless also to the Bembex, who came back, felt it for an instant and left it disdainfully. The little break in her custody of it had rendered her suspicious of it.[253]

Let us end this chapter by the history of the larva. Its monotonous life offers nothing remarkable during the two weeks while it eats and grows. Then comes making a cocoon. The slight development of silk-producing organs does not allow of a dwelling of pure silk, like those of the Ammophila and Sphegidæ—made of several wrappers which protect the larva, and later the nymph, from damp in the ill-protected, shallow burrow during autumn rains and winter snows. Yet this Bembex burrow is in worse conditions than those of the Sphex, being made at a depth of only a few inches in very permeable soil. To fashion a sufficient shelter the larva supplements by its industry the small amount of silk at its disposal. With grains of sand artistically put together and connected by silky matter, it constructs a most solid cocoon—impenetrable to damp.

Three general methods are employed by fossorial Hymenoptera to construct the dwelling in which metamorphosis is to take place. Some hollow burrows at a great depth under a shelter, and then the cocoon consists of a single wrapper, so thin as to be transparent. Such is the case with Philanthidæ and Cerceris. Others are content with a shallow burrow in open ground; but in that case they have silk enough for manifold wrappings of the cocoon, as with Sphegidæ, Ammophila, and Scolia; or if the quantity be insufficient, they use agglutinated sand—as, for instance, the Bembex and Palarus. One might take a Bembex cocoon for a solid kernel, so compact and resistant is it. The form is cylindrical—one end rounded, the other pointed. [254]The length is about two centimetres. Outside it is slightly wrinkled and coarse, but within the walls are smoothed by a fine varnish.

Rearing at home has enabled me to follow every detail of the construction of this curious piece of architecture—a real strong box which can brave all the severity of the weather. First of all the larva pushes away the remains of its feast into a corner of the cell, or the compartment arranged for it in a box with paper partitions. Having cleared a space, it affixes to the walls of its abode threads of a beautiful white silk, forming a spidery web which keeps the heap of food-remains at a distance, and serves as scaffolding for the work to come.

This work consists of a hammock, suspended far from anything that can defile it, in the centre of threads stretched from wall to wall. Fine, beautiful white silk is the only material used. The shape is that of a sack open at one end, with a wide circular orifice, closed at the other and ending in a point; a fisherman’s basket gives a very fair idea of it. Then the edges of the aperture are permanently kept apart by numerous threads fastened to the neighbouring walls. The tissue of the bag is extremely fine, allowing all that the grub does to be seen.

Things had been in this state since the previous evening, when I heard the larva scratching in the box. On opening, I found my captive busy scratching the cardboard walls with the tips of its mandibles, its body half out of the bag. Already it had made considerable progress, and a heap of little fragments were piled before the opening of the hammock, to be utilised later. For lack of other [255]materials it would doubtless have used these scrapings for its constructions, but I thought it better to provide according to its tastes and give it sand. Never did Bembex larva build with such sumptuous material. I poured out for my prisoner sand for drying writing,—sand well sprinkled with gilded grains of mica,—before the opening of the bag, which was in a horizontal position, suitable to the work which would follow. The larva, half out of its hammock, chose its sand almost grain by grain, routing in the heap with its mandibles, and, if one too bulky presented itself, it was seized and cast aside. The sand being sorted, the larva introduced a certain quantity with its mouth into the silken fabric, then retired into its sack and began spreading the materials in a uniform layer on the inner surface, then glued together various grains and inlaid them in the fabric, with silk for cement. The outer surface was constructed more slowly. These grains were carried singly and fixed on with silk gum.

This first deposit of sand only concerns the anterior part of the cocoon—that half which ends in the opening. Before turning round to work at the back part, the larva renews its store of materials and takes certain precautions, so as not to be embarrassed in its masonry. The sand heaped before the entrance might slip inside and hinder the builder in so narrow a space. The grub foresees this, and glues some grains together, making a coarse curtain of sand, which stops up the orifice, imperfectly indeed, but enough for the purpose. These precautions taken, the grub labours at the back part of the cocoon. From time to time it turns round to get [256]fresh materials from outside, tearing away a corner of the protecting curtain, and through this window grasping the materials needed. The cocoon is still incomplete—wide open at the upper end and without the spherical cap needed to close it. For this final bit of work the grub provides itself abundantly with sand, and then pushes away the heap before the entrance. A silken cap is now woven and fitted close to the mouth of this primitive basket. On this silken foundation are deposited, one by one, the sand grains kept in the interior and cemented with silk-spittle. This lid completed, the larva has only to give the last finish to the interior of the dwelling and glaze the walls with varnish, to protect its tender skin from the roughness of the sand.

The hammock of pure silk and the cap which later closes it are evidently only scaffolding intended to support the masonry of sand and to give it a regular curve. One might compare them to the constructions used by builders when making an arch or vault. The work being completed, the silken support disappears, partly lost in the masonry, and partly destroyed by contact with rough earth, and no trace remains of the ingenious method employed to put together a construction perfectly regular, yet made of a material so little coherent as is sand. The spherical cap which closes the original basket is a separate work, adjusted to the main body of the cocoon. However well the two pieces are fitted and soldered, the solidity is not such as the larva would obtain had it built the whole dwelling continuously. Thus, on the circumference of the cover there is a circular line less capable of resistance, but this is not [257]a fault of construction. On the contrary it is a fresh perfection. The insect would experience grave difficulty in issuing from its strong box, so thick are the walls, did not the line of junction, weaker than the rest, apparently save much effort, as it is usually along this line that the cover is detached when the perfect Bembex emerges.

I have called the cocoon a strong box. It is indeed a solid article, both from its shape and the nature of its materials. Landslips or falling sand cannot alter its form, since the strongest pressure of one’s fingers cannot always crush it. Thus it matters little to the larva if the ceiling of its burrow, dug in loose soil, should sooner or later fall in, and it need not fear, even should a passing foot press down the thin covering of sand; it runs no risks when once enclosed in its stout shelter. Nor does damp endanger it. I have immersed Bembex cocoons for a fortnight in water without finding any trace of damp inside them. Ah! why cannot we have such waterproof for our dwellings? To sum up: the cocoon, of graceful oval shape, appears rather the product of patient art than the work of a grub. For any one not behind the scenes, the cocoons which I saw in process of construction with the sand from my inkstand might well have been precious articles of some unknown industry—great beads starred with golden dots on a ground of lapis lazuli, destined for the necklace of some Polynesian belle.

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