THE BEMBEX

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. THE BEMBEX

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 [221]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.[222]

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 [223]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 [224]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.[225]

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 [226]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 [227]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.

Let us examine the burrow of a Hymenopteron, which provides beforehand for its larvæ. If we choose the moment when the insect enters with a captive, we shall find in the cell a certain number of victims already stored, but never a larva—not even an egg, for this is only laid when the provisions are complete. The egg deposited, the cell is closed, and the mother returns no more. It is, therefore, only in burrows where the mother’s visits are no longer needed that one can find larvæ amid the larger or smaller heap of food. Visit, on the other hand, the dwelling of a Bembex as she enters with the produce of her chase, and you are sure to find a larva, larger or smaller, amid the remains of food already devoured. The ration now brought is to continue a repast which has been going on for several days, and is to be prolonged upon the produce of future expeditions. If we can make this examination towards the end of the larva’s upbringing,—an advantage which I have enjoyed at pleasure,—we shall find upon a great heap of fragments a portly larva, to which the mother is still bringing food. The Bembex only ceases to do so and to leave the cell definitely when the larva, distended by a wine-coloured pap, refuses to eat, and reclines, thoroughly stuffed, on the remains of wings and feet of the game which it has devoured.[228]

Each time that she penetrates into the burrow on returning from the chase, the mother brings but a single fly. Were it possible by means of the remains contained in a cell where the larva is full grown to count the victims served up, one would at least know how often the Hymenopteron visited its burrow after the egg is laid. Unfortunately, these broken meats—munched and munched again in moments of scarcity—are for the most part unrecognisable. But on opening a cell with a less advanced nursling, one can examine the provisions, some of the prey being yet whole or nearly so, and others, more numerous, being trunks in sufficiently good preservation to be distinguishable. Incomplete as it is, the enumeration thus obtained strikes one with surprise, as showing what activity the Hymenopteron must display to satisfy the demands of such a table. Here is one of the bills of fare observed.

At the end of July around the larva of Bembex Julia, which had almost reached the third of its full size, I found the prey of which the following is the list:—Six Echinomyia rubescens—two whole and four in pieces; four Syrphus corollæ—two whole, two in fragments; three Gonia atra—all intact, and one just brought by the mother, which had enabled me to discover the burrow; two Pollenia ruficollis—one whole, one attacked; a Bombylius reduced to pulp; two Echinomyia intermedia in bits; and finally two Pollenia floralis, also in bits—total, twenty. Certainly we have here a bill of fare as abundant as varied, but as the larva had only attained to a third of its complete size, the entire bill of fare might well amount to sixty articles.[229]

The verification of this magnificent sum-total is easily obtained. I myself will undertake the maternal cares of the Bembex, and feed the larva until it is thoroughly satisfied. I place the cell in a little cardboard box furnished with a layer of sand. On this bed is placed the larva with due regard to its delicate epidermis. Around it, without omitting a single fragment, I arrange the provender with which it was supplied, and return home with the box still in my hand, to avoid any shake which might turn it topsy-turvy and endanger my charge during a journey of several miles. Any one who had seen me on the dusty road to Nîmes, exhausted with fatigue and bearing religiously in my hand, as the only result of my painful journey, a wretched grub, distending itself with a heap of flies, would assuredly have smiled at my simplicity. The journey was achieved without hindrance; when I got home the larva was peacefully consuming its flies as if nothing had happened. On the third day the provisions taken from the burrow were finished, and the grub with its pointed mouth was searching in the heap of remains without finding anything to its taste. The dry, horny, juiceless pieces which it got hold of were rejected with disgust. The moment had come for me to continue the food supply. The first Diptera within reach must content my prisoner; I slew them by squeezing them between my fingers, but did not crush them. Three Eristalis tenax composed the first ration, together with a Sarcophaga. In twenty-four hours all were devoured. The next day I provided two Eristalis and four house-flies. This sufficed for that day, but nothing was left over. I [230]went on thus for a week, giving the grub each morning a larger ration. On the ninth day it refused to eat and began to spin its cocoon. The bill of fare for the week’s high feeding amounted to sixty-two items, chiefly Eristalis and house-flies, which, added to the twenty items found entire or in fragments in the cell, formed a total of eighty-two.

Possibly I may not have brought up my larva with the wholesome frugality which the mother would have shown; there may have been some waste in the daily rations, provided all at once and left entirely to the discretion of the grub. I fancied that in some particulars things did not go on exactly as in the cell, for my notes have such details as: “In the alluvial sands of the Durance I discovered a burrow into which Bembex oculata had taken a Sarcophaga agricola. At the bottom of the gallery was a larva, numerous fragments, and some Diptera entire—namely, four Sphærophoria scripta, one Onesia viarum, and two Sarcophaga agricola, counting that which the Bembex had brought under my very eyes.” Now it must be remarked that one half of this game, the Sphærophoria, was quite at the bottom of the cell—under the very jaws of the larva, while the other half was still in the gallery—on the threshold of the cell—consequently out of the grub’s reach, as it could not leave its place. It would seem that when game abounds, the mother disposes provisionally of her captures on the threshold of the cell, and forms a reserve on which she draws as need arises, especially on rainy days, when all labour is at a standstill. This economy in distributing food would prevent the waste unavoidable with my larva perhaps too sumptuously [231]treated. I subtract then from the sum obtained, and reduce it to sixty pieces of medium size, between that of the house-fly and Eristalis tenax. This would be about the number of Diptera given by the mother to the larva when the prey is middle-sized, as is the case with all the Bembecids of my district except B. rostrata and B. bidentata, which especially favour the gadfly. For these the number of slain would be from one to two dozen, according to the size of the Dipteron, which varies greatly in the gadfly species.

In order not to return to the kind of provisions, I give a list of the Diptera observed in the burrows of the six kinds of Bembex, which are the subject of this essay.

(1) B. olivacea, Rossi. Once only have I seen this species, at Cavaillon, preying on Lucilia Cæsar. The five next are common round Avignon.

(2) B. oculata, Jur. The Dipteron upon which the egg is laid is generally a Sphærophoria, especially S. scripta; sometimes it is a Geron gibbosus. Further provender consisted in Stomoxys calcitrans, Pollenia ruficollis, P. rudis, Pipiza nigripes, Syrphus corollæ, Onesia viarum, Calliphora vomitoria, Echinomyia intermedia, Sarcophaga agricola, Musca domestica. The usual food was Stomoxys calcitrans, of which I have found fifty or sixty in a single burrow.

(3) Bembex tarsata, Lat. It, too, lays its egg on Sphærophoria scripta; but it also hunts Anthrax flava, Bombylius nitidulus, Eristalis æneus, E. sepulchralis, Merodon spinipes, Syrphus corollæ, Helophilus trivittatus, Zodion notatum. Its favourite prey consists in Bombylius and Anthrax.[232]

(4) Bembex Julii (a new species). The egg hatches either on a Sphærophoria or a Pollenia floralis, and the provender is a mixture of Syrphus corollæ, Echinomyia rubescens, Gonia atra, Pollenia floralis, P. ruficollis, Clytia pellucens, Lucilia Cæsar, Dexia rustica, Bombylius.

(5) Bembex rostrata. This is above all a captor of gadflies. It lays its egg on a Syrphus corollæ, or a Lucilia Cæsar, but then only brings to the larva large game belonging to the various kinds of Tabanus.

(6) Bembex bidentata. Another ardent hunter of gadflies. I have never seen it with other game, and do not know on what the egg is laid.

This variety of provisions shows that the Bembecids have no exclusive tastes, and attack one and all of the species of Diptera which are offered by the chances of the chase. They seem, however, to have some favourites—one species especially choosing Bombylius, another Stomoxys, and a third and fourth, Gadflies.

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