THE BEMBEX

One of my favourite spots for the observations which I will now describe is not far from Avignon, on the right bank of the Rhône, opposite the mouth of the Durance. It is the Bois des Issarts. Let not the reader mistake the value of this word bois, which usually suggests a carpet of cool moss and the shade of tall trees, with a dim light filtering through the leaves. The scorched plains where the Cicada grates out his ditty on the pale olive-tree know none of these delicious retreats filled with cool shadow.

The Bois des Issarts is a coppice of holm-oaks no higher than one’s head and sparingly distributed in scanty clumps which, even at their feet, hardly temper the force of the sun’s rays. When I used to settle myself in some part of the coppice suitable for my observations, on certain afternoons in the dog-days of July and August, I had the shelter of a large umbrella, which later, in the most unexpected fashion, lent me a very precious aid of a different kind, as my story will show in good time. If I neglected to furnish myself with this embarrassing adjunct to a long walk, my only resource against sunstroke was to lie down at full length behind some sandy knoll; and, when the veins in my temples were throbbing to bursting point, my last hope lay in putting my head down a Rabbit-burrow. Such are one’s means of keeping cool in the Bois des Issarts.

The soil not occupied by those clumps of woody vegetation is almost bare and consists of fine, dry, very loose sand, which the wind heaps into little dunes wherever the stems and roots of the holm-oak interfere with its dissemination. The sides of these sand-dunes are generally very smooth, because of the extreme lightness of the materials, which slide down into the smallest depression and of their own accord restore the evenness of the surface. You need but push your finger into the sand and take it out again to bring about an immediate landslip which fills up the hole and restores things to their original condition without leaving a visible trace. But, at a certain depth, which varies according to the more or less recent date of the last rains, the sand retains a lingering dampness which keeps it in its place and gives it a consistency that enables it to have small excavations made in it without a subsequent collapse of walls and roof. A blazing sun, a gloriously blue sky, sandy slopes that yield without the least difficulty to the strokes of the Wasp’s rake, game galore for the grub’s food, a peaceful site hardly ever disturbed by the foot of man: all the good things are combined in this Bembex paradise. Let us watch the industrious insect at work.

If the reader will sit with me under the umbrella, or consent to share my Rabbit-burrow, this is the sight which he is invited to behold, at the end of July: a Bembex (B. rostrata) arrives suddenly, I know not whence, and alights, without preliminary investigations or the least hesitation, at a spot which to my eyes differs in no respect from the rest of the sandy surface. With her fore-tarsi, which are armed with rows of stiff hairs and suggest at the same time a broom, a brush and a rake, she works at clearing her subterranean dwelling. The insect stands on its four hind-legs, holding the two at the back a little wide apart, while the front ones alternately scratch and sweep the shifting sand. The precision and quickness of the performance could not be greater if the circular movement of the tarsi were worked by a spring. The sand, shot backwards under the abdomen, passes through the arch of the hind-legs, gushes like a fluid in a continuous stream, describes its parabola and falls to the ground some seven or eight inches away. This spray of dust, kept up evenly for five or ten minutes at a time, is enough to show the dazzling rapidity of the tools employed. I know no other example of this swiftness, which nevertheless in no way detracts from the easy grace and the free movement of the insect, as it advances and retires first on this side, then on that, without discontinuing its parabolic streams of sand.

The soil excavated is of the lightest kind. As the Wasp digs, the sand near by slips back and fills the cavity. Amongst the rubbish that falls are tiny bits of wood, decayed leaf-stalks and particles of grit larger than the rest. The Bembex takes them up in her mandibles and carries them away, moving backwards as she goes; then she returns to her sweeping, but never going to any length and making no attempt to bury herself underground. What is her object in thus labouring entirely on the surface? It would be impossible to tell from this first glance; but, after spending many days with my beloved Wasps and grouping together the scattered facts resulting from my observations, I seem to catch a glimpse of the reason for the present proceedings.

The Wasp’s nest is certainly there, a few inches below the ground; in a little cell dug in the cool, firm sand lies an egg, perhaps a grub for which the mother caters from day to day, bringing it Flies, the unvarying food of the Bembex in their first state. The mother has to be able at any moment to enter the nest, as she flies up carrying in her legs the nurseling’s daily portion of game, even as the bird of prey enters its eyrie with the food for its young in its talons. But, while the bird returns to a home on some inaccessible ledge of rock, with no difficulty to overcome but that of the weight and encumbrance of the captured prey, the Bembex has each time to undertake rough miner’s work and open up anew a gallery blocked and closed by the mere fact that the sand gives way as the insect proceeds. In that underground dwelling, the only room with steady walls is the spacious cell where the larva lives amid the remnants of its fortnight’s feast; the narrow corridor which the mother enters to reach the flat at the back or to come out and go hunting collapses each time, at least in the front part dug out of very dry sand, which repeated exits and entrances make looser still. Each time therefore that the Wasp goes in or out, she has to clear herself a passage through the débris.

Going out presents no difficulty, even should the sand retain the consistency which it might have at the start, when first disturbed: the insect’s movements are free, it is safe under cover, it can take its time and use its tarsi and mandibles without undue hurry. Going in is a very different matter. The Bembex is hampered by her prey, which her legs hold clasped to her body; and the miner is thus deprived of the free use of her tools. And a still graver circumstance is this: brazen parasites, veritable bandits in ambush, crouch here and there in the neighbourhood of the burrow, spying on the mother Wasp as she makes her laborious entrance, so that they may rush in and lay their egg on the piece of game at the very moment when it is about to disappear down the corridor. If they succeed, the Wasp’s nurseling, the son of the house, will perish, starved by its gluttonous fellow-boarders.

The Bembex seems aware of these dangers and makes arrangements for her entrance to be effected swiftly, without serious obstacles—in short, for the sand blocking the door to yield to a mere push of her head, aided by a brisk sweep of her front tarsi. With this object, the material at the approaches to the home are subjected to a sort of sifting. At leisure moments, under a kindly sun, when the larva has its food and does not need her attentions, the mother rakes the ground in front of her door; she removes little bits of wood, any extra-large particles of gravel, any leaves that might get in the way and bar her passage at the dangerous moment of her return. The Bembex whom we have just seen so zealously employed was busy at this work of sifting: to facilitate the access to her home, the materials of the corridor have to be dug up, carefully sorted and rid of anything likely to obstruct the road. Who indeed can tell whether, by that nimble eagerness, that joyous activity, the insect is not expressing in its own way its maternal satisfaction, its happiness in watching over the roof of the cell to which the precious egg has been entrusted?

As the Wasp is confining herself to her duties outside the house, without trying to penetrate into the sand, everything must be in order inside and there is no hurry about anything. We should only wait in vain: the insect would tell us nothing more for the time being. Let us therefore examine the underground dwelling. If we scrape the dune lightly with the blade of a knife at the point where the Bembex was busiest, we soon discover the entrance-corridor, which, though blocked for part of the way down, is nevertheless recognizable by the distinctive appearance of the materials moved. This passage, which is as wide as one’s finger and straight or winding, longer or shorter according to the nature and the accidents of the ground, measures eight to twelve inches. It leads to a single chamber, hollowed in the damp sand, whose walls are not coated with any kind of mortar likely to prevent a subsidence or to lend a polish to the rough surface. The ceiling will do, if it can hold out while the larva is growing up; it does not matter what falls in afterwards, when the larva is enclosed in its stout cocoon, a sort of safe which we shall see it building. The workmanship of the cell, therefore, is very rustic: the whole thing is reduced to a rough excavation, of no definite shape, with a low roof and space enough to contain two or three walnuts.

In this retreat lies a piece of game, one only, quite small and quite insufficient for the greedy nurseling which it is meant to feed. It is a golden-green Fly, a Green-bottle (Lucilia Cæsar),1 who lives on putrid flesh. The Fly served up as food is absolutely motionless. Is she quite dead, or only paralysed? This question will be cleared up later. For the moment we will note the presence, on the side of the game, of a cylindrical egg, white, very slightly curved and a couple of millimetres2 long. It is the egg of the Bembex. As we expected from the mother’s behaviour, there is nothing urgent indoors: the egg is laid and provided with a first ration apportioned to the requirements of the feeble grub which will hatch twenty-four hours hence. The Bembex had no need to re-enter the underground passage for some time and was confining herself to keeping a good look-out all round, or perhaps to digging fresh burrows and continuing to lay her eggs, one by one, each in a cell to itself.

This peculiarity of beginning the provisioning with a single head of small game is not confined to the Rostrate Bembex. All the other species do the same thing. If we open the cell of any Bembex shortly after the egg is laid, we shall always find the tiny cylinder glued to the side of a Fly, who constitutes the entire provision; moreover, this initial ration is invariably small, as though the mother went in search of the tenderest mouthfuls for the feeble nurseling. Besides, another reason, the abiding freshness of the food, might easily prompt her to make this choice. We will look into that later. This first portion, always a scanty one, varies greatly in nature, according to the frequency of this or that kind of game in the neighbourhood of the nest. It is sometimes a Green-bottle, sometimes a Stomoxys, or some small Eristalis, sometimes a dainty Bee-fly clad in black velvet; but the most usual dish is a slim-bellied Sphærophoria.

This general fact, to which there is no exception, of the victualling of the egg with a single Fly, a ration infinitely too small for a larva blessed with a voracious appetite, at once puts us on the track of the most remarkable habit of the Bembex. Wasps whose larvæ live on prey heap up in each cell the number of victims necessary for the rearing of the grub; they lay the egg on one of the bodies and close the dwelling, which they do not enter again. From that moment the larva hatches and develops alone, having before it from the very beginning the whole stock of provisions which it is to consume. The Bembex form an exception to this rule. The cell is first stocked with a single head of game, always small in size, and the egg is laid on it. When that is done, the mother leaves the burrow, which closes of itself; besides, before going away, the insect is careful to rake over the outside, so as to smooth the surface and hide the entrance from any eye but her own.

Two or three days elapse; the egg hatches and the little larva eats up the choice ration served to it. Meanwhile the mother remains in the neighbourhood and you see her sometimes feeding herself by sipping the sugary exudations of the field eringo, sometimes settling happily on the burning sand, no doubt watching the outside of the house. Every now and again she sifts the sand at the entrance; then she flies away and disappears, perhaps to dig other cells elsewhere and to stock them in the same way. But, however long she may stay away, she never forgets the young larva so scantily provided for; the instinct of a mother tells her the hour when the grub has finished its food and is calling for fresh nourishment. She therefore returns to the nest, of which she is wonderfully capable of discovering the invisible entrance; she goes down into the earth, this time carrying a bulkier piece of game. After depositing her prey, she again leaves the house and waits outside till the moment arrives to serve a third course. This moment is not slow in coming, for the larva devours its food with a lusty appetite. Again the mother appears with fresh provisions.

During nearly a fortnight, while the larva is growing up, the meals thus follow in succession, one by one, as needed, and coming closer together as the nurseling waxes bigger. Towards the end of the fortnight it takes all the mother’s activity to satisfy the appetite of the glutton, who crawls heavily along with his great lumbering belly, amid the scorned leavings: rejected wings and legs and horny abdominal segments. You see her at every moment returning with a recent capture, at every moment setting out again upon the chase. In short, the Bembex brings up her family from day to day, without storing up provisions in advance, just as the bird does, which feeds its nestlings from hand to mouth. Of the many proofs that are evidence of this method of upbringing, a very singular method for a Wasp who feeds her offspring on prey, I have already mentioned the presence of the egg in a cell containing no provisions but one small Fly, never more. And here is another one, which can be verified at any time.

Let us look into the burrow of a Wasp who stocks her grubs’ provisions in advance: if we select the moment when the insect is going in with its prey, we shall find in the cell a certain number of victims, the commencement of a larder, but never at that time a grub, nor even an egg, for this is not laid until the provisions are quite complete. When the egg is laid, the cell is closed and the mother does not return to it. It is therefore only in burrows where the mother’s visits are no longer necessary that we can find larvæ side by side with larger or smaller stocks of food. On the other hand, let us inspect the home of a Bembex at the moment when she is entering with the fruits of her hunting. We are certain of finding in the cell a larva, big or little as the case may be, among remnants of provisions already consumed. The portion which the mother is now bringing is therefore intended to prolong a meal which has already lasted several days and which is to continue for some time further with the produce of future hunting expeditions. Should we be fortunate enough to make this search towards the end of the larva’s infancy—an advantage which I have enjoyed as often as I wished to—we shall find, on a copious heap of remnants, a large and portly grub, to which the mother is still bringing fresh victuals. The Bembex does not cease her catering and does not leave the cell for good until the larva, distended by a purply paste, refuses its food and lies down, stuffed to repletion, on the jumble of legs and wings of the game which it has devoured.

Each time that the mother enters the burrow on returning from the chase, she brings but a single Fly. If it were possible, by counting the remnants contained in a cell whose occupant is full-grown, to tell the number of victims supplied to the larva, we should know how often at the least the Wasp visited her burrow after laying the egg. Unfortunately, these broken victuals, chewed and chewed again at moments of scarcity, are for the most part unrecognizable. But, if we open a cell with a less forward nurseling, the provisions lend themselves to examination, some of them being still whole or nearly whole, while others, more numerous, are represented by fragments in a state of preservation that enables them to be identified. Incomplete though it be, the list obtained under these conditions is surprising and shows what activity the Wasp must display to satisfy the needs of such a table. I will set forth one of the bills of fare which I have observed.

At the end of September, around the larva of a Jules’ Bembex (Bembex Julii),3 which has reached almost a third of the size which it will finally attain, I find the following heads of game: six Echinomyia rubescens (two whole and four in pieces); four Syrphus corollæ (two complete, the other two broken up); three Gonia atra (all three untouched: one of them had that moment been brought along by the mother, which led to my discovering the burrow); two Pollenia rufescens (one untouched, the other partly eaten); one Bombylius (reduced to pulp); two Echinomyia intermedia (in bits); and two Pollenia floralis (likewise in bits): twenty pieces in all. This certainly makes a both plentiful and varied bill of fare; but, as the larva was only a third of its ultimate size, the complete menu might easily number as many as sixty items.

It is not at all difficult to verify this sumptuous figure: I will myself take the place of the Bembex in her maternal functions and supply the larva with food till it is ready to burst. I move the cell into a little cardboard box which I furnish with a layer of sand. I place the larva on this bed, with all due consideration for its delicate skin. Around it, without omitting a single fragment, I arrange the provisions with which it was supplied. Then I go home, still holding the box in my hand, to avoid any shaking which might turn the house upside down and endanger my charge during a walk of several miles. Any one who had met me on the dusty Nîmes Road, dropping with fatigue and religiously carrying in my hand, as the sole fruit of my laborious trip, an ugly grub battening on a heap of Flies, would certainly have smiled at my simplicity.

The journey was effected without damage: when I reached home, the larva was placidly eating its Flies as though nothing had happened. On the third day of captivity the provisions taken from the burrow were finished; the grub was rummaging with its pointed mouth among the heap of remains without finding anything to suit it; the dry particles taken hold of, all horny, juiceless bits, were rejected with disgust. The moment has come for me to continue the food supply. The first Flies within reach shall form my prisoner’s diet. I kill them by pressing them in my fingers, but without crushing them. The first ration consists of three Eristalis tenax and one Sarcophaga.4 This is all gobbled up in twenty-four hours. Next day I provide two Eristales, or Drone-flies, and four House-flies. It was enough for the day, but left nothing over. I went on like this for eight days, giving the grub a larger portion every morning. On the ninth day the larva refused all food and began to spin its cocoon. The full record of this eight days’ feast amounts to sixty-two pieces, composed mainly of Drone-flies and House-flies, which, added to the twenty items found whole or in pieces in the cell, brings up the total to eighty-two.

It is possible that I did not rear my larva with the wholesome frugality and the wise economy which the mother would have shown; there was perhaps some waste in the daily provisions served all at one time and left entirely to the grub’s discretion. In some respects I feel inclined to believe that things do not happen just like that in the maternal cell, for my notes contain such details as the following. In the alluvial sands of the Durance I discover a burrow which the Wasp (Bembex oculata) has just entered with a Sarcophaga agricola. Inside I find a larva, numerous fragments and a few whole Flies, namely, four Sphærophoria scripta, one Onesia viarum and two Sarcophaga agricola, including the one which the Bembex has just brought along before my eyes. Now it is worthy of remark that half of this game, namely, the Sphærophoriæ, is right at the end of the cell, under the larva’s very teeth, whereas the other half is still in the passage, on the threshold of the cell, and therefore beyond the reach of the grub, which is unable to change its position. It seems to me then that, when game is plentiful, the mother lays her captures on the threshold of the cell for the time and forms a reserve on which she draws as and when necessary, especially on rainy days when all labour is at a standstill.

Thus practised with economy, the distribution of food would save a waste which I was not able to prevent with my larva, treated I dare say too sumptuously. I therefore lower the figure obtained and reduce it to some sixty pieces, of middling size, between that of the House-fly and of the Eristalis tenax. This would about represent the number of Flies supplied by the mother to the larva when the prey is of a moderate size, as is the case with all the Bembex of my district except the Rostrate Bembex (B. rostrata) and the Two-pronged Bembex (B. bidentata), who have a preference for Gad-flies. With them, the number of victims would be from one to two dozen, according to the size of the Fly, which varies greatly in the different species of Gad-flies.

To avoid reopening this question of the nature of the provisions, I will here give a list of the Flies observed in the burrows of the six species of Bembex that form the subject of this essay.

1. Bembex olivacea, Rossi. I only once saw this species, at Cavaillon, feeding on Green-bottles. The five other species are common in the Avignon neighbourhood.

2. Bembex oculata, Jur. The Fly carrying the egg is most often a Sphærophoria, especially S. scripta; sometimes it is a Geron gibbosus. The later provisions include Stomoxys calcitrans, Pollenia ruficollis, P. rudis, Pipiza nigripes, Syrphus corollæ, Onesia viarum, Calliphora vomitoria,5 Echinomyia intermedia, Sarcophaga agricola and Musca domestica.6 The usual fare consists of Stomoxys calcitrans, of which I have many a time found fifty or sixty in a single burrow.

3. Bembex tarsata, Lat. This one also lays her egg on Sphærophoria scripta. She next hunts: Anthrax flava, Bombylius nitidulus, Eristalis æneus, E. sepulchralis, Merodon spinipes, Syrphus corollæ, Helophilus trivittatus and Zodion notatum. Her favourite game consists of Bombylii, or Bee-flies, and Anthrax-flies.7

4. Bembex Julii (sp. nov.). The egg is laid on a Sphærophoria or on a Pollenia floralis. The provisions are a hotchpotch of Syrphus corollæ, Echinomyia rubescens, E. intermedia, Gonia atra, Pollenia floralis, P. ruficollis, Clytia pellucens, Lucilia Cæsar, Dexia rustica and Bombylius.

5. Bembex rostrata, Fab. This is preeminently a consumer of Gad-flies. She lays her egg on a Syrphus corollæ or a Lucilia Cæsar, after which she feeds her larva exclusively on big game belonging to the various species of the genus Tabanus.

6. Bembex bidentata, V. L. Another ardent huntress of Gad-flies. I have never seen her pursue other game and I do not know on what Fly the egg is laid.

This great variety of provisions shows that the Bembex have no exclusive tastes and fall upon any species of Flies, indifferently, which the hazards of the chase place within their reach. They seem nevertheless to entertain a few preferences. Thus one species feeds more particularly on Bee-flies, a second on Stomoxys-flies, a third and a fourth on Gad-flies.