THE ADVENTURES OF A GRUB

The high banks of sandy clay in the country round about Carpentras are the favourite haunts of a host of Bees and Wasps, those lovers of a sunny aspect and of soil that is easy to dig in. Here, in the month of May, two Bees, both of them Mason-bees, builders of subterranean cells, are especially abundant. One of them builds at the entrance of her dwelling an advanced fortification, an earthly cylinder, wrought in open work and curved, of the width and length of a man’s finger. When it is peopled with many Bees one stands amazed at the elaborate ornamentation formed by all these hanging fingers of clay.

The other Bee, who is very much more frequently seen and is called Anthophora pilipes, leaves the opening of her corridor bare. The chinks between the stones in old walls and abandoned hovels, or exposed surfaces of sand stone or marl, are found suitable for her labours; [158]but the favourite spots, those to which the greatest number of swarms resort, are straight stretches of ground exposed to the south, such as occur in the cuttings of deeply-sunken roads. Here, over areas many yards in width, the wall is drilled with a multitude of holes, which give to the earthy mass the look of some enormous sponge. These round holes might have been made with a gimlet, so regular are they. Each is the entrance to a winding corridor, which runs to the depth of four or five inches. The cells are at the far end. If we wish to watch the labours of the industrious Bee we must visit her workshop during the latter half of May. Then—but at a respectful distance—we may see, in all its bewildering activity, the tumultuous, buzzing swarm, busied with the building and provisioning of the cells.

But it has been most often during the months of August and September, the happy months of the summer holidays, that I have visited the banks inhabited by the Anthophora. At this season all is silent near the nests: the work has long been completed: and numbers of Spiders’ webs line the crevices or plunge their silken tubes into the Bees’ corridors. That is no reason, however, for hastily abandoning the city that was once so full of life and bustle, and now appears deserted. A few inches below the surface, thousands of grubs are imprisoned in their cells of clay, resting until the coming [159]spring. Surely these grubs, which are paralysed and incapable of self-defence, must be a temptation—fat little morsels as they are—to some kind of parasite, some kind of insect stranger in search of prey. The matter is worth inquiring into.

Two facts are at once noticeable. Some dismal-looking Flies, half black and half white, are flying indolently from gallery to gallery, evidently with the object of laying their eggs there. Many of them are hanging dry and lifeless in the Spiders’ webs. At other places the entire surface of a bank is hung with the dried corpses of a certain Beetle, called the Sitaris. Among the corpses, however, are a few live Beetles, both male and female. The female Beetle invariably disappears into the Bees’ dwelling. Without a doubt she, too, lays her eggs there.

If we give a few blows of the pick to the surface of the bank we shall find out something more about these things. During the early days of August this is what we shall see: the cells forming the top layer are unlike those at a greater depth. The difference is owing to the fact that the same establishment is used by two kinds of Bee, the Anthophora and the Osmia.

The Anthophoræ are the actual pioneers. The work of boring the galleries is wholly theirs, and their cells are right at the end. If they, for any reason, leave the [160]outer cells, the Osmia comes in and takes possession of them. She divides the corridors into unequal and inartistic cells by means of rough earthen partitions, her only idea of masonry.

The cells of the Anthophora are faultlessly regular and perfectly finished. They are works of art, cut out of the very substance of the earth, well out of reach of all ordinary enemies; and for this reason the larva of this Bee has no means of spinning a cocoon. It lies naked in the cell, whose inner surface is polished like stucco.

In the Osmia’s cells, however, means of defence are required, because they are at the surface of the soil, are roughly made, and are badly protected by their thin partitions. So the Osmia’s grubs enclose themselves in a very strong cocoon, which preserves them both from the rough sides of their shapeless cells and from the jaws of various enemies who prowl about the galleries. It is easy, then, in a bank inhabited by these two Bees, to recognise the cells belonging to each. The Anthophora’s cells contain a naked grub: those of the Osmia contain a grub enclosed in a cocoon.

Now each of these two Bees has its own especial parasite, or uninvited guest. The parasite of the Osmia is the black-and-white Fly who is to be seen so often at the entrance to the galleries, intent on laying her eggs within them. The parasite of the Anthophora is the [161]Sitaris, the Beetle whose corpses appear in such quantities on the surface of the bank.

If the layer of Osmia-cells be removed from the nest we can observe the cells of the Anthophora. Some will be occupied by larvæ, some by the perfect insect, and some—indeed many—will contain a singular egg-shaped shell, divided into segments with projecting breathing-pores. This shell is extremely thin and fragile; it is amber-coloured, and so transparent that one can distinguish quite plainly through its sides a full-grown Sitaris, struggling as though to set herself at liberty.

What is this curious shell, which does not appear to be a Beetle’s shell at all? And how can this parasite reach a cell which seems to be inaccessible because of its position, and in which the most careful examination under the magnifying-glass reveals no sign of violence? Three years of close observation enabled me to answer these questions, and to add one of its most astonishing chapters to the story of insect life. Here is the result of my inquiries.

The Sitaris in the full-grown state lives only for a day or two, and its whole life is passed at the entrance to the Anthophora’s galleries. It has no concern but the reproduction of the species. It is provided with the usual digestive organs, but I have grave reasons to doubt whether it actually takes any nourishment whatever. [162]The female’s only thought is to lay her eggs. This done, she dies. The male, after cowering in a crevice for a day or two, also perishes. This is the origin of all those corpses swinging in the Spiders’ web, with which the neighbourhood of the Anthophora’s dwelling is upholstered.

At first sight one would expect that the Sitaris, when laying her eggs, would go from cell to cell, confiding an egg to each of the Bee-grubs. But when, in the course of my observations, I searched the Bees’ galleries, I invariably found the eggs of the Sitaris gathered in a heap inside the entrance, at a distance of an inch or two from the opening. They are white, oval, and very small, and they stick together slightly. As for their number, I do not believe I am exaggerating when I estimate it at two thousand at least.

Thus, contrary to what one was to some extent entitled to suppose, the eggs are not laid in the cells of the Bee; they are simply dumped in a heap inside the doorway of her dwelling. Nay more, the mother does not make any protective structure for them; she takes no pains to shield them from the rigours of winter; she does not even attempt to stop up the entrance-lobby in which she has placed them, and so protect them from the thousand enemies that threaten them. For as long [163]as the frosts of winter have not arrived these open galleries are trodden by Spiders and other plunderers, for whom the eggs would make an agreeable meal.

The better to observe them, I placed a number of the eggs in boxes; and when they hatched out about the end of September I imagined they would at once start off in search of an Anthophora-cell. I was entirely wrong. The young grubs—little black creatures no more than the twenty-fifth of an inch long—did not move away, though provided with vigorous legs. They remained higgledy-piggledy, mixed up with the skins of the eggs whence they came. In vain I placed within their reach lumps of earth containing open Bee-cells: nothing would tempt them to move. If I forcibly removed a few from the common heap they at once hurried back to it in order to hide themselves among the rest.

At last, to assure myself that the Sitaris-grubs, in the free state, do not disperse after they are hatched, I went in the winter to Carpentras and inspected the banks inhabited by the Anthophoræ. There, as in my boxes, I found the grubs all piled up in heaps, all mixed up with the skins of the eggs.

I was no nearer answering the question: how does the Sitaris get into the Bees’ cells, and into a shell that does not belong to it?[164]

The appearance of the young Sitaris showed me at once that its habits must be peculiar. It could not, I saw, be called on to move on an ordinary surface. The spot where this larva has to live evidently exposes it to the risk of many dangerous falls, since, in order to prevent them, it is equipped with a pair of powerful mandibles, curved and sharp; robust legs which end in a long and very mobile claw; a variety of bristles and probes; and a couple of strong spikes with sharp, hard points—an elaborate mechanism, like a sort of ploughshare, capable of biting into the most highly polished surface. Nor is this all. It is further provided with a sticky liquid, sufficiently adhesive to hold it in position without the help of other appliances. In vain I racked my brains to guess what the substance might be, so shifting, so uncertain, and so perilous, which the young Sitaris is destined to inhabit. I waited with eager impatience for the return of the warm weather.

At the end of April the young grubs imprisoned in my cages, hitherto lying motionless and hidden in the spongy heap of egg-skins, suddenly began to move. They scattered, and ran about in all directions through the boxes and jars in which they have passed the winter. [165]Their hurried movements and untiring energy showed they were in search of something, and the natural thing for them to seek was food. For these grubs were hatched at the end of September, and since then, that is to say for seven long months, they had taken no nourishment, although they were by no means in a state of torpor. From the moment of their hatching they are doomed, though full of life, to an absolute fast lasting for seven months; and when I saw their excitement I naturally supposed that an imperious hunger had set them bustling in that fashion.

The food they desired could only be the contents of the Anthophora’s cells, since at a later stage the Sitaris is found in those cells. Now these contents are limited to honey and Bee-grubs.

I offered them some cells containing larvæ: I even slipped the Sitares into the cells, and did all sorts of things to tempt their appetite. My efforts were fruitless. Then I tried honey. In hunting for cells provisioned with honey I lost a good part of the month of May. Having found them I removed the Bee-grub from some of them, and laid the Sitaris-grub on the surface of the honey. Never did experiment break down so completely! Far from eating the honey, the grubs became entangled in the sticky mass and perished in it, suffocated. “I have offered you larvæ, cells, [166]honey!” I cried in despair. “Then what do you want, you fiendish little creatures?”

Well, in the end I found out what they wanted. They wanted the Anthophora herself to carry them into the cells!

When April comes, as I said before, the heap of grubs at the entrance to the Bees’ cells begins to show signs of activity. A few days later they are no longer there. Strange as it may appear, they are all careering about the country, sometimes at a great distance, clinging like grim death to the fleece of a Bee!

When the Anthophoræ pass by the entrance to their cells, on their way either in or out, the young Sitaris-grub, who is lying in wait there, attaches himself to one of the Bees. He wriggles into the fur and clutches it so firmly that he need not fear a fall during the long journeys of the insect that carries him. By thus attaching himself to the Bee the Sitaris intends to get himself carried, at the right moment, into a cell supplied with honey.

One might at first sight believe that these adventurous grubs derive food for a time from the Bee’s body. But not at all. The young Sitares, embedded in the fleece, at right angles to the body of the Anthophora, head inwards, tail outwards, do not stir from the spot they have selected, a point near the Bee’s shoulders. [167]We do not see them wandering from spot to spot, exploring the Bee’s body, seeking the part where the skin is most delicate, as they would certainly do if they were really feeding on the insect. On the contrary, they are always fixed on the toughest and hardest part of the Bee’s body, a little below the insertion of the wings, or sometimes on the head; and they remain absolutely motionless, clinging to a single hair. It seems to me undeniable that the young Sitares settle on the Bee merely to make her carry them into the cells that she will soon be building.

But in the meantime the future parasites must hold tight to the fleece of their hostess, in spite of her rapid flights among the flowers, in spite of her rubbing against the walls of the galleries when she enters to take shelter, and in spite, above all, of the brushing which she must often give herself with her feet, to dust herself and keep spick and span. We were wondering a little time ago what the dangerous, shifting thing could be on which the grub would have to establish itself. That thing is the hair of a Bee who makes a thousand rapid journeys, now diving into her narrow galleries, now forcing her way down the tight throat of a flower.

We can now quite understand the use of the two spikes, which close together and are able to take hold of hair more easily than the most delicate tweezers. We [168]can see the full value of the sticky liquid that helps the tiny creature to hold fast; and we can realise that the elastic probes and bristles on the legs serve to penetrate the Bee’s down and anchor the grub in position. The more one considers this arrangement, which seems so useless as the grub drags itself laboriously over a smooth surface, the more does one marvel at all the machinery which this fragile creature carries about to save it from falling during its adventurous rides.

One 21st of May I went to Carpentras, determined to see, if possible, the entrance of the Sitaris into the Bee’s cells.

The works were in full swing. In front of a high expanse of earth a swarm of Bees, stimulated by the sun, was dancing a crazy ballet. From the tumultuous heart of the cloud rose a monotonous, threatening murmur, while my bewildered eye tried to follow the movements of the throng. Quick as a lightning-flash thousands of Anthophoræ were flying hither and thither in search of booty: thousands of others, also, were arriving, laden with honey, or with mortar for their building.

At that time I knew comparatively little about these insects. It seemed to me that any one who ventured [169]into the swarm, or—above all—who laid a rash hand on the Bees’ dwellings, would instantly be stabbed by a thousand stings. I had once observed the combs of the Hornet too closely; and a shiver of fear passed through me.

Yet, to find out what I wished to know, I must needs penetrate that fearsome swarm; I must stand for whole hours, perhaps all day, watching the works I intended to upset; lens in hand, I must examine, unmoved amid the whirl, the things that were happening in the cells. Moreover, the use of a mask, of gloves, of a covering of any kind, was out of the question, for my fingers and eyes must be absolutely free. No matter: even though I should leave the Bee’s nest with my face swollen beyond recognition, I was determined that day to solve the problem that had puzzled me too long.

Having caught a few stray Anthophoræ with my net, I satisfied myself that the Sitaris-larvae were perched, as I expected, on the Bees.

I buttoned my coat tightly and entered the heart of the swarm. With a few blows of the mattock I secured a lump of earth, and to my great surprise found myself uninjured. A second expedition, longer than the first, had the same result: not a Bee touched me with her sting. After this I remained permanently in front of the nest, removing lumps of earth, spilling the honey, [170]and crushing the Bees, without arousing anything worse than a louder hum. For the Anthophora is a pacific creature. When disturbed in the cells it leaves them hastily and escapes, sometimes even mortally wounded, without using its venomous sting except when it is seized and handled.

Thanks to this unexpected lack of spirit in the Mason-bee, I was able for hours to investigate her cells at my leisure, seated on a stone in the midst of the murmuring and distracted swarm, without receiving a single sting, though I took no precautions whatever. Country folk, happening to pass and seeing me seated thus calmly amid the Bees, stopped aghast to ask me if I had bewitched them.

In this way I examined the cells. Some were still open, and contained only a more or less complete store of honey. Others were closely sealed with an earthen lid. The contents of these varied greatly. Sometimes I found the larva of a Bee; sometimes another, fatter kind of larva; at other times honey with an egg floating on the surface. The egg was of a beautiful white, and was shaped like a cylinder with a slight curve, a fifth or sixth of an inch in length—the egg of the Anthophora.

In a few cells I found this egg floating all alone on the surface of the honey: in others, very many others, I saw, [171]lying on the Bee’s egg as though on a sort of raft, a young Sitaris-grub. Its shape and size were those of the creature when it is hatched. Here, then, was the enemy within the gates.

When and how did it get in? In none of the cells was I able to detect any chink by which it could have entered: they were all sealed quite tightly. The parasite must have established itself in the honey-warehouse before the warehouse was closed. On the other hand, the open cells, full of honey but as yet without an egg, never contain a Sitaris. The grub must therefore gain admittance either while the Bee is laying the egg, or else afterwards, while she is busy plastering up the door. My experiments have convinced me that the Sitaris enters the cell in the very second when the egg is laid on the surface of the honey.

If I take a cell full of honey, with an egg floating in it, and place it in a glass tube with some Sitaris-grubs, they very rarely venture inside it. They cannot reach the raft in safety: the honey that surrounds it is too dangerous. If one of them by chance approaches the honey it tries to escape as soon as it sees the sticky nature of the stuff under its feet. It often ends by falling back into the cell, where it dies of suffocation. It is therefore certain that the grub does not leave the fleece of the Bee [172]when the latter is in her cell or near it, in order to make a rush for the honey; for this honey would inevitably cause its death, if it so much as touched the surface.

We must remember that the young Sitaris which is found in a closed cell is always placed on the egg of the Bee. This egg not only serves as a raft for the tiny creature floating on a very treacherous lake, but also provides it with its first meal. To get at this egg, in the centre of the lake of honey, to reach this raft which is also its first food, the young grub must somehow contrive to avoid the fatal touch of the honey.

There is only one way in which this can be done. The clever grub, at the very moment when the Bee is laying her egg, slips off the Bee and on to the egg, and with it reaches the surface of the honey. The egg is too small to hold more than one grub, and that is why we never find more than one Sitaris in a cell. Such a performance on the part of a grub seems extraordinarily inspired—but then the study of insects constantly gives us examples of such inspiration.

When dropping her egg upon the honey, then, the Anthophora at the same time drops into her cell the mortal enemy of her race. She carefully plasters the lid which closes the entrance to the cell, and all is done. A second cell is built beside it, probably to suffer the same fate; and so on until all the parasites sheltered by [173]her fleece are comfortably housed. Let us leave the unhappy mother to continue her fruitless task, and turn our attention to the young larva which has so cleverly secured for itself board and lodging.

Let us suppose that we remove the lid from a cell in which the egg, recently laid, supports a Sitaris-grub. The egg is intact and in perfect condition. But now the work of destruction begins. The grub, a tiny black speck which we see running over the white surface of the egg, at last stops and balances itself firmly on its six legs; then, seizing the delicate skin of the egg with the sharp hooks of its mandibles, it tugs at it violently till it breaks and spills the contents. These contents the grub eagerly drinks up. Thus the first stroke of the parasite’s mandibles is aimed at the destruction of the Bee’s egg.

This is a very wise precaution on the part of the Sitaris-grub! It will have to feed on the honey in the cell: the Bee’s grub which would come out of the egg would also require the honey: there is not enough for two. So—quick!—a bite at the egg, and the difficulty is removed.

Moreover, another reason for the destruction of the egg is that special tastes compel the young Sitaris to make its first meals of it. The tiny creature begins by greedily drinking the juices which the torn wrapper of [174]the egg allows to escape. For several days it continues to rip the envelope gradually open, and to feed on the liquid that trickles from it. Meanwhile it never touches the honey that surrounds it. The Bee’s egg is absolutely necessary to the Sitaris-grub, not merely as a boat, but also as nourishment.

At the end of a week the egg is nothing but a dry skin. The first meal is finished. The Sitaris-grub, which is now twice as large as before, splits open along the back, and through this slit the second form of this singular Beetle falls on the surface of the honey. Its cast skin remains on the raft, and will presently disappear with it beneath the waves of honey.

Here ends the history of the first form adopted by the Sitaris.