Fabre's Book of Insects by Jean-Henri Fabre - HTML preview

CHAPTER XVI
THE ANTHRAX FLY

I
A STRANGE MEAL

I made the acquaintance of the Anthrax in 1855 at Carpentras, when I was searching the slopes of which I have already told you, the slopes beloved of the Anthophora-bees. Her curious pupa, so powerfully equipped to force an outlet for the perfect insect, which is incapable of the least effort, seemed worthy of investigation. For that pupa is armed with a ploughshare in front, a trident at its tail, and rows of harpoons on its back, with which to rip open the Osmia-bee’s cocoon and break through the hard crust of the hill-side.

Let us, some day in July, knock away the pebbles that fasten the nests of the Mason-bees to the sloping ground on which they are built. Loosened by the shock, the dome comes off cleanly, all in one piece. Moreover—and this is a great advantage—the cells are all exposed at the base of the nest, for at this point they have no other wall than the surface of the pebble. Without any scraping, which would be wearisome work for us and dangerous to the Bees, we have all the cells before our eyes, together   with their contents—a silky, amber-yellow cocoon, as delicate and transparent as the skin of an onion. Let us split the dainty wrappers with the scissors, cell by cell, one after another. If fortune be at all kind, as it always is to the persevering, we shall end by finding cocoons harbouring two larvæ together, one more or less faded in appearance, the other fresh and plump. We shall also find some, no less plentiful, in which the withered larva is accompanied by a family of little grubs wriggling uneasily round it.

It is easy to see that a tragedy is happening under the cover of the cocoon. The flabby, faded larva is the Mason-bee’s. A month ago, in June, having finished its ration of honey, it wove itself a silken sheath in which to take the long sleep that precedes its transformation. It was bulging with fat, and was a rich and a defenceless morsel for any enemy that could reach it. And enemies did reach it. In spite of obstacles that might well seem insurmountable, the wall of mortar and dome-shaped cover, the enemy grubs appeared in the secret retreat, and began to eat the sleeper. Three different species take part in this murderous work, often in the same nest, in adjoining cells. We will concern ourselves only with the Anthrax Fly.

The grub, when it has eaten its victim and is left alone in the Mason-bee’s cocoon, is a naked worm, smooth, legless, and blind. It is creamy-white, and each of its   segments or divisions forms a perfect ring, very much curved when at rest, but almost straight when disturbed. Including the head I can count thirteen segments, well-marked in the middle of the body, but in the fore-part difficult to distinguish. The white, soft head shows no sign of any mouth, and is no bigger than a tiny pin’s head. The grub has four pale red stigmata, or openings through which to breathe, two in front and two behind, as is the rule among Flies. It has no walking-apparatus whatever; it is absolutely incapable of shifting its position. If I disturb its rest, it curves and straightens itself alternately, tossing about violently where it lies; but it does not manage to progress.

But the most interesting point about the grub of the Anthrax is its manner of eating. A most unexpected fact attracts our attention: the curious ease with which this larva leaves and returns to the Bee-grub on which it is feeding. After watching flesh-eating grubs at hundreds and hundreds of meals, I suddenly find myself confronted with a manner of eating that is entirely unlike anything I ever saw before.

This, for instance, is the Amophila-grub’s way of devouring its caterpillar. A hole is made in the victim’s side, and the head and neck of the grub dives deep into the wound. It never withdraws its head, never pauses to take breath. The voracious animal always goes forward, chewing, swallowing, digesting, until the caterpillar’s   skin is empty. Once the meal is begun, the creature does not budge as long as the food lasts. If moved by force it hesitates, and hunts about for the exact spot where it left off eating; for if the caterpillar be attacked at a fresh point it is liable to go bad.

In the case of the Anthrax-grub there is none of this mangling, none of this persistent clinging to the original wound. If I tease it with the tip of a pointed brush it at once retires, and there is no wound to be seen on the victim, no sign of broken skin. Soon the grub once more applies its pimple-head to its meal, at any point, no matter where, and keeps itself fixed there without any effort. If I repeat the touch with the brush I see the same sudden retreat and the same calm return to the meal.

The ease with which this larva grips, leaves, and regrips its victim, now here, now there, and always without a wound, shows that the mouth of the Anthrax is not armed with fangs that can dig into the skin and tear it. If the flesh were gashed by pincers of any kind, one or two attempts would be necessary before they could leave go or take hold again; and besides, the skin would be broken. There is nothing of the kind: the grub simply glues its mouth to its prey, and withdraws it. It does not chew its food like the other flesh-eating grub: it does not eat, it inhales.

This remarkable fact led me to examine the mouth   under the microscope. It is a small conical crater, with yellowish-red sides and very faint lines running round it. At the bottom of this funnel is the opening of the throat. There is not the slightest trace of mandibles or jaws, or any object capable of seizing and grinding food. There is nothing at all but the bowl-shaped opening. I know of no other example of a mouth like this, which I can only compare to a cupping-glass. Its attack is a mere kiss, but what a cruel kiss!

To observe the working of this curious machine I placed a new-born Anthrax-grub, together with its prey, in a glass tube. Here I was able to watch the strange repast from beginning to end.

The Anthrax-grub—the Bee’s uninvited guest—is fixed by its mouth or sucker to any convenient part of the plump Bee-grub. It is ready to break off its kiss suddenly, should anything disturb it, and to resume it as easily when it wishes. After three or four days of this curious contact the Bee-grub, formerly so fat, glossy, and healthy, begins to look withered. Her sides fall in, her fresh colour fades, her skin becomes covered with little folds, and she is evidently shrinking. A week is hardly passed when these signs of exhaustion increase to a startling degree. The victim is flabby and wrinkled, as though borne down by her own weight. If I move her from her place she flops and sprawls like a half-filled indiarubber bottle. But the kiss of the Anthrax goes on   emptying her: soon she is but a sort of shrivelled bladder, growing smaller and smaller from hour to hour. At length, between the twelfth and fifteenth day, all that remains of the Mason-bee’s larva is a little white grain, hardly as large as a pin’s head.

If I soften this small remnant in water, and then blow into it through a very fine glass tube, the skin fills out and resumes the shape of the larva. There is no outlet anywhere for the compressed air. It is intact: it is nowhere broken. This proves that, under the cupping-glass of the Anthrax, the skin has been drained through its pores.

The devouring grub, in making its attack, chooses its moment very cunningly. It is but an atom. Its mother, a feeble Fly, has done nothing to help it. She has no weapons; and she is quite incapable of penetrating the Mason-bee’s fortress. The future meal of the Anthrax has not been paralysed, nor injured in any way. The parasite arrives—we shall presently see how; it arrives, scarcely visible, and having made its preparations it installs itself upon its monstrous victim, whom it is going to drain to the very husk. And the victim, though not paralysed nor in any way lacking in vitality, lets it have its way, and is sucked dry without a tremor or a quiver of resistance. No corpse could show greater indifference to a bite.

Had the Anthrax-grub appeared upon the scene   earlier, when the Bee-grub was eating her store of honey, things would surely have gone badly with it. The victim, feeling herself bled to death by that ravenous kiss, would have protested with much wriggling of body and grinding of mandibles. The intruder would have perished. But at the hour chosen so wisely by it all danger is over. Enclosed in her silken sheath, the larva is in the torpid state that precedes her transformation into a Bee. Her condition is not death, but neither is it life. So there is no sign of irritation when I stir her with a needle, nor when the Anthrax-grub attacks her.

There is another marvellous point about the meal of the Anthrax-grub. The Bee-grub remains alive until the very end. Were she really dead it would, in less than twenty-four hours, turn a dirty-brown colour and decompose. But during the whole fortnight that the meal lasts, the butter-colour of the victim continues unaltered, and there is no sign of putrefaction. Life persists until the body is reduced to nothing. And yet, if I myself give her a wound, the whole body turns brown and soon begins to rot. The prick of a needle makes her decompose. A mere nothing kills it; the atrocious draining of its strength does not.

The only explanation I can suggest is this, and it is no more than a suggestion. Nothing but fluids can be drawn by the sucker of the Anthrax through the unpierced skin of the Bee-grub: no part of the breathing-apparatus   or the nervous system can pass. As these two essentials remain uninjured, life goes on until the fluid contents of the skin are entirely exhausted. On the other hand, if I myself injure the larva of the Bee, I disturb the nervous or the air-conducting system, and the bruised part spreads a taint all over the body.

Liberty is a noble possession, even in an insignificant grub; but it has its dangers everywhere. The Anthrax escapes these dangers only on the condition of being, so to speak, muzzled. It finds its own way into the Bee’s dwelling, quite independently of its mother. Unlike most of the other flesh-eating larvæ it is not fixed by its mother’s care at the most suitable spot for its meal. It is perfectly free to attack its prey where it chooses. If it had a set of carving-tools, of jaws and mandibles, it would meet with a speedy death. It would split open its victim and bite it at random, and its food would rot. Its freedom of action would kill it.

II
THE WAY OUT

There are other grub-eaters which drain their victims without wounding them, but not one, among those I know, reaches such perfection in this art as the Anthrax-grub. Nor can any be compared with the Anthrax as regards the means brought into play in order to leave the   cell. The others, when they become perfect insects, have implements for mining and demolishing. They have stout mandibles, capable of digging the ground, of pulling down clay partition-walls, and even of grinding the Mason-bee’s tough cement to powder. The Anthrax, in her final form, has nothing like this. Her mouth is a short, soft proboscis, good at most for soberly licking the sugary fluid from the flowers. Her slim legs are so feeble that to move a grain of sand would be too heavy a task for them, enough to strain every joint. Her great stiff wings, which must remain full-spread, do not allow her to slip through a narrow passage. Her delicate suit of downy velvet, from which you take the bloom by merely breathing on it, could not withstand the contact of rough tunnels. She is unable to enter the Mason-bee’s cells to lay her egg, and equally unable to leave it when the time comes to free herself and appear in broad daylight.

And the grub, for its part, is powerless to prepare the way for the coming flight. That buttery little cylinder, owning no tools but a sucker so flimsy and small that it is barely visible through the magnifying-glass, is even weaker than the full-grown insect, which at least flies and walks. The Mason-bee’s cell seems to this creature like a granite cave. How can it get out? The problems would be insoluble to these two incapables, if nothing else played its part.  

Among insects the pupa—the transition stage, when the creature is no longer a grub but is not yet a perfect insect—is generally a striking picture of complete weakness. A sort of mummy, tightly bound in swaddling-clothes, motionless and unconscious, it awaits its transformation. Its tender flesh is hardly solid; its limbs are transparent as crystals, and are held fixed in their place, lest a movement should disturb the work of development. In the same way, to secure his recovery, a patient whose bones are broken is held bound in the surgeon’s bandages.

THE ANTHRAX FLY

Her delicate suit of downy velvet, from which you take the bloom by merely breathing on it, could not withstand the contact of rough tunnels

Well, here, by a strange reversal of the usual state of things, a stupendous task is laid upon the pupa of the Anthrax. It is the pupa that has to toil, to strive, to exhaust itself in efforts to burst the wall and open the way out. To the pupa falls the desperate duty, to the full-grown insect the joy of resting in the sun. The result of these unusual conditions is that the pupa possesses a strange and complicated set of tools that is in no way suggested by the grub nor recalled by the perfect Fly. This set of tools includes a collection of ploughshares, gimlets, hooks, spears, and other implements that are not found in our trades nor named in our dictionaries. I will do my best to describe the strange gear.

By the time that July is nearly over the Anthrax has finished eating the Bee-grub. From that time until the following May it lies motionless in the Mason-bee’s cocoon, beside the remains of its victim. When the fine   days of May arrive it shrivels, and casts its skin; and it is then that the pupa appears, fully clad in a stout, reddish, horny hide.

The head is round and large, and is crowned on top and in front with a sort of diadem of six hard, sharp, black spikes, arranged in semi-circle. This sixfold ploughshare is the chief digging-implement. Lower down the instrument is finished off with a separate group of two small black spikes, placed close together.

Four segments in the middle of the body are armed on the back with a belt of little horny arches, set in the skin upside down. They are arranged parallel to one another, and are finished at both ends with a hard, black point. The belt forms a double row of little thorns, with a hollow in between. There are about two hundred spikes on the four segments. The use of this rasp, or grater, is obvious: it helps the pupa to steady itself on the wall of the gallery as the work proceeds. Thus anchored on a host of points the brave pioneer is able to hit the obstacle harder with its crown of awls. Moreover, to make it more difficult for the instrument to recoil, there are long, stiff bristles, pointing backwards, scattered here and there among the rows of spikes. There are some also on other segments, and on the sides they are arranged in clusters. Two more belts of thorns, less powerful than the others, and a sheaf of eight spikes at the tip of the body—two of which are longer than the   rest—completes the strange boring-machine that prepares an outlet for the feeble Anthrax.

About the end of May the colouring of the pupa alters, and shows that the transformation is close at hand. The head and fore-part of the creature become a handsome, shiny black, prophetic of the black livery worn by the coming insect. I was anxious to see the boring-tools in action, and, since this could not be done in natural conditions, I confined the Anthrax in a glass tube, between two thick stoppers of sorghum-pith. The space between the stoppers was about the same size as the Bee’s cell, and the partitions, though not so strong as the Bee’s masonry, were firm enough to withstand considerable effort. On the other hand the side-walls, being of glass, could not be gripped by the toothed belts, which made matters much harder for the worker.

No matter: in the space of a single day the pupa pierced the front partition, three-quarters of an inch thick. I saw it fixing its double ploughshare against the back partition, arching itself into a bow, and then suddenly releasing itself and striking the stopper in front of it with its barbed forehead. Under the blows of the spikes the pith slowly crumbled to pieces, atom by atom. At long intervals the method of work changed. The animal drove its crown of awls into the pith, and fidgeted and swayed about for a time; then the blows began again.   Now and then there were intervals of rest. At last the hole was made. The pupa slipped into it, but did not pass through entirely. The head and chest appeared beyond the hole, but the rest of the body remained held in the tunnel.

The glass cell certainly puzzled my Anthrax. The hole through the pith was wide and irregular: it was a clumsy breach and not a gallery. When made through the Mason-bee’s walls it is fairly neat, and exactly of the animal’s diameter. For narrowness and evenness in the exit-tunnel are necessary. The pupa always remains half-caught in it, and even pretty securely fixed by the graters on its back. Only the head and chest emerge into the outer air. A fixed support is indispensable, for without it the Anthrax could not issue from her horny sheath, unfurling her great wings and drawing out her slender legs.

She therefore remains steadily fixed by the graters on her back, in the narrow exit-gallery. All is now ready. The transformation begins. Two slits appear on the head: one along the forehead, and a second, crossing it, dividing the skull in two and extending down the chest. Through this cross-shaped opening the Anthrax Fly suddenly appears. She steadies herself upon her trembling legs, dries her wings and takes to flight, leaving her cast skin at the doorway of the gallery. The sad-coloured   Fly has five or six weeks before her wherein to explore the clay nests amid the thyme and to take her small share of the joys of life.

III
THE WAY IN

If you have paid attention to this story of the Anthrax Fly, you must have noticed that it is incomplete. The Fox in the fable saw how the Lion’s visitors entered his den, but did not see how they went out. With us the case is reversed: we know the way out of the Mason-bee’s fortress, but we do not know the way in. To leave the cell whose owner it has eaten, the Anthrax becomes a boring-tool. When the exit-tunnel is opened this tool splits like a pod bursting in the sun, and from the strong framework there escapes a dainty Fly. A soft bit of fluff that contrasts strangely with the roughness of the prison whence it comes. On this point we know pretty well what there is to know. But the entrance of the grub into the cell puzzled me for a quarter of a century.

It is plain that the mother cannot place her egg in the Bee’s cell, which is closed and barricaded with a cement wall. To pierce it she would have to become a boring-tool once more, and get into the cast-off rags which she left at the doorway of the exit-tunnel. She would have   to become a pupa again. For the full-grown Fly has no claws, nor mandibles, nor any implement capable of working its way through the wall.

Can it be, then, the grub that makes its own way into the storeroom, that same grub that we have seen sucking the life out of the Bee’s larva? Let us call the creature to mind: a little oily sausage, which stretches and curls up just where it lies, without being able to shift its position. Its body is a smooth cylinder, its mouth a circular lip. It has no means whatever of moving; not even a hair or a wrinkle to enable it to crawl. It can do nothing but digest its food. It is even less able than the mother to make its way into the Mason-bee’s dwelling. And yet its provisions are there: they must be reached: it is a matter of life and death. How does the Fly set about it? In the face of this puzzle I resolved to attempt an almost impossible task and watch the Anthrax from the moment it left the egg.

Since these Flies are not really plentiful in my own neighbourhood I made an expedition to Carpentras, the dear little town where I spent my twentieth year. The old college where I made my first attempts as a teacher was unchanged in appearance. It still looked like a penitentiary. In my early days it was considered unwholesome for boys to be gay and active, so our system of education applied the remedy of melancholy and gloom. Our houses of instruction were above all houses   of correction. In a yard between four walls, a sort of bear-pit, the boys fought to make room for their games under a spreading plane-tree. All round it were cells like horseboxes, without light or air: those were the class-rooms.

I saw, too, the shop where I used to buy tobacco as I came out of the college; and also my former dwelling, now occupied by monks. There, in the embrasure of a window, sheltered from profane hands, between the closed outer shutters and the panes, I kept my chemicals—bought for a few sous saved out of the housekeeping money. My experiments, harmless or dangerous, were made on a corner of the fire, beside the simmering broth. How I should love to see that room again, where I pored over mathematical problems; and my familiar friend the blackboard, which I hired for five francs a year, and could never buy outright for want of the necessary cash!

But I must return to my insects. My visit to Carpentras, unfortunately, was made too late in the year to be very profitable. I saw only a few Anthrax Flies hovering round the face of the cliff. Yet I did not despair, because it was plain that these few were not there to take exercise, but to settle their families.

So I took my stand at the foot of the rock, under a broiling sun, and for half a day I followed the movements of my Flies. They flitted quietly in front of the slope, a few inches away from the earthly covering.   They went from one Bee’s nest to another, but without attempting to enter. For that matter, the attempt would be useless, for the galleries are too narrow to admit their spreading wings. So they simply explore the cliff, going to and fro, and up and down, with a flight that was now sudden, now smooth and slow. From time to time I saw one of them approach the wall and touch the earth suddenly with the tip of her body. The proceeding took no longer than the twinkling of an eye. When it was over the insect rested a moment, and then resumed flight.

I was certain that, at the moment when the Fly tapped the earth, she laid her eggs on the spot. Yet, though I rushed forward and examined the place with my lens, I could see no egg. In spite of the closest attention I could distinguish nothing. The truth is that my state of exhaustion, together with the blinding light and scorching heat, made it difficult for me to see anything. Afterwards, when I made the acquaintance of the tiny thing that comes out of that egg, my failure no longer surprised me: for even in the leisure and peace of my study I have the greatest difficulty in finding the infinitesimal creature. How then could I see the egg, worn out as I was under the sun-baked cliff?

None the less I was convinced that I had seen the Anthrax Flies strewing their eggs, one by one, on the spots frequented by the Bees who suit their grubs. They   take no precaution to place the egg under cover, and indeed the structure of the mother makes any such precaution impossible. The egg, that delicate object, is laid roughly in the blazing sun, among grains of sand, in some wrinkle of the chalk. It is the business of the young grub to manage as best it can.

The next year I continued my investigations, this time on the Anthrax of the Chalicodoma, a Bee that abounds in my own neighbourhood. Every morning I took the field at nine o’clock, when the sun begins to be unendurable. I was prepared to come back with my head aching from the glare, if only I could bring home the solution of my puzzle. The greater the heat, the better my chances of success. What gives me torture fills the insect with delight; what prostrates me braces the Fly.

The road shimmers like a sheet of molten steel. From the dusty, melancholy olive-trees rises a mighty, throbbing hum, the concert of the Cicadæ, who sway and rustle with increasing frenzy as the temperature increases. The Cicada of the Ash adds its strident scrapings to the single note of the Common Cicada. This is the moment! For five or six weeks, oftenest in the morning, sometimes in the afternoon, I set myself to explore the rocky waste.

There were plenty of the nests I wanted, but I could not see a single Anthrax on their surface. Not one settled in front of me to lay her egg. At most, from time   to time, I could see one passing far away, with an impetuous rush. I would lose her in the distance; and that was all. It was impossible to be present at the laying of the egg. In vain I enlisted the services of the small boys who keep the sheep in our meadows, and talked to them of a big black Fly and the nests on which she ought to settle. By the end of August my last illusions were dispelled. Not one of us had succeeded in seeing the big black Fly perching on the dome of the Mason-bee.

The reason is, I believe, that she never perches there. She comes and goes in every direction across the stony plain. Her practised eye can detect, as she flies, the earthen dome which she is seeking, and having found it she swoops down, leaves her egg on it, and makes off without setting foot on the ground. Should she take a rest it will be elsewhere, on the soil, on a stone, on a tuft of lavender or thyme. It is no wonder that neither I nor my young shepherds could find her egg.

Meanwhile I searched the Mason-bees’ nests for grubs just out of the egg. My shepherds procured me heaps of the nests, enough to fill baskets and baskets; and these I inspected at leisure on my work-table. I took the cocoons from the cells, and examined them within and without: my lens explored their innermost recesses, the sleeping larva, and the walls. Nothing, nothing, nothing! For a fortnight and more nests were searched   and rejected, and heaped up in a corner. My study was crammed with them. In vain I ripped up the cocoons; I found nothing. It needed the sturdiest faith to make me persevere.

At last I saw, or seemed to see, something move on the Bee’s larva. Was it an illusion? Was it a bit of down stirred by my breath? It was not an illusion; it was not a bit of down; it was really and truly a grub! But at first I thought the discovery unimportant, because I was so greatly puzzled by the little creature’s appearance.

In a couple of days I was the owner of ten such worms and had placed each of them in a glass tube, together with the Bee-grub on which it wriggled. It was so tiny that the least fold of skin concealed it from my sight. After watching it one day through the lens I sometimes failed to find it again on the morrow. I would think it was lost: then it would move, and become visible once more.

For some time the belief had been growing in me that the Anthrax had two larval forms, a first and a second, the second being the form I knew, the grub we have already seen at its meals. Was this new discovery, I asked myself, the first form? Time showed me that it was. For at last I saw my little worms transform themselves into the grub I have already described, and make their first start at draining their victims with kisses.   A few moments of satisfaction like those I then enjoyed make up for many a weary hour.

This tiny worm, the first form or “primary larva” of the Anthrax, is very active. It tramps over the fat sides of its victim, walking all round it. It covers the ground pretty quickly, buckling and unbuckling by turns, very much after the manner of the Looper-caterpillar. Its two ends are its chief points of support. When walking it swells out, and then looks like a bit of knotted string. It has thirteen rings or segments, including its tiny head, which bristles in front with short, stiff hairs. There are four other pairs of bristles on the lower surface, and with the help of these it walks.

For a fortnight the feeble grub remains in this condition, without growing, and apparently without eating. Indeed, what could it eat? In the cocoon there is nothing but the larva of the Mason-bee, and the worm cannot eat this before it has the sucker or mouth that comes with the second form. Nevertheless, as I said before, though it does not eat it is far from idle. It explores its future dish, and runs all over the neighborhood.

Now, there is a very good reason for this long fast. In the natural state of the Anthrax-grub it is necessary. The egg is laid by the mother on the surface of the nest, at a distance from the Bee’s larva, which is protected by a thick rampart. It is the business of the new-born   grub to make its way to its provisions, not by violence, of which it is incapable, but by patiently slipping through a maze of cracks. It is a very difficult task, even for this slender worm, for the Bee’s masonry is exceedingly compact. There are no chinks due to bad building, no cracks due to the weather. I see but one weak point, and that only in a few nests: it is the line where the dome joins the surface of the stone. This weakness so seldom occurs that I believe the Anthrax-grub is able to find an entrance at any spot on the dome of the Bee’s nest.

The grub is extremely weak, and has nothing but invincible patience. How long it takes to work its way through the masonry I cannot say. The work is so laborious and the worker so feeble! In some cases I believe it may be months before the slow journey is accomplished. So it is very fortunate, you see, that this first form of the Anthrax, which exists only in order to pierce the walls of the Bees’ nest, should be able to live without food.

At last I saw my young worms shrink, and rid themselves of their outer skin. They then appeared as the grub I knew and was so anxiously expecting, the grub of the Anthrax, the cream-colored cylinder with the little button of a head. Fastening its round sucker to the Bee-grub, it began its meal. You know the rest.

Before taking leave of this tiny animal let us dwell   for a moment on its marvellous instinct. Pi