The Anthrax Fly: 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. Picture it as having just left the egg, just awakened to life under the fierce rays of the sun. The bare stone is its cradle; there is no one to welcome it as it enters the world, a mere thread of half-solid substance. Instantly it starts on its struggle with the flint. Obstinately it sounds each pore of the stone; it slips in, crawls on, retreats, begins again. What inspiration urges it towards its food, what compass guides it? What does it know of those depths, or of what lies in them? Nothing. What does the root of a plant know of the earth’s fruitfulness? Again, nothing. Yet both the root and the worm make for the nourishing spot, Why? I do not understand. I do not even try to understand. The question is far above us.

We have now followed the complete history of the Anthrax. Its life is divided into four periods, each of which has its special form and its special work. The primary larva enters the Bees’ nest, which contains provisions; the secondary larva eats those provisions; the pupa brings the insect to light by boring through the enclosing wall; the perfect insect strews its eggs. Then the story starts afresh.