Built on small pebbles

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. EXPERIMENTS

XXI. EXPERIMENTS

Built on small pebbles which one can carry whither one will, remove, or interchange, without disturbing either the work of the constructor or the quiet of the inhabitants of the cells, the nests of Chalicodoma muraria lend themselves readily to experiment—the only method capable of throwing a little light on the nature of instinct. Profitably to study the physical faculties of the animal it is not enough to know how to turn to account such circumstances as a happy chance may offer to the observer: one must be capable of originating others, and vary them as much as possible and submit them to mutual control; in short, to give science a solid basis of fact one must experiment. Then some day will vanish before the evidence of exact documents the fantastic legends which cumber our books, such as the Scarabæus inviting his comrades to help in dragging his ball out of a rut, or a Sphex cutting up a fly to carry it in spite of the wind, and much more which is misused by those who desire to see in the animal world that which is not there. Thus, too, will materials be prepared which, used sooner or later by a learned hand, will cast premature and baseless theories back into oblivion.

Réaumur generally confines himself to stating facts as they offered themselves to him in the normal course of things, and does not attempt to penetrate further into the powers of the insect by means of conditions brought about artificially. In his day there was everything to do, and the harvest was so great that the illustrious reaper hurried on to what was most urgent,—the gathering of it in, leading his successors to examine grain and ear in detail. Nevertheless, he mentions an experiment made on Chalicodoma muraria by his friend Du Hamel. The nest was placed in a glass funnel, the mouth of which was closed by a piece of gauze. Three males were hatched, which, though they had penetrated mortar hard as a stone, either did not attempt to pierce the thin gauze, or thought it beyond their power to do so. All three died under the glass. Insects generally only know how to execute that which they need to do in the common order of nature, adds Réaumur.

For two reasons the experiment does not satisfy me. First of all, to give gauze to be pierced by insects with tools made to pierce lumps as hard as tufa does not seem a happy idea; you cannot expect a navvy’s pickaxe to do the same work as the scissors of a seamstress. Secondly, the transparent glass prison seems ill chosen. As soon as it had opened a way through the thickness of its earthen dome, the insect found itself in daylight, and to it daylight means final deliverance and freedom. It strikes against an invisible obstacle—the glass, and glass [291]does not suggest an obstacle to it. Beyond, it sees a free space bathed in sunshine. It exhausts itself in efforts to fly there, unable to comprehend the uselessness of struggling against this strange, invisible barrier, and perishes, obstinate and exhausted, without a glance at the gauze which closes the conical tube. The experiment must be repeated under better conditions.

The obstacle I selected was common gray paper—opaque enough to keep the insect in the dark—thin enough not to offer serious resistance to the prisoner’s efforts. As there is a vast difference by way of obstacle between a paper partition and a vault of unbaked clay, let us see first if Chalicodoma muraria knows how, or rather if it is able, to pierce such a barrier. The two mandibles—pickaxes adapted to pierce hard mortar—are they also scissors capable of cutting thin material? That is the point to be ascertained.

In February, when the insect is already in the perfect state, I withdrew a certain number of cocoons uninjured from their cells, and placed each separately in a piece of reed, closed at one end naturally, open at the other. The pieces of reed represented the nest-cells. The cocoons were introduced so that the head of the insect should turn to the opening. Finally, my artificial cells were closed in various ways. Some had a stopper of kneaded earth, which, when dry, answered in thickness and consistency to the mortar of the nest; others were shut by a cylinder of Sorghum vulgare at least a centimetre thick, and others with a stopper of gray paper, solidly fixed by its edges. All these bits of [292]cane were arranged side by side, vertically, in a box, with the artificial roof at the top, so that the insects were in the exact position they had in a nest. To open them they must do as they would had I not intervened—break through the wall overhead. I protected all with a large bell glass, and awaited the month of May when they would emerge.

The result greatly surpassed my expectations. The earthen stopper made by me was pierced with a round hole, noways differing from that made by the mason bee through its mortar dome. The vegetable barrier, so new to my prisoner,—namely, the Sorghum cylinder,—was likewise opened by a hole, apparently made by a single effort, and the gray paper allowed the insect to pass, not by bursting through, but once more by a neat round hole. So my bees were capable of work for which they were not created. To issue from their reed cells they did what probably none of their race ever did before; they perforated the Sorghum pith and made a hole in the paper just as they would have done with their natural clay ceiling. When the moment came to free themselves, the nature of the obstacle was no hindrance so long as it was not too strong for them, and thenceforward the plea of incapacity could not be evoked where a mere paper barrier was in question.

At the same time as the reed cells, two intact nests on their pebbles were placed under the glass bell. On one I pressed closely a sheet of gray paper over the mortar dome, so that to come forth the insect must first pierce the dome and then the paper, no space being left between them; while a little cone of gray paper was gummed on the stone [293]round the other nest, so that, as in the first case, there was a double barrier, an earthen and a paper one, with, however, this difference—that the two barriers were not close together, there being a space between them of about a centimetre at the base, and increasing as the cone rises. The results of these two experiments were quite unlike. The Hymenoptera from the nest where paper had been applied to the dome came forth by piercing the double barrier, the outer one being pierced by a clean round hole, as in the reed cells closed in the same way. For the second time it is shown that if the bee is stopped by a paper barrier, the cause is not incapacity to deal with such an obstacle. On the other hand, after they had pierced their earthen vault, the dwellers in the second nest who found the sheet of paper a little way off, made no attempt to overcome the obstacle over which they would so easily have triumphed had it been attached to the nest. They died under the cover without an effort for freedom. So had perished Réaumur’s bees under his glass tube when there was but a bit of gauze between them and freedom. This fact appears to me rich in consequences. What! Here are strong insects which find penetrating tufa mere play, and a stopper of thin wood or a sheet of paper quite easy to pierce, new as these are to them, and yet these vigorous insects let themselves stupidly perish imprisoned in a cone of paper which they might have torn to bits with one bite of their mandibles. They might—but they never dreamed of doing so. The motive of their dull inertness can be only this—the insect is excellently endowed with tools and instinctive [294]faculties, in order to accomplish the final act of its metamorphosis, i.e. issuing from the cocoon or cell. Its mandibles furnish it with scissors, file, pick, and lever to cut, gnaw, and pull down not only its cocoon and wall of mortar, but any other barrier not too tenacious which may be substituted for the natural wall of its nest. Moreover,—and this is a chief condition, without which its outfit would be useless,—there is, I will not say the will to use these tools, but an inward stimulus inviting it to employ them. The hour to come forth having arrived, this stimulus awakens, and the insect sets to work to bore a passage.

In that case it matters little whether the material to be pierced is natural mortar, Sorghum pith, or paper. The imprisoning cover will not resist long. It even matters little if the obstacle be thickened and a paper barrier be added to the earthen one. Both count as one if there be no interval between them, and the insect passes through them because this coming forth seems to it a single action. With the paper cone, whose wall is at a short distance, the conditions are changed, although the total thickness of barrier is really the same. The insect has done all that it was destined to do in order to free itself. To move freely on the mortar dome means to it that deliverance is achieved. It has bored its way out; the work is accomplished. But round the nest another barrier presents itself—the paper wall. To pierce through, the action already accomplished must be repeated—that action which the insect has to perform but once in its life. It must double that which naturally is but single; and it [295]cannot, simply because it has not the will to do it. It perishes for lack of the smallest ray of intelligence. Yet in this singular intellect it is the fashion nowadays to see a rudiment of human reason! The fashion will pass and the facts remain, bringing us back to the good old ideas of the soul and its immortal destinies.

Réaumur relates, too, how his friend Du Hamel, having seized a mason bee with his pincers when it had entered half-way into its cell, head first, to fill it with bee-bread, carried it into a room at a considerable distance from the spot where he caught it. The bee escaped and flew through the window. Du Hamel immediately returned to the nest. The mason bee reached it almost at the same time, and resumed work. It only seemed a little wilder, says the narrator.

Why were you not with me, venerated master, on the banks of the Aygues, with their stretches of pebbles, dry for three parts of the year, and an enormous torrent when it rains? I would have shown you something far better than the fugitive escaped from your pincers. You should have seen, and shared my surprise thereat, not the short flight of a mason bee, which, carried into a room near at hand, escapes and returns straight home in a neighbourhood familiar to her, but long journeys by unknown ways. You would have seen the bee, carried away by me to a long distance, return with a geographical precision which the swallow would not disown, or the martin, or the carrier-pigeon, and you would have asked yourself, as I did, what inexplicable knowledge of the map of the country guides this mother [296]in seeking her nest. Let us come to the facts. We must repeat on the mason bee my earlier experiments with the Cerceris—namely, carrying the insect in darkness far from the nest, marking and setting it free. In case any one should wish to repeat the experiment, I will explain my method of operation, which may make it easier for a beginner. The insect destined for a long journey must of course be captured with certain precautions. No nippers, no pincers which might maim a wing, strain it, and endanger power of flight. While the bee is absorbed in work within her cell, I cover the latter with a little glass tube. As she flies out she goes into this, and thus, without touching her, I can transfer her to a twist of paper and close it quickly. A botanical tin serves as a means of transporting the captives, each in its paper prison.

It is on the spots chosen as starting-places that the most delicate operation takes place—namely, marking each captive before freeing her. I use chalk powdered fine and moistened with a strong solution of gum arabic. Dropped somewhere on the insect with a straw, it leaves a white mark, which dries quickly and adheres to the bee’s fleece. If a mason bee has to be marked, so as to distinguish her from another in an experiment of short duration, such as I shall presently describe, I only touch the tip of the abdomen with a straw charged with colour while the insect is half inside the cell, head down-wards. The bee does not notice the slight touch and works on undisturbed; but the mark is not very durable, nor at a spot favourable for its preservation, since the bee frequently brushes her body to [297]detach pollen, and sooner or later effaces it. It is therefore in the very middle of the thorax—between the wings—that I drop the gummed chalk.

In such work it is hardly possible to wear gloves. The fingers require all their dexterity to seize the mason bee with sufficient delicacy, and to master her struggles without rough pressure. It is evident that if nothing else be gained, one is sure of stings; with a little address they can generally be avoided, but not always; one must take them with resignation. Besides, a mason bee’s sting is by no means so painful as that of a hive bee. The white spot dropped on the thorax—off goes the mason bee, and the mark dries as she goes.

The first time I tried the experiment I took two mason bees busy at their nests on the boulders covering the alluvial lands along the Aygues, not far from Serignan, and carried them to my home at Orange, where I freed them after marking each. According to the Ordnance map the distance between the two places is about four kilometres in a right line. The captives were freed in the evening at an hour when bees begin to leave off work, so it was likely that my two would spend the night somewhere near.

The next morning I returned to the nests. It was still too cold, and work was suspended. When the dew was dried the masons set to work. I saw a bee, but without the white spot, taking pollen to one of the two nests whence had come the travellers whom I expected. A stranger, having found the cell unoccupied, and having expatriated the owner, had established herself there, unaware that it was [298]the property of another. Perhaps she had been storing it since the previous evening. Towards ten o’clock, at the hottest time, suddenly the proprietor arrived. Her rights as first occupier were inscribed as far as I was concerned in irrefutable characters in white chalk on her thorax. Here was one of my travellers come back.

Over waves of corn, over fields of red sainfoin, she had accomplished the four kilometres, and returned to her nest after collecting booty on the way, for she came,—worthy creature that she was!—all yellow underneath with pollen. To return from the verge of the horizon was a marvel, but to do so with a well-furnished pollen brush was really sublime economy! A journey, even if compulsory, is always for a bee an opportunity of collecting food. She found the stranger in her nest. “What’s all this? You just wait!” and fell furiously on the other, who perhaps had thought no wrong. Then there were hot pursuits through the air. From time to time the two hovered almost motionless, facing one another with a couple of inches between them, doubtless measuring each other with their eyes, and humming abuse at one another. Sometimes one, sometimes the other alighted on the nest in question. I expected to see a wrestle, and stings used; but I was mistaken. The duties of maternity spoke too imperiously to allow them to risk life, and wipe out the injury in a mortal duel. All was limited to hostile demonstrations and a few tussles leading to nothing.

However, the proprietor seemed to draw redoubled courage and strength from consciousness of her [299]rights. She encamped permanently on the nest and received the other bee each time that she ventured to approach with an irritated quiver of the wings in token of just indignation. The stranger finally withdrew discouraged, and instantly the mason resumed work as actively as if she had not undergone the chances and changes of a long journey.

Yet another word as to rights of property. While a mason bee is absent it is not unusual for some homeless vagabond to visit the nest, take a liking to it, and set to work, sometimes at the same cell, sometimes at the next, if there are several free, as often happens with old nests. When the first occupant returns she does not fail to drive away the intruder, who always ends by getting the worst of it, so lively and invincible is the real owner’s sense of property. Reversing the savage Prussian maxim, “Strength before right,” here right comes before strength; otherwise the constant retreat of the intruder would be quite inexplicable, since the latter’s strength is in no way inferior to that of the real owner. If she has less audacity it must come from not feeling braced by the sovereign strength of being right, which decides among equals, even in the brute creation.

The second of my two travellers did not appear, either on the day when the first came, nor later. I decided to make another experiment—this time with five subjects. Place of starting and arrival, distance and hours, were the same. I found three at the nests on the following day; two were missing.

It is therefore quite clear that Chalicodoma [300]muraria carried away four kilometres, and, set free where she certainly could never have been before, can return home. But why did one out of two, and two out of five, fail to do so? What one could do, why not another? Are they not equally gifted with the faculty which guides them through the unknown? Is it not rather inequality in the power of flight? I recollected that my Hymenoptera did not all fly off with the same energy; hardly were some out of my fingers, launching themselves impetuously into the air, than I lost sight of them, while others let themselves drop a few paces off after a short flight. It seems certain that these had suffered during the journey—perhaps from the concentrated heat in the furnace of my box, or I may have harmed the jointure of the wings while marking them—an operation difficult to perform when one has to avoid being stung. These are maimed, weak creatures—unable to go on with all sail spread, as they ought, for this journey. The experiment must be tried again, only counting those bees which instantly leave my fingers with a swift, strong flight. We shall omit those which hesitate or linger close by on some bush. Moreover, I will do my best to compute the time employed in returning to the nest.

Such an experiment requires a considerable number of subjects, as the weak and maimed, who may be many, must be rejected. Chalicodoma muraria cannot furnish the quantity needed; it is not common enough, and I am anxious not to disturb the small people by the Aigues whom I want for other observations later. Fortunately I have near my house, under the projecting edge of the roof of a shed, a magnificent [301]colony of Chalicodoma sicula in full activity. I can draw at pleasure on the populous city. The insect is small—less than half the size of C. muraria; no matter—all the more merit if it can traverse the four kilometres which I have in reserve for it, and find its nest. I took forty, isolating them as usual in paper cones.

A ladder was placed against the wall in order to reach the nest; it was to be used by my daughter Aglaë, to allow her to mark the exact instant when the first one returned. I set the clock on the mantelpiece and my watch together, that I might compare the moment of departure and arrival. Then I carried off my forty captives to the spot where Chalicodoma muraria works beside the Aygues. The expedition had a double scope—to observe Réaumur’s mason bee and set the Sicilian one free. The latter would have to fly back four kilometres.

At length my prisoners were released—all marked with a large white dot in the middle of the thorax. It is not for nothing that one successively handles forty wrathful Hymenoptera which forthwith unsheath and make play with their poisoned stings. Before the mark could be made, too often the stab was given, and my burning fingers moved in self-defence sometimes against my will; I handled them with more consideration for myself than for the insect, and sometimes squeezed my bees too hard. To experiment in order to lift a small corner of the veil that covers a truth is a beautiful and noble thing, which can enable one to brave many perils, yet surely one may show a little impatience if in a brief space of time one’s finger tips get stung forty times. [302]If any one should reproach me for my clumsy handling, I would suggest that he make the experiment, and then judge how far the situation was pleasant.

In short, either from the fatigue of the journey, or because I pressed too hard and injured some articulations, out of my forty Hymenoptera only twenty flew off strongly and unhesitatingly; the rest strayed over the herbage near at hand, unable to keep their balance, or remained on the willows where I had put them, refusing to fly even when excited by a straw. These faint-hearted ones, these maimed ones, these incapables hurt by my fingers, must be struck off the list. Twenty started with an unhesitating flight. That was amply sufficient.

At the moment of departure there was nothing special in the direction taken—nothing of that straight line to the nest which the Cerceris took in a like case. As soon as they were free the Chalicodoma fled scared—one in this direction, one to a completely opposite point; but, as far as their fiery flight allowed, I think I saw a rapid return of those bees which had flown in the wrong direction for their nests, and most seemed to go to that side of the horizon. I leave this point with the doubts unavoidable with regard to insects lost sight of at some twenty metres distance. So far the experiment had been favoured by calm weather, but now things grew complicated. The heat was stifling, and the sky grew stormy. Rather a strong wind rose, blowing from the south—the very direction which my bees should take to return home. Could they overcome this contrary current and cleave this aerial torrent [303]with their wings? If they try it they must keep close to the ground, as I saw those Hymenoptera doing which continued to work, but it appeared out of the question to soar into the high regions where they might obtain a clear acquaintance with the surrounding country. It was therefore with great apprehension as to the success of my experiment that I returned to Orange after again trying to learn some secret from the bees on the Aygues pebbles.

Hardly had I entered my house when I saw Aglaë, flushed with excitement. “Two,” she cried—“two came at twenty minutes to three, all laden with pollen!” A friend chanced to have come in—a grave legal personage, who, hearing what was on hand, forgot the Code and stamped paper, and insisted on also watching for the arrival of my homing pigeons. The result interested him more than did the lawsuit about the partition wall. In a Senegalian sun and furnace heat reflected from the wall, every five minutes did he mount the ladder bareheaded, with no other protection against sunstroke than his thick, gray locks. Instead of the single watcher whom I had posted I found two good pairs of eyes watching the bees’ return. I had freed them about two o’clock, and the first two returned to the nest at twenty minutes to three, so that three-quarters of an hour had sufficed for travelling four kilometres,—a very striking result, especially if we remember that the bees worked on the road, as was proved by the pollen on their bodies, and besides they must have been hindered by having the wind against them. Two more came back under my eyes, and they had signs of having worked on the way by their load of [304]pollen. As it was growing late, observations could not be continued. When the sun goes down the mason bees leave the nest and take refuge I know not where—here and there—perhaps under roof tiles and in little shelters in walls. I could not count on the arrival of the others until work was resumed in full sunshine.

The next day, when sunshine recalled the scattered workers, I again counted the bees with white dots on their thorax. My success surpassed all my hopes; I counted fifteen—fifteen of the deported bees storing or building as if nothing had happened! Then the storm, which had threatened more and more, burst, and a succession of rainy days stopped all further observations.

Such as it was, the experiment sufficed. Out of twenty bees which seemed fit for the journey when released, fifteen at least had come back—two in the first hour, and three in the course of the evening, and the rest next morning. They had come back in spite of having the wind against them, and—a yet greater difficulty—in spite of their unfamiliarity with the place whither I had transported them. There could be no question that it was for the first time that they saw the osier beds of the Aygues which I had chosen as the starting-place. Never on their own account had they gone so far afield, for they find all they want by way of building material and food close to my shed. The road at the foot of the wall furnishes mortar; the meadows round my house offer nectar and pollen. Economical of time as they are, they would not fly four kilometres to procure what abounds close to the nests. I see [305]them daily taking material from the road, and making a harvest on the meadow flowers, especially on Salvia. According to all appearance they do not fly beyond a circle of a hundred metres. How then did my exiles return? What guided them? Not memory, certainly, but some special faculty, which we can only recognise by its astonishing effects without pretending to explain it, so far outside our own psychology is it.

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