THE RETURN TO THE NEST

The Ammophila digging her well late in the day leaves her work after stopping the entrance with a stone, flits away from one flower to another, goes into a new neighbourhood, and yet next day can return with a caterpillar to the abode hollowed out the evening before, notwithstanding her want of acquaintance with the locality—often new to her; the Bembex, loaded with prey, alights with almost mathematical precision on the threshold of a dwelling blocked by sand and rendered uniform with the rest of the sandy surface. Where my sight and memory are at fault, theirs have a certainty verging on infallibility. One would say that the insect possessed something more subtle than mere recollection—a kind of intuition of locality with which nothing in us corresponds—in short, an indefinable faculty which I call memory for lack of any other expression by which to designate it. The unknown cannot be named. In order to throw if possible a little light on this point in the psychology of animals I instituted a series of experiments, which I will now describe.

The first had for its subject Cerceris tuberculata, [259]which hunts the Cleonus. About 10 a.m. I took a dozen females busy at the same bank and of the same colony, either hollowing or storing burrows. Each was enclosed in a twist of paper, and all were put into a box. About two kilometres from the nests I freed my captives, first marking them with a white dot in the middle of the thorax by means of a straw dipped in an indelible colour, in order to recognise them later. They flew on every side—some here, some there, but only a few paces, alighting on blades of grass and passing their forelegs over their eyes for a moment, as if dazzled by the bright sunshine to which they were suddenly restored. Then they took flight—some earlier, some later; and one and all took unhesitatingly a straight line south, i.e. in the direction of their home. Five hours later I returned to the common territory of the nests. Almost directly I saw two of my white-dotted Cerceris working at their burrows. Soon a third came in, with a weevil between her feet. A fourth soon followed—four out of twelve in a quarter of an hour was enough for conviction; I judged it useless to wait longer; what four could do, the others could, if indeed they had not already done it, and one may very well suppose that the eight absentees were out hunting, or perhaps had retired into the depths of their burrows. Thus, carried to a distance of two kilometres, in a direction and by a way which they could not possibly perceive from the depths of their paper prison, the Cerceris—at all events part of them—had returned home.

I do not know to what distance they go hunting; possibly they know the country round for some two kilometres. In that case they would not have been [260]far enough away, and came home by their local knowledge. The experiment had to be tried again at a greater distance, and from a point which the Cerceris could not possibly know.

I therefore took nine females from the colony whence I had got them in the morning; three of these had been already experimented upon. Again they were conveyed in a dark box—each imprisoned in a paper twist. The starting-place was to be the neighbouring town of Carpentras, about three kilometres from the burrows. I meant to release them not amid fields as before, but in a street in the midst of a populous quarter, where the Cerceris, with their rustic habits, had assuredly never penetrated. As the hour was late I put off the experiment, and my captives spent the night in their prison cells.

The next morning, towards eight o’clock, I marked them with a double white spot on the thorax to distinguish them from those of the evening before, which had only one, and set them free successively in the middle of the street. Each mounted vertically, as if to get as soon as possible from between the houses and gain a wide horizon, then rising above the roofs, instantly and energetically turned its flight south. And it was from the south that I brought them into the town, and their burrows are south. Nine times with my nine prisoners did I obtain this striking result—that an insect quite beyond its bearings should not hesitate a moment what direction to take to regain its nest.

Some hours later I too was at the burrows. I saw several of my yesterday’s Cerceris with a white dot, but none of the last set free. Had they been [261]unable to find the way back? Were they out hunting, or in their galleries recovering from the excitement of such a trial? I do not know. The next morning I came again, and had the satisfaction of finding at work, as active as if nothing extraordinary had happened to them, five Cerceris with two white dots. Three kilometres of distance,—the town with its houses, roofs, and smoky chimneys—all so novel to my rustic Cerceris,—had proved no obstacle to their return home.

Taken from its brood and carried an enormous distance, the pigeon returns promptly to its dovecote. If one were to consider the length of journey in proportion to the size of the animal, how superior to the pigeon is the Cerceris carried away three kilometres and returning to its burrow! The size of the insect does not equal a cubic centimetre, while that of the pigeon must quite equal the cube of a decimetre, if it does not exceed it. The bird, a thousand times larger than the Hymenopteron, ought, in order to rival it, to find its dovecote at a distance of 3000 kilometres—thrice the greatest length of France from north to south. I do not know if a carrier-pigeon has ever shown such prowess, but wing-power and yet more lucidity of instinct cannot be measured by yards. Nor can we here consider the question of size, and one can only see in the insect a worthy rival to the bird without deciding which has the advantage.

Are the two guided by memory when placed by man beyond their bearings and carried to great distances—into regions with which they are unacquainted and in unknown directions? Is memory [262]as quick when, having reached a certain height at which they can in some sort take their bearings, they launch themselves with all their power of wing towards that part of the horizon where are their nests? Is it memory which traces their aerial way across regions seen for the first time? Evidently not. It is not possible to recollect the unknown. The Hymenopteron and the bird know nothing of their surroundings; nothing can have taught them the general direction which they followed when carried thither, for it was in the darkness of a closed box that the journey was made. Locality, orientation,—all is unknown, and yet they find their way. They have then as guide something better than simple memory—a special faculty, a kind of topographic consciousness of which we can form no idea, possessing nothing analogous to it.

I am now about to establish experimentally how subtle and precise is this faculty in the narrow cycle where it is applied, and also how limited and obtuse when it has to move out of habitual conditions. Such is the invariable antithesis of instinct.

A Bembex, actively engaged in feeding her larva, has left her burrow. She will return immediately with the product of the chase. The entrance is carefully stopped with sand—swept backward by the insect before departing. Nothing distinguishes it from the rest of the sandy surface. But this offers no difficulty to the Hymenopteron, who finds her doorway again with a sagacity which I have already described. Let us plan some treachery; let us perplex her by altering the state of the place. I cover the entrance with a flat stone as large as [263]my hand. She soon returns. The complete change made upon her threshold during her absence does not seem to cause her the slightest hesitation; at all events she alights immediately upon the stone, and tries for an instant to hollow it, not at a chance spot, but exactly over the opening of her burrow. Quickly turned aside from this attempt by the hardness of the obstacle, she traverses the stone in every direction, goes round it, slips underneath, and begins to dig in the precise direction of her dwelling.

The flat stone is too trifling an obstacle to disconcert the clever fly; let us find something better. I did not allow the Bembex to continue her excavation, which I saw would soon prove successful, and drove her far off with my handkerchief. The absence of the frightened insect for a considerable time allowed me to prepare my snares leisurely. What materials must now be employed? In these improvised experiments one must know how to turn all things to profit. Not far off on the high road is the fresh dropping of a beast of burden; here is wood for our arrow. The dropping was collected, crushed, and spread in a layer at least an inch thick on the threshold of the burrow and its surroundings over more than a quarter of a yard. Assuredly here was such a façade as never Bembex knew. Colour, the nature of the material, the effluvium,—all combined to deceive the Hymenopteron. Can she take this stretch of manure—this dung—for the front of the dwelling? She does! Here she comes; studying from above the unusual condition of the place, and settling in the middle of the layer, just opposite the entrance, routing about, making a way [264]through the fibrous mass, and penetrating to the sand, she promptly discovers the mouth of the passage. I stop and drive her away a second time.

Is not the precision with which the Bembex settles before her dwelling, though masked in a way so novel, a proof that sight and memory are not in such a case the only guides? What further can there be? Smell, perhaps. That is very doubtful, for the emanations from the dung could not baffle the perspicacity of the insect. Nevertheless, let us try another odour. I happen to have with me, as part of my entomological outfit, a little phial of ether. The covering of manure is swept off and replaced by a cushion of moss, not very thick, but covering a wide surface, on which I pour the contents of my phial the moment I see the Bembex coming. The over-strong emanations keep her off, but only for an instant. She alights on the moss, still reeking of ether, traverses the obstacle and penetrates to her dwelling. The etherised effluvia did not disturb her any more than did those of the manure; something surer than smell tells where her nest is.

The antennæ have been often suggested as the seat of a special sense to guide insects. I have already shown how the suppression of these organs appears to offer no obstacle to the researches of the Hymenoptera. Let us try once more in wider conditions. The Bembex is caught, its antennæ amputated to the roots, and is then released. Stung by the pain—wild with terror at being held between my fingers—the insect flies off swifter than an arrow. I had to wait a whole hour, uncertain as to its [265]return. However, it came, and with its invariable precision alighted quite close to its doorway, whose look I had changed for the fourth time, having covered the site with a large mosaic of pebbles the size of a nut. My work, which, compared to the Bembex, surpassed what for us are the Megalithic monuments of Brittany, or the lines of Menhirs at Carnac, was powerless to deceive the mutilated insect. Though deprived of antennæ it found the entrance in the midst of my mosaic as easily as would have done an insect under other conditions. This time I let the faithful mother go home in peace.

The site transformed four times over, the outworks of the abode changed in colour, scent, and material, the pain of a double wound,—all failed to disconcert the Hymenopteron or even to make her doubtful as to the precise locality of her doorway. I had exhausted my stratagems, and understood less than ever how the insect, if it have no special guide in some faculty unknown to us, can find its way when sight and smell are baffled by the artifices of which I have spoken. Some days later an experience gave me the opportunity to take up the problem from a new point of view. The Bembex burrow had to be bared in its whole extent, without quite destroying it, to which operation its shallowness and almost horizontal direction, and the light soil in which it was made, lent themselves readily. The sand was gradually scraped off with the blade of a knife, and thus, deprived of roof from end to end, the underground abode became a semi-canal or conduit, straight or curved, some eight inches long, open where was the entrance, and ending [266]in a cul-de-sac where lay the larva amid its food.

The dwelling was uncovered in full sunshine; how would the mother behave on her return? Let us consider the question scientifically. The observer may be greatly embarrassed: what I have already seen leads me to expect it. The mother’s impulse is to bring food to her larva, but to reach this larva she must first find the door. Grub and entrance are the points which appear to deserve being separately examined; therefore I take away grub and food, and the end of the passage is cleared. There is nothing more to do but arm one’s self with patience.

At last the Bembex arrives and makes straight for her absent door, only the threshold of which remains. There for a good hour did I see her dig, sweep the surface, send the sand flying, and persist, not in making a new gallery, but in seeking the loose sand barrier which should yield to the mere pressure of her head and let her pass easily. Instead of loose materials she finds firm soil not yet disturbed. Warned by this resistance she limits her efforts to exploring the surface, always close to where the door should be, only allowing herself to deviate a few inches. She returns to sound and sweep places already sounded and swept some twenty times, unable to leave her narrow circle, so obstinately convinced is she that the door must be there and nowhere else. With a straw I pushed her gently and repeatedly to another point. She would have none of it, and came back at once to where the door ought to have been. Now and then the gallery, turned into a semi-canal, appeared to attract [267]her attention, but very faintly. She would go a few steps along it, still raking, and then return to the entrance. Two or three times I saw her go the whole length of the gallery and reach the cul-de-sac where the larva should be, do a little careless raking, and hurry back where the entrance used to be, and continue searching with a patience which exhausted mine. More than an hour had passed, and still she sought on the site whence the door had disappeared.

What would happen in the presence of the larva? That was the second part of the question. To continue the experiment with the same Bembex would not have offered sufficient guarantee, as the creature, rendered more obstinate by her vain search, seemed possessed by a fixed idea, and this would have interfered with the facts which I wanted to prove. I required a new subject, concerned solely with the impulses of the actual moment. An opportunity soon came. The burrow was uncovered, as I have just said; but I did not touch the contents; larva and food were left in their places,—all was in order inside, the roof only was wanting. Well, with this open dwelling, whose every detail the eye could embrace,—vestibule, gallery, cell at the far end, with the grub and its heap of provender,—this dwelling turned into a roofless gallery at the end of which the larva was moving restlessly, under the hot sun, its mother continued the manœuvres already described. She alighted just where the entrance had been, and there it was that she hunted about and swept the sand—there that she always returned after some hasty attempt elsewhere in a circuit of a few [268]inches. No exploration of the gallery—no anxiety for the distressed larva; though the grub, whose delicate skin has just exchanged the gentle moisture of a cave for burning sunshine, is writhing on its heap of chewed Diptera, the mother takes no notice of it. For her it is no more than any one of the objects strewn on the sand,—a little pebble, a clod, a scrap of dried mud,—nothing more. It is undeserving of attention. This tender, faithful mother, who wears herself out in efforts to reach her nursling’s cradle, cares nothing just now but for her entrance door—the door she is used to. That which goes to her maternal heart is the longing to find the well-known passage. Yet the way is open; nothing holds her back, and under her eyes wriggles the grub, the final object of her anxiety. With one spring she would be at the side of the unhappy larva who so needs help. Why does she not rush to her beloved nursling? She could dig a new habitation and get it swiftly underground. But no—she persists in seeking a way which no longer exists, while her son is grilled under her eyes. I was boundlessly surprised by this obtuse maternity, since maternity is the most powerful and most fertile in resource of all feelings which move the animal. Hardly could I have believed my eyes but for endless experiments on the Cerceris and Philanthidæ, as well as on Bembecidæ of different species. Stranger still, the mother, after long hesitation, at length entered the unroofed passage—all that was left of the corridor. She advanced, drew back, and gave a few careless sweeps without stopping. Guided by vague recollections, and perhaps by the smell of [269]venison exhaled from the heap of Diptera, she came occasionally as far as the end of the gallery, the very spot where lay the larva. Mother and son had met. At this moment of reunion after long anxiety, were there earnest solicitude, sign of tenderness, or of maternal joy? Whoever thinks so has only to repeat my experiment to convince himself of the contrary. The Bembex did not recognise her larva at all; it was a worthless thing, in her way,—nothing but an embarrassment. She walked over it and trampled it unheeding, as she hurried backwards and forwards. If she wanted to dig at the bottom of the cell, she rudely kicked it behind her,—pushed, upset, expelled it, as she might have treated a large bit of gravel which got in her way while at work. Thus maltreated, the larva bethought itself of defence. I have seen it seize her by one tarsus with no more ceremony than she would have shown in biting the foot of a Dipteron caught by her. The struggle was sharp, but at last the fierce mandibles let go, and the mother flew wildly away with her sharpest hum. This unnatural scene of the son biting the mother, and perhaps even trying to eat her, is unusual, and brought about by circumstances which the observer is not always able to conjure up. What one can always witness is the profound indifference of the Hymenopteron for its offspring, and the brutal disdain with which that inconvenient heap, the grub, is treated. Once she has raked out the far end of the passage, which is done in a moment, the Bembex returns to her favourite point, the threshold, to resume her useless researches. As for the grub, it continues to struggle [270]and wriggle wherever the maternal kicks may have landed it. It will perish unaided by its mother, who could not recognise it because she was unable to find the passage she was used to. If we return to-morrow, we shall find it in the gallery, half-broiled by the sun, and already a prey to the flies—once its own prey.

Such is the connection in acts of instinct; one leading to the next in an order that the most serious circumstances have no power to alter. After all, what was the Bembex seeking? Her larva, evidently. But to reach this larva she had to enter the burrow, and to enter the burrow she had to find the door, and the mother persists in seeking this door while the gallery lay open with provender and larva all before her. The ruined abode, the endangered family, were for the moment unimportant; all she could think of was the familiar passage reached through loose sand. Let all go—habitation and inhabitant—if this passage be not found! Her actions are like a series of echoes, awaking one another in a fixed order, the following one only sounding when the preceding has sounded. Not because there was any obstacle; the burrow was all open, but for want of the usual entrance the first action could not take place. That decides everything; the first echo is mute, and so all the rest are silent. What a gulf between intelligence and instinct! Through the ruins of the shattered dwelling a mother guided by intelligence rushes straight to her son; guided by instinct she stops obstinately where once was the door.