I myself was at the burrows a few hours later. [[309]]I saw several of yesterday’s Cerceres, recognizing them by the one white spot on the thorax; but I saw none of those whom I had just let loose. Had they not been able to find their home again? Were they hunting? Or were they hiding in their galleries to recover from the excitement of such a trial? I do not know. Next day I paid a fresh visit; and this time I had the satisfaction of finding at work, as active as though nothing out of the way had happened, five of the Cerceres with two white spots on the thorax. A journey of quite two miles, the town with its houses, its roofs, its smoky chimneys, all things so new to these utter rustics, had not prevented them from going back to the nest.
When taken from his brood and carried to enormous distances, the Pigeon returns promptly to the dovecote. If we wanted to work out a proportion between the length of the journey and the size of the creature, how greatly superior to the Pigeon would be the Cerceris, who finds her burrow after being carried a distance of two miles! The bulk of the insect is not a cubic centimetre,[2] whereas that of the Pigeon must be quite a cubic decimetre,[3] if not more. The bird, being a thousand times larger than the Wasp, ought therefore, in order to rival her, to [[310]]find the dovecote at a distance of two thousand miles, which is thrice the greatest length of France from north to south. I do not know that a Carrier-pigeon has ever performed such a feat. But power of flight and, still less, lucidity of instinct are qualities that cannot be measured by the yard. Comparative size cannot here be taken into consideration; and we must just look upon the insect as a worthy rival of the bird, without deciding which of the two has the advantage.
In returning to the dovecote and the burrow, when man has artificially made them lose their bearings and carried them to great distances, in unfamiliar directions and into regions which they have not yet visited, are the Pigeon and the Cerceris guided by recollection? Is memory their compass when, on reaching a certain height, whence they can, so to speak, pick up the scent after a fashion, they dart with all their power of wing towards the horizon where their nests are? Is it memory that traces their road through the air, across regions which they are seeing for the first time? Obviously not: there can be no recollection of the unknown. The Wasp and the bird are unacquainted with the country around; nothing can have told them the general direction in which they were moved, for the journey was made in the darkness of a closed [[311]]basket or a box. Locality, relative position: everything is unknown to them; and yet they find their way. They therefore have something better than mere memory as a guide: they have a special faculty, a sort of topographical sense of which we cannot possibly form an idea, having nothing similar ourselves.
I will show by experiment how subtle and precise this faculty is within its narrow province, and also how obtuse and dull it becomes when driven to depart from the usual conditions in which it acts. This is the invariable antithesis of instinct.
A Bembex, actively engaged in feeding her larva, leaves the burrow. She will return presently with the produce of the chase. The entrance is carefully stopped up with sand, which the insect has swept there backwards before going away; there is nothing to distinguish it from other points of the sandy surface; but this does not trouble the Wasp, who finds her door with a skill which I have already emphasized. Let us devise some insidious plot and change the conditions of the locality in order to perplex the insect. I cover the entrance with a flat stone, the size of my hand. The Wasp soon arrives. The great change effected on her threshold during her absence appears to cause her not the slightest hesitation; at least, the Bembex at [[312]]once alights upon the stone and tries, for an instant, to dig into it, not at random but at a spot corresponding with the opening of the burrow. The hardness of the obstacle soon dissuades her from her enterprise. She then runs about the stone in every direction, goes all round it, slips underneath and begins to dig in the exact direction of her dwelling.
The flat stone is not enough to mislead our wide-awake friend; we must find something better. To cut things short, I do not allow the Bembex to continue her excavations, which, I can see, will soon prove successful; I drive her off with my handkerchief. The fairly long absence of the frightened insect will give me time to prepare my snares at leisure. What materials shall I employ now? In these improvised experiments we must know how to turn everything to use. Not far off, on the high-road, are the fresh droppings of some beast of burden. The very thing! The droppings are collected, broken up, crumbled and then spread in a layer at least an inch thick on the threshold of the burrow and all around, covering about a quarter of a square yard. This certainly is a house-front the like of which no Bembex ever knew. The colouring, the nature of the materials, the stercoral effluvia all combine to mystify the Wasp. Will she take all this—that [[313]]expanse of manure, that dung—for the front of her door? Why, yes: here she comes! She inspects the unwonted condition of the place from above and settles in the middle of the layer, just opposite the entrance. She digs, makes a hole through the stringy mass and reaches the sand, where she at once finds the orifice of the passage. I stop her and drive her away a second time.
Is not the precision with which the Wasp alights just in front of her door, though this be masked in a way so new to her, a proof that sight and memory are not her only guide? What else can there be? Could it be scent? It is very doubtful, for the emanations from the droppings have not been able to baffle the insect’s perspicacity. Still, let us try a different smell. I happen to have on me, as part of my entomological luggage, a small phial of ether. I sweep away the sheet of manure and replace it by a blanket of moss, not very thick, but spreading to a considerable distance; and I pour the contents of my phial on it as soon as I see the Bembex arrive. The ethereal fumes, at first too strong, keep the Wasp away, but only for a moment. Then she alights on the moss, which still exhales a very perceptible smell of ether, passes through the obstacle and makes her way indoors. The ethereal effluvia put her out no [[314]]more than did the stercoral effluvia. Something surer than scent tells her where her nest lies.
The antennæ have often been suggested as the seat of a special sense able to guide insects. I have already shown how the amputation of those organs seems in no way to impede the Wasp’s investigations. Let us try once more, under more complicated conditions. I seize the Bembex, cut off her antennæ at the roots, and at once release her. Goaded by pain, maddened at having been imprisoned in my fingers, the insect darts off faster than an arrow. I have to wait for a good hour, very uncertain as to whether it will come back. The Wasp arrives however and, with her unvarying precision, alights quite close to her door, whose appearance I have changed for the fourth time. The site of the nest is now covered with a spreading mosaic of pebbles the size of a walnut. My work, which, as regards the Bembex, surpasses what the megalithic monuments of Brittany or the rows of menhirs at Carnac are to us, is powerless to deceive the mutilated insect. Though deprived of her antennæ, the Wasp finds her entrance in the middle of my mosaic as easily as the same insect, supplied with those organs, would have done under other conditions. This time I let the faithful mother go indoors in peace. [[315]]
Four successive alterations in the site; changes in the colour, the smell, the materials of the outside of the home; lastly, the pain of a double wound: all had failed to baffle the Wasp or even to make her waver as to the precise locality of her door. I had come to the end of my stratagems and understood less than ever how the insect, if it possess no special guide in some faculty unknown to us, can find its way when sight and scent are baffled by the artifices which I have mentioned.
A few days later, a lucky experiment reopened the question and allowed me to study it under another aspect. In this case we uncover the Bembex’ burrow all the way along, without changing its appearance too much, an operation made easier by the shallowness of the burrow, its almost horizontal direction, and the lack of consistency of the soil in which it is dug. With this object we scrape the sand away gradually with a knife. Thus deprived of its roof from end to end, the underground dwelling becomes an open trench, a conduit, straight or curved, some eight inches long, open at the spot where the entrance-door used to be and finishing in a blind alley at the other end, where the larva lies amid its victuals.