Let us continue our series of tests with the Mason-bee of the Walls. Thanks to its position on a pebble which we can move at will, the nest of this Bee lends itself to most interesting experiments. Here is the first: I shift a nest from its place, that is to say, I carry the pebble which serves as its support to a spot two yards away. As the edifice and its base form but one, the removal is performed without the smallest disturbance of the cells. I lay the boulder in an exposed place where it is well in view, as it was on its original site. The Bee returning from her harvest cannot fail to see it.
In a few minutes, the owner arrives and goes straight to where the nest stood. She hovers gracefully over the vacant site, examines and alights upon the exact spot where the stone used to lie. Here she walks about for a long time, making persistent searches; then the Bee takes wing and flies away to some distance. Her absence is of short duration. Here she is back again. The search is resumed, walking and flying, and always on the site which the nest occupied at first. A fresh fit of exasperation, that is to say, an abrupt flight across the osier-bed, is followed by a fresh return and a renewal of the vain search, always upon the mark left by the shifted pebble. These sudden departures, these prompt returns, these persevering inspections of the deserted spot continue for a long time, a very long time, before the Mason is convinced that her nest is gone. She has certainly seen it, has seen it over and over again in its new position, for sometimes she has flown only a few inches above it; but she takes no notice of it. To her, it is not her nest, but the property of another Bee.
Often the experiment ends without so much as a single visit to the boulder which I have moved two or three yards away: the Bee goes off and does not return. If the distance be less, a yard for instance, the Mason sooner or later alights on the stone which supports her abode. She inspects the cell which she was building or provisioning a little while before, repeatedly dips her head into it, examines the surface of the pebble step by step and, after long hesitations, goes and resumes her search on the site where the home ought to be. The nest that is no longer in its natural place is definitely abandoned, even though it be but a yard away from the original spot. Vainly does the Bee settle on it time after time: she cannot recognize it as hers. I was convinced of this on finding it, several days after the experiment, in just the same condition as when I moved it. The open cell half-filled with honey was still open and was surrendering its contents to the pillaging Ants; the cell that was building had remained unfinished, with not a single layer added to it. The Bee, obviously, may have returned to it; but she had not resumed work upon it. The transplanted dwelling was abandoned for good and all.
I will not deduce the strange paradox that the Mason-bee, though capable of finding her nest from the verge of the horizon, is incapable of finding it at a yard's distance: I interpret the occurrence as meaning something quite different. The proper inference appears to me to be this: the Bee retains a rooted impression of the site occupied by the nest and returns to it with unwearying persistence even when the nest is gone. But she has only a very vague notion of the nest itself. She does not recognize the masonry which she herself has erected and kneaded with her saliva; she does not know the pollen-paste which she herself has stored. In vain she inspects her cell, her own handiwork; she abandons it, refusing to acknowledge it as hers, once the spot whereon the pebble rests is changed.
Insect memory, it must be confessed, is a strange one, displaying such lucidity in its general acquaintance with locality and such limitations in its knowledge of the dwelling. I feel inclined to call it topographical instinct: it grasps the map of the country and not the beloved nest, the home itself. The Bembex-wasps (Cf. "Insect Life": chapters 16 to 19.—Translator's Note.) have already led us to a like conclusion. When the nest is laid open, these Wasps become wholly indifferent to the family, to the grub writhing in agony in the sun. They do not recognize it. What they do recognize, what they seek and find with marvellous precision, is the site of the entrance-door of which nothing at all is left, not even the threshold.
If any doubts remained as to the incapacity of the Mason-bee of the Walls to know her nest other than by the place which the pebble occupies on the ground, here is something to remove them: for the nest of one Mason-bee, I substitute that of another, resembling it as closely as possible in respect to both masonry and storage. This exchange and those of which I shall speak presently are of course made in the owner's absence. The Bee settles without hesitation in this nest which is not hers, but which stands where the other did. If she was building, I offer her a cell in process of building. She continues the masonry with the same care and the same zeal as if the work already done were her own work. If she was fetching honey and pollen, I offer her a partly-provisioned cell. She continues her journeys, with honey in her crop and pollen under her belly, to finish filling another's warehouse. The Bee, therefore, does not suspect the exchange; she does not distinguish between what is her property and what is not; she imagines that she is still working at the cell which is really hers.
After leaving her for a time in possession of the strange nest, I give her back her own. This fresh change passes unperceived by the Bee: the work is continued in the cell restored to her at the point which it had reached in the substituted cell. I once more replace it by the strange nest; and again the insect persists in continuing its labour. By thus constantly interchanging the strange nest and the proper nest, without altering the actual site, I thoroughly convinced myself of the Bee's inability to discriminate between what is her work and what is not. Whether the cell belong to her or to another, she labours at it with equal zest, so long as the basis of the edifice, the pebble, continues to occupy its original position.
The experiment receives an added interest if we employ two neighbouring nests the work on which is about equally advanced. I move each to where the other stood. They are not much more than thirty inches a part. In spite of their being so near to each other that it is quite possible for the insects to see both homes at once and choose between them, each Bee, on arriving, settles immediately on the substituted nest and continues her work there. Change the two nests as often as you please and you shall see the two Mason-bees keep to the site which they selected and labour in turn now at their own cell and now at the other's.
One might think that the cause of this confusion lies in a close resemblance between the two nests, for at the start, little expecting the results which I was to obtain, I used to choose the nests which I interchanged as much alike as possible, for fear of disheartening the Bees. I need not have taken this precaution: I was giving the insect credit for a perspicacity which it does not possess. Indeed, I now take two nests which are extremely unlike each other, the only point of resemblance being that, in each case, the toiler finds a cell in which she can continue the work which she is actually doing. The first is an old nest whose dome is perforated with eight holes, the apertures of the cells of the previous generation. One of these cells has been repaired; and the Bee is busy storing it. The second is a nest of recent construction, which has not received its mortar dome and consists of a single cell with its stucco covering. Here too the insect is busy hoarding pollen-paste. No two nests could present greater differences: one with its eight empty chambers and its spreading clay dome; the other with its single bare cell, at most the size of an acorn.
Well, the two Mason-bees do not hesitate long in front of these exchanged nests, not three feet away from each other. Each makes for the site of her late home. One, the original owner of the old nest, finds nothing but a solitary cell. She rapidly inspects the pebble and, without further formalities, first plunges her head into the strange cell, to disgorge honey, and then her abdomen, to deposit pollen. And this is not an action due to the imperative need of ridding herself as quickly as possible, no matter where, of an irksome load, for the Bee flies off and soon comes back again with a fresh supply of provender, which she stores away carefully. This carrying of provisions to another's larder is repeated as often as I permit it. The other Bee, finding instead of her one cell a roomy structure consisting of eight apartments, is at first not a little embarrassed. Which of the eight cells is the right one? In which is the heap of paste on which she had begun? The Bee therefore visits the chambers one by one, dives right down to the bottom and ends by finding what she seeks, that is to say, what was in her nest when she started on her last journey, the nucleus of a store of food. Thenceforward she behaves like her neighbour and goes on carrying honey and pollen to the warehouse which is not of her constructing.