The perforator of clay nests has, so it seems to me, delivered us from an old physiological fallacy. She would deserve studying, if for no other result than this; but her interest is far from being exhausted. Let us look at her from another point of view, whose full importance will not be apparent until the end; let us speak of something which I was very far from suspecting when I was so assiduously watching the nests of my Mason-bees. The same cell can receive the Leucopsis' probe a number of times, at intervals of several days. I have said how I used to mark in black the exact place at which the laying-implement had entered and how I wrote the date of the operation beside it. Well, at many of these already visited spots, concerning which I possessed the most authentic documents, I saw the insect return a second, a third and even a fourth time, either on the same day or some while after, and drive its inoculating-thread in again, at precisely the same place, as though nothing had happened. Was it the same individual repeating her operation in a cell which she had visited before but forgotten, or different individuals coming one after the other to lay an egg in a compartment thought to be unoccupied? I cannot say, having neglected to mark the operators, for fear of disturbing them. As there is nothing, except the mark of my pencil, a mark devoid of meaning to the insect, to indicate that the auger has already been at work there, it may easily happen that the same operator, finding under her feet a spot already exploited by herself but effaced from her memory, repeats the thrust of her tool in a compartment which she believes herself to be discovering for the first time. However retentive its memory for places may be, we cannot admit that the insect remembers for weeks on end, as well as point by point, the topography of a nest covering a surface of some square yards. Its recollections, if it have any, serve it badly; the outward appearance gives it no information; and its drill enters wherever it may happen to discover a cell, at points that have already perhaps been pierced several times over. It may also happen--and this appears to me the most frequent case--that one exploiter of a cell is succeeded by a second, a third, a fourth and others still, all fired with the newcomer's zeal because their predecessors have left no trace of their passage. In one way or another, the same cell is exposed to manifold layings, though its contents, the Chalicodoma-grub, be only the bare ration of a single Leucopsis-grub.