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, 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 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.