Fiddler Crabs Use the Visual Horizon to Distinguish Predators from Conspecifics: A Review of the Evidence

Male fiddler crabs, Uca pugilator (Crustacea: Decapoda), respond to conspecifics by claw waving, and to predators by freezing or escape. In field experiments it was found that this distinction was not made on the basis of angular size and speed, nor was shape important. The remaining possibilities were either the absolute size of the stimulus, determined from angular size and distance, or the position of the stimulus relative to the horizon. To distinguish between these, a crab was placed in a glass dish, and moved black stimuli on a white background, at a distance of 22 cm. Stimuli below the crab's horizon hardly ever evoked escape. However, identical stimuli partially or wholly above the crab's horizon produced escape responses whose frequency varied with the angular size of the stimulus. Halving the distance of the stimulus showed that it was angular and not absolute size that determines escape frequency; and experiments with a tilted horizon showed that it is the position of the stimulus relative to the eye equator that is important, rather than the geographical horizon itself. It has been concluded that crabs categorize stimuli as dangerous or otherwise by their position relative to the crabs’ visual horizon.

VI. On the formation of definite figures by the deposition of dust

When trying some experiments which had an object other than that described in the following communication, it was noticed that a fine powder when allowed to settle on a slightly warmed plate produced figures which were remarkably clear and definite. So striking and peculiar were these figures, and so simple were the conditions of their formation, that a careful study of them was undertaken. These figures are so clear and sharp that it is easy to obtain exact photographic records of them, an important point, for, at present, it does not seem possible to offer a simple explanation of the complicated relationships which exist between the external conditions and the figures formed. Sensitive as these figures are to outside influences, the forms they assume are very characteristic of different conditions, are perfectly constant, and are easily produced. The general method of obtaining these figures is as follows:—The plate on which the figure is to be deposited is best supported on three pins about 1½ to 2 inches high, and the dust most convenient to use is that made by burning magnesium ribbon. It is kindled and allowed to burn in a receiver. A circular glass dish with straight sides, about 4 inches high and 9 inches in diameter, is a convenient form of vessel to use; and if the vessel be large enough (there should be about 2 inches between the plate and the inside of the receiver); the shape and the material of this dust containing vessel is not of much consequence. After the magnesium has burnt out, this receiver is allowed to stand for a minute or so, and it is then placed over the plate on its stand and allowed to remain there for six to seven minutes. On removing it a clear and definite figure will be found to have formed on the plate. If the plate has been a square one, then a cross consisting of four rays, each starting from a corner and meeting, but not necessarily joining, in the centre, is produced. If the corners be varnished or covered by a small piece of tinfoil (fig. 1) the cross is still formed.