THE VISUAL CONTROL OF BEHAVIOR IN FIDDLER-CRABS .1. RESOLUTION, THRESHOLDS AND THE ROLE OF THE HORIZON

The distribution of resolution around the eye of the fiddler crab Uca pugilator was measured using the pseudopupil, and the results were compared with various behavioural thresholds. As noted previously (Zeil et al. 1986) the eye has a band of greatly increased vertical resolution around the middle with inter-ommatidial angles as small as 1 degrees (Figs. 1-3). In life this band images the horizon. The horizontal resolution is nearly constant throughout the eye. Males notice females when they subtend 2 to 3 inter-ommatidial angles, and distinguish males from females when they subtend 6 to 7 ommatidia (Fig. 6). Potential predators are detected when they subtend only one inter-ommatidial angle, and move through a similar angle. Using models, it was found that males respond to moving objects of the right size as though they were conspecifics, although they do not resemble crabs in shape, It is concluded that only one feature distinguishes other crabs from predators: whether they appear below or above the crab's horizon.