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Abstract

1. Pitch and roll eye movements of three species of crabs (Heloecius cordiformis, Mictyris longicarpus, Pachygrapsus marmoratus) were recorded in response to visual stimuli. 2. The ‘flat world’ species (Heloecius, Mictyris) with a zone of high vertical acuity around the equator of the eye (Zeil et al. 1986) turn the eye stalks towards a horizontal contrast line presented close to their specialized zone and pursue it for some degrees when it rotates away (Figs. 5a, b, 6a, b, 8, 10a). We interpret this as fixation of the horizon and discuss its relevance for the proposed mechanisms for ‘vision in a flat world’ (Zeil et al. 1986; Nalbach and Nalbach 1987). 3. Both types of crabs, the ‘flat world’ and the ‘complex world’ species, stabilize their eyes against dynamic perturbations not only via mechanosensory systems (Fig. 7, Nalbach et al. 1989) but also via an optokinetic system (Figs. 1, 2). In roll even nystagmic saccades can be elicited which, however, occur more unreliably and are about four times slower than in yaw. They are missing completely in pitch. 4. Regional distribution of optokinetic sensitivity over the eye in pitch and roll (Figs. 3, 4) is discussed as adaptation to image flow occurring under natural conditions. 5. The results obtained with the rock crab,Pachygrapsus (Fig. 4), suggest that movement detectors looking into opposite directions in the visual field might interact specifically to extract global rotational image flow.