Abstract
1.
The way the sensitivity of the optokinetic response varies with azimuth and elevation was studied in several species of brachyuran crabs:Mictyris longicarpus andHeloecius cordiformis, both inhabiting mud flats;Carcinus maenas andPachygrapsus marmoratus, inhabiting densely structured environments.Mictyris is an exclusively forward walking crab, the other species usually run sideways.
2.
All species investigated respond maximally to lateral optokinetic stimulation. When they see with both eyes, variation of optokinetic sensitivity with azimuth is less pronounced than in monocular crabs.
3.
Species which live in flat environments only respond to optokinetic stimulation along and above the equator of the eye, species living in densely structured environments also respond to movement below the equator.
4.
In “flat world crabs”, the upper half of the eye views relatively distant objects. Moving their eyes only in response to motion above the horizon gives these crabs a way of avoiding the rotational component of the visual flow-field generated during locomotion. Thus the separation of the rotational from the translational component of the visual flow field is achieved without performing complicated computations but by taking advantage of the geometry of the situation.
