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Abstract

In arena experiments with the walking fruit fly, we found a remarkable persistence of orientation toward a landmark that disappeared during the fly's approach. The directional stability achieved by 'after-fixation' allows a fly to continue pursuit under natural conditions, where a selected target is frequently concealed by surrounding structures. The persistence of after-fixation was investigated in Buridan's paradigm, where a fly walks persistently back and forth between two inaccessible landmarks. Upon disappearance of a selected target, the flies maintained their intended course for more than 15 body lengths of approximately 2.5 mm in about 50 of the trials. About 13 even exceeded 75 body lengths. About 88 of the approaches clustered in equal portions around peaks at 2.4 s and 8.6 s. About 12 of the approaches persisted even longer. In contrast, a single peak at about 2.2 s is sufficient to describe the persistence of orientation in a random walk. The ability to pursue an invisible landmark is disturbed neither by a transient angular deviation from the course toward this landmark, when this target disappeared, nor by a distracting second landmark. Accordingly, after-fixation seems to require an internal representation of the direction toward the concealed target, and idiothetical course control to maintain this direction.