Fig 2. The four behavioral options exhibited by a fly upon approaching the doorway.

A distance of 8.5mm or less from the doorway’s center defines proximity to it. Path segments traced by the fly are colored from yellow (the beginning of the segment) to light blue (the end of the segment). The fly approaches the doorway (a) from the small arena but does not cross to the large arena, (b) from the small arena crossing into the large arena (cutting through), (c) from the large arena crossing into the small arena (d), from the large arena but does not cross to the small arena (holding back).