The visual system of the honeybee (Apis mellifera): The maximum length of the orientation detector

Bees were trained to discriminate between two or more black bars and similar bars at right angles, presented on a vertical surface. The positions of the bars were shifted every 5 min to prevent their locations being used as cues. The experiments exploit the fact that bees do not discriminate the global orientation of a straight line of small black squares that are individually resolved, because the local responses to equal lengths of edges at right angles cancel out, and each square has no residual orientation cue. The experiments measure the resolution of this effect by control of the width of the gaps between the squares. At the limit the unit orientation detectors cannot span the gaps. Training with vertical or with horizontal bars in separate experiments, and testing with vertical or horizontal lines of squares, shows that the vertical gaps in horizontal rows are detected with better resolution than horizontal gaps in vertical rows. The results show that unit orientation detectors span not more than 3 ommatidia.