Discrimination of coloured stimuli by honeybees: Alternative use of achromatic and chromatic signals

Using a Y-maze experimental set-up, honeybees Apis mellifera were trained to a coloured disc presented against an achromatic background. In subsequent tests they were given a choice between the trained disc and an alternative disc that differed either in its chromatic properties, or in the amount of achromatic green contrast that it produced against the background. Tests were conducted in two experimental situations: one in which discs subtended a visual angle of 30 degrees (as viewed by the bee at the decision point in the Y-maze), and another in which the angle was 6.5 degrees or 5 degrees (depending on the experiment). At the visual angle of 30 degrees, the bees' choice behaviour was governed by the differences in chromatic properties, and not by the differences in the amount of green contrast. With the 6.5 degrees- and 5 degrees-discs, on the other hand, it was governed by the differences in the amount of green contrast, and not by the differences in chromatic properties. Consequently, in the present discrimination task, bees use either chromatic or achromatic cues, depending on the visual angle subtended by the stimuli at the eye. Results of a further experiment, in which the trained disc was tested against discs that produced various amounts of green contrast, confirm the above conclusion and show, in addition, that bees learn the green-contrast difference between a trained and a non-rewarded alternative.