How bees detect coloured targets using different regions of their compound eyes

Honeybees Apis mellifera detect coloured targets presented to the frontal region of their compound eyes using their colour vision system at larger visual angles (alpha > 15 degrees), and an achromatic visual system based on the long-wave photoreceptor type at smaller visual angles (5 degrees < alpha < 15 degrees). Here we examine the capability of the dorsal, ventral and frontal regions of the eye for colour detection. The minimum visual angle alpha(min) at which the bees detect a stimulus providing both chromatic contrast and receptor-specific contrasts to the three receptor types varies for the different regions of the eye: 7.1 +/- 0.5 degrees for the ventral region, 8.2 +/- 0.6 degrees for the dorsal region and 4.0 +/- 0.5 degrees for the frontal region. Flight trajectories show that when the target was presented in the horizontal plane, bees used only the ventral region of their eyes to make their choices. When the targets appeared dorsally, bees used the frontodorsal region. This finding suggests that pure dorsal detection of coloured targets is difficult in this context. Furthermore, alpha(min) in the ventral plane depends on receptor-specific contrasts. The absence of S-receptor contrast does not affect the performance (alpha(min) = 5.9 +/- 0.5 degrees), whilst the absence of M- and L-receptor contrast significantly impairs the detection task. Minimal visual angles of 10.3 +/- 0.9 degrees and 17.6 +/- 3 degrees, respectively, are obtained in these cases. Thus, as for many visual tasks, the compound eye of the honeybee shows a regionalisation of colour detection that might be related to peripheral or central specialisations.