Regional differences in the preferred e-vector orientation of honeybee ocellar photoreceptors

Abstract

In addition to compound eyes, honeybees (Apis mellifera) possess three single lens eyes called ocelli located on the top of the head. Ocelli are involved in head-attitude control and in some insects have been shown to provide celestial compass information. Anatomical and early electrophysiological studies have suggested that UV and blue-green photoreceptors in ocelli are polarization sensitive. However, their retinal distribution and receptor characteristics have not been documented. Here, we used intracellular electrophysiology to determine the relationship between the spectral and polarization sensitivity of photoreceptors and their position within the visual field of the ocelli. We first determined a photoreceptor’s spectral response through a series of monochromatic flashes (340 - 600 nm). We found UV and Green receptors, with peak sensitivities at 360 nm and 500 nm respectively. We subsequently measured polarization sensitivity at the photoreceptor’s peak sensitivity wavelength by rotating a polarizer with monochromatic flashes. Polarization sensitivity (PS) values were significantly higher in UV receptors (3.8±1.5, N=61) than Green receptors (2.1±0.6, N=60). Interestingly, most receptors with receptive fields below 35° elevation were sensitive to vertically polarized light while the receptors with visual fields above 35° were sensitive to a wide range of polarization angles. These results agree well with anatomical measurements showing differences in rhabdom orientations between dorsal and ventral retinae. We discuss the functional significance of the distribution of polarization sensitivities across the visual field of ocelli by highlighting the information the ocelli are able to extract from the bee’s visual environment.