IS THE MEMBRANE VOLTAGE-AMPLIFIER OF DRONE PHOTORECEPTORS USEFUL AT PHYSIOLOGICAL LIGHT INTENSITIES

Free-flying drones (Apis mellifera) were attracted by pheromone to an elevated observation site where they looked for small distant objects, a visual task similar to nuptial pursuit. The brightness of the sky a gainst which drones performed this task was measured as the radiance o ver a range of wavelengths covering the absorbance spectra of the prin cipal drone rhodopsin and its metarhodopsin. This brightness varied re latively little, by just over one log unit. In laboratory experiments on the intact drone, the amplitude of photoreceptor responses to small decrements in intensity showed a marked maximum when the background i ntensity depolarized the cells to about -35 mV. The radiance of this o ptimal background was only 0.2 log units greater than the sky radiance at which most drones were seen. It is argued that because of probable systematic errors the true difference may be less. It is known that i n superfused retinal slices voltage-dependent sodium channels amplify small signals under these stimulus conditions. We therefore conclude t hat this amplification is useful for the major visual activity of the drone.