Polarization pattern: Light scattered in a blue sky forms a pattern of partially polarized ultraviolet light. This depends on the position of the sun, and is invisible to human eyes. Each lens unit of a compound eye has a UV receptor, and a UV filter oriented differently in each of these units, so a bee is able to detect this polarization pattern. A small piece of blue sky is enough for a bee to recognize the pattern changes occurring over the course of a day. This provides not only directional but also information on the time of day.