Abstract
Light perception is a highly useful skill. Like other animals, we humans rely on vision to navigate, to locate food and mates, and to avoid predators. But biological applications of light perception go well beyond vision — from basic light-avoidance to circadian rhythms1. What's more, photoreceptive cells are located not only in the eyes, but also in various non-ocular locations, ranging from the skin in molluscs2 to the hypothalamus deep within a bird's brain3. Even overtly eyeless animals, such as the soil-dwelling nematode Caenorhabditis elegans, possess photosensitive neurons that help them to avoid the daylight4.