Male Japanese yellow swallowtail butterflies have light-sensitive cells on their genitalia. That may seem like a strange place for what are essentially simple eyes, but apparently they’re crucial for what many humans might consider an essentially simple task: successful mating. Neuroethologist Kentaro Arikawa and his colleagues at the Yokohama City University in Yokohama teased out the cells’ function in a non-volunteer force of male butterflies, either by destroying the cells outright (with heat) or painting them black. Only one in four males without the cells successfully copulated, while two-thirds of intact males finished the task. Arikawa suspects that the cells’ detection of the change from light to dark in their immediate vicinity is necessary to spur the male into action. Normally, mating males receive light by the genitalia when they are searching the female genitalia. But when they are nicely aligned with the female, and in the correct position to mate, the photoreceptor cells are totally covered and the response stops, says Arikawa. When you block the photoreceptor, the response drop cannot be experienced by the male. The male will continue searching the genitalia--unable to ‘see’ that he is correctly aligned--until he eventually gives up.