It may sound freaky, but the Japanese yellow swallowtail butterfly can actually 'see' with its genitals.
Twenty years ago, biologist Kentaro Arikawa discovered that the genitals of the male and female adults contain cells that respond to light.
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These photoreceptive cells aren't used 'for' sex, but they do shed light on the process - literally. Yellow swallowtails mate facing away from each other, so the male uses his light-sensitive organ to help line up with the female. If the photoreceptors are covered up, mating success decreases.
Females, meanwhile, use their light-sensitive cells to help position their 'ovipositor' (egg-laying organ). Pregnant females will lay their eggs on citrus leaves, but if their light-sensitive cells are blacked out, they struggle to detect the leaf surface or attach their eggs.
Similar structures have since been found in other butterfly species, so the phenomenon could be more widespread than initially thought.
