Animals get all the cool accoutrements — tails, wings, prehensile penises — and humans all know which one they’re most jealous of. If you want proof, ask any elementary schooler, most stoned college kids, and certain tags on Tumblr. But the one creature I’ll bet no human has ever, ever envied is the Japanese swallowtail butterfly. Okay, it has pretty wings, but it also has eyes in its butt. Pass.
The butterfly, Papilio xuthus, has two light-sensing neurons called photoreceptors in its rear end, right near its junk. (In fact, I’m being a little euphemistic when I say that it sees “with its butt.” In the scientific literature, they are pretty consistently called “genital photoreceptors.”) The photoreceptors aren’t eyes in the classical sense, but they do respond to light, and they can “see” well enough to do their job: ensuring that the female’s ovipositor — the thing that deposits eggs — is in the correct position. Male butterflies have rear-end photoreceptors too, though I’m not sure why.
(UPDATE: Ferris Jabr of Scientific American explained why! “Males rely on light-detection when aligning their genitals with the female’s during mating; they do it back to back, facing away.” So it’s the same idea — making it easier to to tricky positioning on bits you can’t see, like those rear-view cameras on fancy cars.)
Man. If crotch-eyes are the price you pay for being a butterfly, then for once I think I’m glad to be a human.
Blindsight: Animals that See Without Eyes [Slide Show], Scientific American.