Most animals are pretty good at avoiding the embarrassing faux pas of having sex with the wrong species.

But what if you're a millipede under a cramped,dark, slippery rock with a dozen or more species all scurrying about,looking for love?

Fortunately,the critters evolved a solution: elaborate male gonopods—literally "genital feet"—with all manner of branches, bumps and bristles.

And even with 12,000 species of millipedes, no two varieties of gonopods are exactly the same.

So the little arthropods can immediately tell if they're consorting with the wrong species—convenient not only for the millipedes,but also for biologists.

"So you just have to pick it up and look at it and you say, yup,that's the species,and you can identify it." That's millipede researcher Petra Sierwald at the Field Museum in Chicago.

The only problem is—no offense to any male millipedes—the gonopods are tiny.

"If you look at a millipede,it's not that big to begin with.

So you can imagine their modified legs ...

are even smaller."Now,before we go any further,you might be wondering, "Who cares about millipedes,let alone their genitals?"And the answer is,not enough of us.