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A spider does what with his genitals?

You probably knew that honeybees typically die after they sting you, as their barbed stingers are torn from their abdomens. But did you know that after mating, some male spiders break off the ends of their palps—the organ used to transfer sperm—inside the female? 

That’s right. And afterward, this mutilated “eunuch” will stick around the female’s web and prevent her from shacking up with his competitors.

In a paper in this month’s issue of the journal Evolution, Jonathan Coddington of the Smithsonian Institution, along with colleagues Matjaz Kuntner and Jutta Schneider, studied the evolution of the genitals of males and females in 32 species of Nephilid spiders, including the golden orb weaver (pictured). Whereas most studies of sexual selection have focused on how males compete with each other for dominance and how females pick their mates, spider genitalia tell another side of the story: that the battle of the sexes continues during and, even after sex. Males want to ensure that their sperm—and only their sperm—fertilizes the females’ eggs, while females would like to keep their options open.

The result, Coddington says, is that male and female genitalia have grown increasingly complex over evolutionary time. Males gradually developed hooks, ridges, and twists on their palps as female genitalia transformed from a slit with a straight, short duct to a series of elaborate chambers making them more difficult to “plug".

“Sex is one of the more powerful driving forces behind evolution,” Coddington says, “and it just so happens that spiders have some of the most bizarre sex of animals on Earth.”

Image of golden orb weaver courtesy of Jonathan Coddington

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