The male orb-web spider has a dangerous natural imperative. He—the little one on the left—has to have sex with the behemoth on the right. And she wants to eat him. What's a guy to do? Use his detachable penis to keep her satisfied while he scurries off, of course.
The male Nephilengys malabarensis is equipped with two sets of genitals known as "palps." These are designed to break off and continue pumping sperms when the male is either eaten or otherwise bails. But why would a male orb-web spider limit himself to pleasing just two ladies?
Daiquin Li, a researcher at the University of Singapore believes it aids in the spider's chances of procreation. "About 30 percent of sperm were transferred to the females before the palp breakoff, and about 70 percent of sperm were still left in the broken palp," Li told LiveScience. "In our experiments, it took about 20 minutes to transfer about 85 percent of [the] sperm." That's an impossibly long time to be riding something five times your size with a hankering for your flesh—like riding a zombie bull for a half hour.
What's more, the male will occasionally break off both palps in what's known as the "eunuch phenomenon." Oddly though, after castration the males become much more aggressive, defending the female from other potential meals suitors. [MSNBC]