The male orb-web spider has detachable genitalia so that he can flee while finishing intercourse so that his mate won't eat him.

You may know that many kinds of spiders have a bad habit of eating their mates after being impregnated. The male orb-web spider has a countermeasure to this. Like some other types of spiders, Nephilengys malabarensis is sexually cannibalistic, meaning the female likes to eat the male after intercourse. 

These spiders have only up to two chances to mate. The males have sperm transferring organs called “palps” that detach either when their bodies are eaten by the females or when they break them off and run away. This isn’t too uncommon in spiders though. Many scientists are perplexed by this because they aren’t able to see how having a break-off reproductive organ is a good thing. 

Evolutionarily speaking, not being able to mate isn’t a very desirable thing, so it’s a little understandable why such a thing would be an advantage. If anything, we can just be glad this isn’t a genetic trait also shared in humans. 

(Source)