That’s a whole new meaning to “absentee father”. Put yourself in a scenario. You get to have sex one time, and if you stick around too long you’ll get eaten. You want to make that one time count, and you’d also like to make sure that no one else got the opportunity to reproduce with your chosen lady.
Solution? If you’re an orb-web spider, it appears that breaking off your penis inside of the female and running away is just what Dr. Evolution ordered. It saves his legs, it prevents other males from copulating, and it allows him to fight to protect her. Even if she does want to eat him in the first place. How romantic.
Violent mating behaviors are actually rather common. Hermaphroditic flatworms engage in “penis fencing" to decide who will be the mother and who will be the father of their offspring. Honeybees’ genitals explode and break off inside the queen in a manner similar to the spider. Bedbugs simply impale the female with their penises and deposit sperm through the opening. And banana slugs have such large penises that if they freak out the female with one too large, they risk getting their organ chewed off.
(via Nature News)