We call those deadly spiders with the red hourglass shapes on their abdomens "black widows" because they eat their mates as part of the sex act. But black widows are far from the only spiders who sexually cannibalize, and some insects and fish do it too. It's just a natural part of some animals' reproductive process.…
The black widow spider practices sexual cannibalism, in which the female eats the male after mating - indeed, that's why they're called black widows in the first place. But males have an unexpected way of avoiding that fate.
Several kinds of female animals, including spiders, are known to devour their male counterparts, often right after they finish having sex. But every so often, the female isn't deadlier than the male, as South America's wolf spider will tell you.