Female mantis are much bigger and stronger thus they practice sexual cannibalism.
Some studies have shown that when in captivity, female spiders eat the male for nutrition.
Female jumping spiders only mate once so they are picky. If they don't like their mate, the male spider gets eaten.
A female green anaconda mates with several males at a time. To get energy they sometimes eat the male anacondas.
Female scorpions give live birth thus to get nutrition they sometimes eat the male scorpion.
Male octopuses after they have injected sperm into the female are often eaten.