Female insects eating males during mating is a minorty activity - the majority of male insects simply die after mating.
Female redback and orb spiders eat their mates after mating.
The female mantis is more likely to eat her mate under laboratory conditions - additional stress makes her more aggressive.
In the wild, a female may eat a mate before they mate - natural selection determining that if he is weak enough to be prey, he was not strong enough to propgate the species.
Scientists believe that the removal of the male's head during mating sets off reactionary stimulation, causing him to ejaculate more fiercely and thus effectively.
Interestingly - out of three-thousand plus species of mantis, only one is renowned for ewating its mate, and then less than thirty-per-cent of matings end in death for the male.
Because of the fasciantion of this ritual, people believe that devouring of the mate is far more common than it actually is in nature.