Wasp spiders: 10-second rule
While some critters enjoy indulging in marathon mating sessions, speed is key for the male wasp spider … in fact, his life depends on it. Male wasp spiders (Argiope bruennichi), capable of mating only twice in a lifetime, must abide by the "10-second rule" to avoid being eaten by their larger, ravenous female partners. According to researchers at Hamburg's Zoological Institute, male wasp spiders that copulate with their siblings like to keep things short and sweet and, in turn, are allowed to live. However, males partnering with non-related lady wasp spiders tend to take their time and wind up the victims of sexual cannibalism. In the twisted world of wasp spider sex, love really does kill, apparently.
Another animal, this one a proper insect, that practices sexual cannibalism is the praying mantis. When in captivity, the she-mantis has been known to sometimes bite her lover's head off while doing the deed. The male can continue to inseminate, even more vigorously, in his decapitated state.