From Nature:
Male spiders that saunter onto a female's web after a rival has spent hours wooing her can quickly copulate without being prematurely eaten by the female. This tactic could lead to small spider suitors seeking out competition with larger rival spiders rather than avoiding it, Canadian researchers say.
The Australian redback spider (Latrodectus hasselti), a member of the black widow family, has a particularly deadly mating ritual. It is one of only a handful of spider species in which the males willingly and actively assist the females with sexual cannibalism — in which the female consumes the male after copulation. In the process of mating, the tiny redback male, whose 4-millimetre body is dwarfed by that of the centimetre-long female, inserts one of his two penis-like organs into one of the female's two sperm-storage sacs. The male then somersaults to place his abdomen over the female's mouthparts, and the female starts eating him as they mate.
More here.