Evolution: sex and cannibalism in redback spiders

Female redback spiders cannibalise mating males. New research has shown they have evolved two separate sperm storage organs, allowing them to make post-copulatory paternity choices. Counter-adaptation by males, to maximise paternity after cannibalism, has led to the evolution of an abdominal constricting mechanism, which enhances short-term survival and mating success after attack by the female and maximises the chances of inseminating both sperm storage sites.