Female redback spiders are not the most sympathetic of lovers: they routinely begin to eat their suitors before they’ve had a chance to finish mating. And now research shows that their internal anatomy also helps them get one over males, by influencing which mates get to fertilize their eggs. The female redback (Latrodectus hasselti) has two organs for storing sperm, called spermathecae, explain Lindsay Snow and Maydianne Andrade, of the University of Toronto in Canada. Although biologists already knew about these twin sperm sacs, they had not investigated how they affect the issue of paternity when a spider mates with more than one male. The two sperm sacs help to prevent a male from stealing a mating advantage simply by being the first to court a female, Snow and Andrade suggest. A male has two sperm-depositing organs, called palps, that correspond to the female’s two sperm sacs, although he can use only one palp in a mating session. A male tends to break off the end of his mating palp inside the female’s sperm sac, partly blocking its entrance, the researchers say. “This functions as a plug, a kind of chastity belt,” explains Paul Hillyard, curator of arachnids at the Natural History Museum in London. Having two sperm sacs may therefore give females extra choice over who fathers her young, Snow and Andrade say. If one of the sacs is blocked by a mate, a subsequent partner can still be given the opportunity to deposit his sperm in the other.