From evolutionary biologist Olivia Judson's great NY Times blog The Wild Side. The story is on about how males of various species tailor their sperm count according to real or perceived competition. I can't resist this tidbit on earthworm virginity:
"An earthworm mating with a worm that has had sex before gives around three times more sperm than it does when mating with a virgin. (Sex in hermaphrodites can take one of two forms, depending on the species. Either one partner plays the male and the other plays the female; then they — usually — swap. Or both animals transfer sperm to each other simultaneously. These earthworms are of the simultaneous type, so an animal will lose both virginities in the same sexual encounter.) How can worms tell if their partners have mated before? It’s not clear; perhaps virgins smell different from other worms."
Proof once again that the we've got nothing, sexually speaking, on the rest of the animal kingdom. There's lots more in her book: “Dr. Tatiana’s Sex Advice to All Creation: The Definitive Guide to the Evolutionary Biology of Sex”