The Mating Game

for a moment, as T. R. Birkhead asks us to do, the seminal fluid of the common housefly. It carries sperm, of course. But it is also the vehicle for a number of very potent chemicals. Some of them mimic substances in the female's body that trigger egg laying; others, like chemical chastity belts, are anti-aphrodisiacs and discourage the female from any future sexual encounters. This co-option of sperm as a weapon in the ongoing conflict over who gets to fertilize whose eggs is part of "an evolutionary arms race," in Birkhead 's words, that is "every bit as ferocious as those between predators and prey and between parasites and their hosts." Female houseflies, in fact, get off relatively lightly. Male fruit flies also inject their mates with a substance, similar to spider venom, that is toxic to other sperm and to the female fruit fly. This toxin significantly shortens the female's life span, but it guarantees that the eggs she lays will be fertilized by the first male to breach her defenses.