Water striders are tiny bugs that skate over water. And they regularly engage in what scientists call "coercive mating," or forced sex. But the females of one water strider species resisted - by evolving very unusual sets of genitals.
Scientists in Korea, studying the mating habits of water striders, noticed that a particular species in that family of insect had a unique mating habit: courtship. Ordinarily, among water striders, the male of the species mounts the female violently, effectively pins her to him by grabbing her by the waist (thorax), overcomes her "resistance" and forcibly penetrates her. But in the water strider species Gerris gracilicornis, the females took evolution to the next level, developing genitals that it was impossible for the males to penetrate without the females' permission. According to study authors Chang Han and Piotr Jablonski:
Due to the morphology of their pre-genital segment, G. gracilicornis females appear to have their genitalia relatively well hidden in comparison to the females of other species. Hence, it is feasible to hypothesize that the vulvar opening, through which the male genitalia enter during intromission, is well shielded behind the pre-genital segment.
In response, male Gerris gracilicornis evolved different mating behaviors.
In an apparent response to the female morphological adaptation, males of this species added to their typically violent coercive mounting of the female new post-mounting, pre-copulatory courtship signals produced by tapping the water's surface with the mid-legs. These courtship signals are intimate in the sense that they are aimed at the female, on whom the male is already mounted. Females respond to the signals by exposing their hidden genitalia for copulatory intromission.
Signals included not just males tapping the water, but tapping the female with their antennae. Far from producing an automatic response, the female water skippers apparently choose to allow copulation to continue on their terms, or refrain from exposing their genitals.
Such evolutionary tactics aren't unknown to scientists. Ducks, who are also known to engage in such coercive mating that female ducks may be injured or drowning, have evolved extremely complicated genitals [NSFW pictures of duck genitalia] in a race to prevent unwanted conception (female ducks) and continue forced mating (male ducks). But the bigger question might be: which species is next?
Related: Duck genitals locked in arms race [Cosmos]
[Image via Macropolous]