Coercive male genital structures expose concealed female genitalia in the water strider Chimarrhometra orientalis Distant, 1879.

Sexual conflict over mating frequency is widespread in animals, driving the evolution of antagonistic traits in both sexes. In insects, this conflict is evident in the evolution of complex genital structures, where males evolve traits that facilitate forceful copulation and females evolve defences to resist unwanted mating. In water striders, such as Chimarrhometra orientalis, both sexes have evolved unique genital morphologies. Females possess concealed genitalia, likely to prevent superfluous copulation, while males have conspicuous antler- and hook-like genital processes. Through detailed behavioural observations and imaging, we suggest that males use these structures to forcefully expose female genitalia and achieve intromission, without any courtship behaviour. This strategy contrasts with behavioural counter-adaptations seen in other water strider species, where males use courtship signals to extract concealed female genitalia. Our findings provide direct evidence of genitalic counter-adaptations to concealed female genitalia in water striders and suggest that such sexually antagonistic evolution can only be revealed through the integration of behavioural and morphological studies. This work highlights the complexity of mating dynamics and underscores the importance of combining close behavioural observation with morphological analysis to understand the evolutionary arms race between the sexes.