Conflict between the sexes over control of copulation may drive the coevolution of elaborate genitalia and other secondary sexual structures. Support for this hypothesis is limited to male adaptations that function to enhance male control over females in copulation (e.g. structures that function to clasp females). Evidence for morphological adaptation in females is critical to the hypothesis, yet lacking. Here, we present the first experimental evidence demonstrating that female abdominal spines in water striders function to increase female control over copulation. By experimentally extending the phenotypic size range of these spines, we show that this morphological adaptation, specific to females, allows them to thwart harassing males, and as a result, reduce the frequency of costly matings to females. This demonstrates the coevolutionary nature of sexual conflict and that females are indeed active participants in the evolutionary conflict over control of reproduction.