Group mating in Cretaceous water striders.

Fossilized mating insects are irreplaceable material for comprehending the evolution of the mating behaviours and life-history traits in the deep-time record of insects as well as the potential sexual conflict. However, cases of mating pairs are particularly rare in fossil insects, especially aquatic or semi-aquatic species. Here, we report the first fossil record of a group of water striders in copulation (including three pairs and a single adult male) based on fossils from the mid-Cretaceous of northern Myanmar. The new taxon, gen. nov., likely represents one of the oldest cases of insects related to the marine environment, such as billabongs formed by the tides. It exhibits conspicuous dimorphism associated with sexual conflict: the male is equipped with a specialized protibial comb as a grasping apparatus, likely representing an adaptation to overcome female resistance during struggles. The paired show smaller males riding on the backs of the females, seemingly recording a scene of copulatory struggles between the sexes. Our discovery reveals a mating system dominated by males and sheds light on the potential sexual conflicts of in the Cretaceous. It indicates the mating behaviour remained stable over long-term geological time in these water-walking insects.