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, <i>Burmogerris</i> 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 <i>Burmogerris</i> 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 <i>Burmogerris</i> in the Cretaceous. It indicates the mating behaviour remained stable over long-term geological time in these water-walking insects.