Locomotion and flow speed preferences in natural habitats by large water striders, Ptilomera tigrina, with micro-morphological adaptations for rowing

Water strider (Gerridae) morphology and behavior have become the focus of interdisciplinary research in biological diversification and bio-inspired technology. However, the diversity of behaviors and morphology of the large-sized Gerridae have not been intensely studied. Here, we provide locomotory behaviors and legs' micro-morphology of the large South-East Asian water strider, Ptilomera tigrina. Using high-speed videography and experiments in natural habitats, as well as scanning electron microscopy of midlegs, we have determined that (1) P. tigrina individuals prefer relatively high flow speeds of 0.15-0.30 m/s, compared to other water striders previously studied, and they are also observed in very high flow speeds of up to 0.6 m/s; (2) they avoid stagnant water, but when on still and very slow flowing water they perform constant back-and-forth rowing using their midlegs; (3) their antipredatory reaction involves repetitive and very fast "protean" movements propelled by the midlegs; (4) their midleg tarsi and tibiae are equipped with brushes of ribbon-like hairs, which are used as paddles for rowing. As the locomotory behaviors and flow-speed preferences by P. tigrina require constant use of midlegs for rowing, the presence of special paddle structures on midlegs illustrates a hypothetical adaptive match between midlegs' locomotory function and their micro-morphology.