Extreme Maneuvering and Hydrodynamics of Rhagovelia Water Striders

Millimeter-sized Rhagovelia are remarkable among water striders because they can stand tempestuous rapids and coastal waters. These outstanding abilities mainly come from special fan-like structures located on their middle legs, which allows them to effect sudden turns. However, it is unknown how capillary phenomena influence fan performance, as well as the hydrodynamics of rapid maneuvering. We found that the fan is hydrophilic, which permits a passive spreading and folding, when placed in or out the water, respectively. Biomechanical and Particle Image Velocimetry (PIV) indicate that Rhagovelia’s turning is driven by a reverse stroke and asymmetric impulse produced by only one leg’s fan. Accordingly, Rhagovelia can turn up to 180 deg in less than ~30 ms, with average speeds and turning rates up to 22 m/s and 4000 deg/s, respectively. Side-view PIV revealed that these insects may be using unsteady hydrodynamics for propulsion. Besides, complex fluid dynamics of Rhagovelia’s natural streams were obtained using in-situ PIV. Thus, Rhagovelia’s extreme turning seems to be driven by a passive fan spreading, an asymmetrical reverse stroke and unsteady hydrodynamics, which can be used in the design of a robot with the capacity to move on unsteady water surfaces.