A morphological innovation, the fan of the water strider <i>Rhagovelia</i> , enables its conquest of turbulent streams

Maneuvering on fast running waters is an energy demanding task, making this niche hard to access. The propelling fan of the waterstrider Rhagovelia is an evolutionary innovation that enhances the maneuverability in these environments. This structure consisting of 20 lamellate branches, each with thinner secondary branches, offers a unique model to understand the tenuous link between morphological innovations and adaptation to new ecological environments. However, the principles of enhanced thrust, i.e. the magnitude of the force transfer between the fan and the fluid, is not understood. We characterized the kinematics of the fan and body of Rhagovelia during the propulsion phase. A simplified mechanical model that replicates the movement of the fan is used in both experiments and fluid dynamic computations. Using Particle Image Velocimetry, we measured the flow velocity field around it and determined the magnitude of forces using a load cell. Finally, we used computational fluid dynamics to resolve the flow around a numerical fan replicate of similar morphology. The computations, once validated by their comparison with experimental results using similitude principles, allowed us to determine the nature of the forces acting on the fan. A large part of the fan's resistive force is due to viscous shear stress. The 75 % porosity lamellate fan functions as a leaky paddle, while producing 80 % of the hydrodynamic drag force of an equivalent membrane oar. The thrust enables the strider to colonize most streams, expanding its niche by a factor of four to five.