Shape-morphing appendages in drag-based underwater rowing by aquatic insects

Many aquatic insects are capable of both terrestrial locomotion (walking) and aquatic locomotion (swimming). These insects use their hind legs as propulsors to navigate underwater, using drag-based rowing at intermediate Reynolds numbers. These hind legs are fringed with many setae (bristles), which extend during the power stroke—increasing the effective surface area—and retract during the recovery stroke. The insects also rotate or “feather” the limb to further differentiate the power and recovery stroke. This shape-morphing quality improves propulsive efficiency, maximizing thrust while reducing drag. However, little is known about the details of the transition between extended and retracted setae, nor the detailed kinematics of the power-recovery cycle. Here we investigate the hind limb shape change of three distinct insect families that use this mechanism to locomote underwater: water boatmen (Corixidae), backswimmers (Notonectidae), and diving beetles (Dytiscidae). We present time-varying measurements of the paddle’s frontal area and stroke kinematics throughout the swimming stroke, identifying similarities and differences across species. We also present preliminary measurements of the flows generated by the paddle as it changes shape. Our results provide a foundation for exploring the forces generated by bristled appendages during underwater locomotion at intermediate Re.