Distinct kinematics and micromorphology for symmetrical rowing and sliding on water in ripple bugs and water striders

The evolution of semiaquatic bugs led to at least two independent origins of symmetrical rowing: a drag-based strategy in Veliidae from fast-flowing waters and a surface-tension-based strategy in Gerridae from still or slow-flowing waters. However, the leg micromorphology and motion patterns underlying these strategies remain underexplored. Using scanning electron microscopy and high-speed video, we compared Rhagovelia distincta (Veliidae), which uses midleg pretarsal fans as oar-like blades, with Gerris latiabdominis (Gerridae), which relies on long, hairy midlegs to generate surface-tension-based thrust. R. distincta performed short, high-frequency strokes and actively controlled its fans, which function as “leaky paddles” exploiting drag, and potentially lift, forces. Fan anatomy further suggests nano-structural adaptations for enhanced mechanical performance. R. distincta also engaged its midleg tarsi in surface-tension-based thrust. In contrast, G. latiabdominis exhibited longer stroke durations and kinematics suited to surface-tension-based propulsion. Both species shared key micromorphological features: ventral longitudinal rows and gaps of midleg setae, with posterior rows particularly robust and nano-grooved in G. latiabdominis. Additionally, both formed ventral “beam-like” structures from overlapping flat-tipped setae on hindlegs—and less prominently on forelegs—used for support and sliding. These findings generate new hypotheses for refining models of locomotion on water surface by insects.