Evolution of distal swimming structures in Gerromorpha: functional use of claws and fan

Distal leg structures of some species of Gerromorpha are highly modified, with unique fan and blade-like structures. Within Gerromorpha, we explore the evolutionary transformations of tarsomeres based on detailed morphological documentation, to clarify the complex morphology and function of the legs. In this infra-order, plesiomorphic conditions of the tarsi are maintained in Mesoveliidae, Hebridae, Hydrometridae, "Macroveliidae" and Hermatobatidae, with apical claws and without specialised tarsal or pretarsal structures. In contrast, highly specialised deeply and symmetrically cleft apical mesotarsomeres, with blade like claws and a feather-shaped empodium, are apomorphic features of Chenevelia stridulans and Rhagovelia antilleana (Veliidae). Using X-ray microtomography, we document to anatomy and infer the function of middle leg tarsomere swimming apparatus for Rhagovelia antilleana, Callivelia conata (Veliidae), Chenevelia stridulans, and Gerris buenoi (Gerridae). We also propose a hypothetical two-component explanation for the release of the fan in Rhagovelia and Chenevelia based on video and scan observations. For Rhagovelia and Chenevelia, the contraction of the femuro-pretarsalis muscle, which is attached by a very long and thin tendon to the conspicuously curved unguitractor plate, releases actively the leaf-like claws from the cleft, as well as the fan-shaped structure. The latter is folded in its resting position but unfolds passively outside of the cleft. The combination of deployable fan and modified claws is a locomotory system driven by a hybrid active-passive mechanism enabling species of these genera to improve their propulsion and colonize fast running water habitats. These structural changes were a unique evolutionary innovation in Gerromorpha, enabling them to cope with specialized habitats and to access new ecological niches.