Locomotion and flow speed preferences in natural habitats by large water striders, Ptilomera tigrina, with micro-morphological adaptations for rowing

Abstract

Water strider (Gerridae) morphology and behavior have become the focus of interdisciplinary research in biological diversification and bio-inspired technology. However, the diversity of behaviors and morphology of the large-sized Gerridae have not been intensely studied. Here, we provide locomotory behaviors and legs’ micro-morphology of the large South-East Asian water strider, Ptilomera tigrina. Using high-speed videography and experiments in natural habitats, as well as scanning electron microscopy of midlegs, we have determined that (1) P. tigrina individuals prefer relatively high flow speeds of 0.15–0.30 m/s, compared to other water striders previously studied, and they are also observed in very high flow speeds of up to 0.6 m/s; (2) they avoid stagnant water, but when on still and very slow flowing water they perform constant back-and-forth rowing using their midlegs; (3) their antipredatory reaction involves repetitive and very fast “protean” movements propelled by the midlegs; (4) their midleg tarsi and tibiae are equipped with brushes of ribbon-like hairs, which are used as paddles for rowing. As the locomotory behaviors and flow-speed preferences by P. tigrina require constant use of midlegs for rowing, the presence of special paddle structures on midlegs illustrates a hypothetical adaptive match between midlegs’ locomotory function and their micro-morphology.

Physics of sliding on water predicts morphological and behavioral allometry in water striders

“…The difference between G. gigas, who lives in slower flowing waters, and P. tigrina, who lives in faster moving water, is consistent with the idea that even though the asymmetric sliding always creates less resistance than the symmetric sliding and does not cause sinking regardless of body mass and leg geometry, P. tigrina does not use the asymmetric sliding (exception shown in Supplementary Movie 4; description of movie in SM7) because of the importance of strong thrust in the very frequent short strides against the fast flowing water in their habitat 47 (see also Supplementary Movie 3 described in SM7). Hence, we propose that the habitat type may affect the evolutionary trajectories shaping the wetted leg geometry in large water striders leading to the asymmetrical locomotion in slow-flowing waters and to the long-foreleg/symmetrical locomotion combination in species from fast currents, where the requirements for frequent and strong thrust may additionally trigger evolution of special micro-structures for rowing 41,47,51 and the associated loss of the midlegs' function of supporting the water strider on water surface 33,47 . If this is correct, then Gerridae illustrate how the physical environment channels the morphological and behavioral evolution 52,53 towards either of the two physically feasible adaptive solutions for locomotion by large-sized water striders.…”

“…S13), that there are two solutions: either a shift to "long-foreleg geometry" by elongation of forelegs (recent studies in the genetics of morphology in Gerridae identified some genes that may be involved in the leg elongation 49,50 while maintaining the standard symmetric locomotion mode like in Ptilomerinae, or a use of asymmetric locomotion mode, like in G. gigas. The difference between G. gigas, who lives in slower flowing waters, and P. tigrina, who lives in faster moving water, is consistent with the idea that even though the asymmetric sliding always creates less resistance than the symmetric sliding and does not cause sinking regardless of body mass and leg geometry, P. tigrina does not use the asymmetric sliding (exception shown in Supplementary Movie 4; description of movie in SM7) because of the importance of strong thrust in the very frequent short strides against the fast flowing water in their habitat 47 (see also Supplementary Movie 3 described in SM7). Hence, we propose that the habitat type may affect the evolutionary trajectories shaping the wetted leg geometry in large water striders leading to the asymmetrical locomotion in slow-flowing waters and to the long-foreleg/symmetrical locomotion combination in species from fast currents, where the requirements for frequent and strong thrust may additionally trigger evolution of special micro-structures for rowing 41,47,51 and the associated loss of the midlegs' function of supporting the water strider on water surface 33,47 .…”

“…4c; Supplementary movie 3). Only when forelegs were handling the food 47 or grooming (Supplementary movie 4), P. tigrina used asymmetric striding mode. Both sexes of the large species with "short-foreleg geometry", G. gigas, used only the asymmetric locomotion mode (Fig.…”

“…Entomological literature suggests that heavy water striders evolved unique foreleg morphology and/or striding behavior in order to support the anterior part of the body on the water. Firstly, disproportionately elongated wetted forelegs in the large-bodied water striders of the genus Ptilomera 43,47 may help to support the anterior body during the thrust and passive sliding phases. However, as this type of morphology is also observed in small species of Gerridae (e.g., in Halobatinae 33,41 ), it may not necessarily be a specific adaptation to heavy body, but rather to the midlegs not being used for support on the water surface.…”

Physics of sliding on water predicts morphological and behavioral allometry across a wide range of body sizes in water striders (Gerridae)

“…The copyright holder for this this version posted October 31, 2022. ; https://doi.org/10.1101/2022.10.28.514156 doi: bioRxiv preprint and strong thrust may additionally trigger evolution of special micro-structures for rowing (Mahadik et al, 2020;Kim et al, 2022;Polhemus and Zettel, 1997) and the associated loss of the midlegs' function of supporting the water strider on water surface (Andersen, 1976;Kim et al, 2022). If this is correct, then Gerridae illustrate how the physical environment channels the morphological and behavioral evolution (Dobzhansky, 1974;Martínez and Moya, 2011) towards either of the two physically feasible adaptive solutions for locomotion by large-sized water striders.…”