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.
Ecological specialists utilize a restricted range of resources and have evolved adaptations to exploit their specialized resources. For example, avian insectivores that feed nestlings with grasshoppers, beetles, or moths perform insect prey preparation before feeding nestlings so that the nestlings are able to swallow the prey. This behavior is generally not expected for soft prey such as earthworms. However, an overview of photographic evidence available online suggested that earthworms are sundered by parents before bringing the prey to the nestlings in a range of species from several families of vermivores worldwide. Reports on the provisioning of nestlings by the vermivores are relatively scant and no report on earthworm sundering has been published. We studied earthworm sundering performed by parents provisioning their broods at four nests of the Fairy Pitta in Korea. The birds sundered earthworms more often when nestlings were smaller and when the earthworm was longer. This is the first quantitative description of earthworm sundering in avian vermivores. We present and evaluate four hypotheses for the function of sundering: provisioning of small nestlings, decreased detectability, hunting multiple prey, and transport of prey. Among these, provisioning of small nestlings seems the most feasible explanation of sundering by the Fairy Pitta as sundering the earthworm allows parents to efficiently provision the younger/smaller nestlings who would have difficulties swallowing unsundered earthworms. This specialized prey preparation technique of vermivores suggests a tight adaptive match between their parental behaviors and their diet (vermivory).
Current theory for surface tension-dominant jumps on water, created for small- and medium-sized water strider species and used in bioinspired engineering, predicts that jumping individuals are able to match their downward leg movement speed to their size and morphology such that they maximize the takeoff speed and minimize the takeoff delay without breaking the water surface. Here, we use empirical observations and theoretical modeling to show that large species (heavier than ~80 mg) could theoretically perform the surface-dominated jumps according to the existing model, but they do not conform to its predictions, and switch to using surface-breaking jumps in order to achieve jumping performance sufficient for evading attacks from underwater predators. This illustrates how natural selection for avoiding predators may break the theoretical scaling relationship between prey size and its jumping performance within one physical mechanism, leading to an evolutionary shift to another mechanism that provides protection from attacking predators. Hence, the results are consistent with a general idea: Natural selection for the maintenance of adaptive function of a specific behavior performed within environmental physical constraints leads to size-specific shift to behaviors that use a new physical mechanism that secure the adaptive function.
Laws of physics shape morphological and behavioral adaptations to locomotion at different body sizes. Water striders serve as a model taxon to study how simple physical constraints of water-surface habitats affect their behavior and morphology, and hydrodynamics of rowing by midlegs on the surface is well understood. However, the physics of the subsequent passive sliding has been less explored. We created a model of sliding on the water surface to simulate the effect of body mass, striding type, and wetted leg lengths on an insect's ability to float on the surface and on the sliding resistance. The model predicts that to support their weight on the surface during sliding, the heavy species should either develop long forelegs that support the frontal part of its body during symmetrical striding (when two midlegs thrust) or use asymmetrical striding (when one forward-extended midleg supports the body while the other midleg and contra-lateral hindleg thrust). These predictions are confirmed by the behavior and morphology of various Gerridae species. Hence, the results illustrate how simple physical processes specific to a certain habitat type have far-reaching consequences for the evolution of morphological and behavioral diversification associated with body size among biological organisms in these habitats.
Current theory for surface tension-dominant jumps on water, created for small and medium size water strider species and used in bio-inspired engineering, predicts that jumping individuals are able to match their downward leg movement speed to their size and morphology such that they maximize the takeoff speed and minimize the takeoff delay without breaking the water surface. Here, we use empirical observations and theoretical modeling to show that large species (heavier than ~80 mg) do not conform to this prediction but switch to using surface-breaking rather than surface tension-dominant jumps in order to achieve jumping performance that appears sufficient for evading attacks from underwater predators. This suggests that natural selection for avoiding predators may break the theoretical scaling relationship between prey size and its jumping performance within one physical mechanism leading to an evolutionary shift to another mechanism of jumping on water.
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