![]() In the present study, we discovered, for the first time, the remarkable phenomenon of the underwater adhesion ability, based on the trapped air bubble, in the terrestrial chrysomelid beetle Gastrophysa viridula ( figure 1). In the case of the midge Clunio, the bubble is stable up to a depth of 3 m. This air bubble is affected by hydrostatic pressure depending on the depth of the animal under water. Backswimmers from the genus Notonecta and midges Clunio are capable of keeping an air bubble attached to the surface for breathing under water. Water-repellent feet of water striders enable locomotion on the water's surface. Some super-hydrophobic plant surfaces have a self-cleaning property. Many plant and animal surfaces have a combination of surface geometry and chemistry to keep the surface water-repellent. In many cases, such biological surfaces are covered by fine microstructures and waxy hydrophobic materials. ![]() Īnimals capable of attachment to the water–solid interface may be capable of controlling the surface wettability. Recently, geometry-related effects have been shown to contribute to underwater adhesion, such as a mushroom-shaped fibrillar microstructure that significantly enhanced a suction effect under water. Nevertheless, nature provides examples of underwater attachment based on complex polymer glues. Underwater adhesion is difficult owing to the problem of displacing water from the adhesive interface and the ability of water to decrease the strength of chemical bonds. Tropical insects that inhabit rain forests may regularly encounter this problem. In nature, plants may be covered by water for quite a long period of time, especially after heavy rain. ![]() However, the ability of terrestrial insects to walk on smooth surfaces under water has not been previously demonstrated. Earlier studies have demonstrated that beetle pads adhere well to dry substrates. Beetle adhesive setae are supplemented with a liquid secretion that is responsible for generating capillary forces on various surfaces. These structures were previously studied by scanning and transmission electron microscopy, and their adhesive forces were measured using force transducers. This ability is attributed to the presence of specialized adhesive setae on their tarsi. Some beetle species can walk freely on flat vertical surfaces such as smooth plant leaves. Inspired by this idea, we designed an artificial silicone polymer structure with underwater adhesive properties. Additional capillary forces are generated by the pad's liquid bridges between the foot and the substrate. Bubbles in contact with the hydrophobic substrate de-wetted the substrate and produced capillary adhesion. Oil-covered hairy pads had a pinning effect, retaining the air bubbles on their feet. Beetle adhesion to hydrophilic surfaces under water was lower than that in air, whereas adhesion to hydrophobic surfaces under water was comparable to that in air. However, our observations showed that these beetles may use air bubbles trapped between their adhesive setae to walk on flooded, inclined substrata or even under water. In general, capillary forces do not contribute to adhesion under water. In air, adhesion is produced by capillary forces between the fluid-covered setae and the substrate. These beetles have adhesive setae on their feet that produce a secretory fluid having a crucial role in adhesion on land. For the first time, we report the remarkable ability of the terrestrial leaf beetle Gastrophysa viridula to walk on solid substrates under water.
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