Elastic hoops jumping on water: A model system for fishing spiders

Small semiaquatic arthropods such as water striders and fishing spiders have amazing maneuverability including jumping on water. The physical principle behind the jump of fishing spiders is discriminated from that of water striders in that the spiders use primarily the water drag, instead of the surface tension, to launch themselves into air. Here we present a simple physical model of the fishing spider’s jump by studying the jump of an elastic hoop off the water surface. An initially compressed hoop that floats on water is set free by removing a constraint, so that the elastic strain energy is converted into the kinetic energy. The hoop pushes the water surface at a similar dynamic condition to fishing spiders. The dynamic interaction of an elastically vibrating hoop and deforming water surface determines the take-off velocity of the hoop, which eventually gives the maximum jump height. Our theory considering both hoop vibration and free-surface flow of water is in good agreement with experiment, revealing that the jump efficiency, defined as the ratio of the hoop’s kinetic energy at take-off to the initial strain energy is approximately only 6%. Our theory can be used to find the optimal design of water jumpers utilizing drag forces.