Robophysical modeling explains why spiders crouch legs when sensing prey on a web

Orb-weaving spiders sense vibrations to detect prey on their webs. They often dynamically crouch their legs during prey sensing. However, the mechanism behind this active sensing behavior is not well understood, due to difficulty in measuring vibrations when the spider and prey are actively moving. Here, we use robophysical modeling to understand this behavior. A simplified spider robot capable of dynamic leg crouching hangs under a physical web and attempts to detect a prey robot on the web. Based on animal observations, we programmed the prey robot to move once but at various amplitudes. When the prey robot moves intensely, it shakes on the web at its own natural frequency, allowing the spider robot to detect it without leg crouching. When the prey robot moves weakly or does not move, the spider robot cannot identify it without leg crouching. But with leg crouching, the spider robot induces both itself and the prey robot to vibrate on the web, each at its natural frequency. Thus, it can detect the prey robot by identifying a frequency different from its own. The spider robot can also differentiate how far the prey robot is, because the latter’s induced shaking is larger when it is closer and smaller when farther. These robophysical modeling results explained our observations that U. diversus spider’s leg crouching is triggered by prey becoming stationary and stops as prey starts to move. Our study suggests that leg crouching may be an active form of “echolocation” (via web vibration) to seek out and detect elusive prey.