How orb-weaving spiders use dynamic leg crouching to sense prey on a web: insights from robophysical modeling

Orb-weaving spiders rely on sensing leg vibrations to detect prey on the web. Biological experiments and computational modeling elucidated the physics of how these spiders use long-timescale web-building behaviors to modulate vibration sensing of prey by controlling web geometry, materials, and tension distribution. By contrast, the physics of how spiders use short-timescale leg behaviors to modulate vibration sensing on a web is less known. Here, we use robophysical modeling to study how dynamic leg crouching common in web spiders contributes to vibration sensing of prey. Following observations in the orb-weaver Uloborus diversus, we created a robophysical model, consisting of a spider robot that can dynamically crouch and sense its leg vibrations and a prey robot that can shake on a horizontal physical web. Without the prey robot, after each dynamic crouch, the spider robot sensed only one dominant frequency—the natural frequency of the spider robot passively vibrating on the web. With the prey robot, after each dynamic crouch, the spider robot sensed two dominant frequencies—the additional one being the natural frequency of the prey robot passively vibrating on its spiral thread. This additional frequency increased as the prey robot became closer from the web center. These features allowed the spider robot to detect prey presence and distance. We found evidence of U. diversus spiders using this strategy. We developed a physics model of a decoupled spider–web–prey system to explain the observations.