Why orb-weaving spiders use leg crouching behavior in vibration sensing of prey on a web: A physical mechanism from robophysical modeling.

One of the key functions of organisms is to sense their physical environment so that they can react upon the sensed information appropriately. All spiders can perceive their environment through vibration sensors in their legs, and most spiders rely on substrate-born vibration sensing to detect prey. Orb-weaving spiders primarily sense leg vibrations to detect and locate prey caught on their wheel-shaped webs. Biological experiments and computational modeling elucidated the physics of how these spiders use long-timescale web-building behaviors, which occur <i>before</i> prey capture, 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 <i>during</i> prey capture is less known. This is in part due to challenges in biological experiments (e.g., having little control over spider behavior, difficulty measuring the whole spider-web-prey system vibrations) and theoretical/computation modeling (e.g., close-form equations intractable for a complex web, high computation cost for simulating vibrations with behaving animals). Here, we use robophysical modeling as a complementary approach to address these challenges and study how dynamic leg crouching behavior common in orb-weaving spiders contributes to vibration sensing of prey on a web. Following observations in the orb-weaver <i>Uloborus diversus</i> from a parallel biological study, we created a robophysical model consisting of a spider robot that can dynamically crouch its legs and sense its leg vibrations and a prey robot that can shake both on a horizontal physical wheel-shaped web. Without the prey robot, after each dynamic crouch, the spider robot sensed leg vibrations with only one dominant frequency-the natural frequency of itself passively vibrating on the web. With the prey robot, after each dynamic crouch, the spider robot sensed leg vibrations with two dominant frequencies-the additional higher frequency being the natural frequency of itself passively vibrating on its spiral thread induced by the spider robot's dynamic crouch. This additional frequency increased as the prey robot became closer from the web center where the spider robot was. These features allowed the spider robot to detect prey presence and distance. We developed a minimalistic physics model that decoupled the spider-web-prey system into two subsystems to explain these observations. Guided by both these results, we found evidence of the same physical mechanism appearing in the web of the <i>U. diversus</i> spider during prey capture in the data from the parallel biological study. Our work demonstrated that robophysical modeling is a useful approach for discovering physical mechanisms of how spiders use short-time scale leg behaviors to enhance vibration sensing of objects on a web and providing new biological hypotheses.