Biomechanics of omnidirectional strikes in flat spiders

Abstract

Many ambush predators attack prey using rapid strikes, but these strikes are typically only anteriorly directed. However, a predator may attack laterally- and posteriorly-oriented prey if it can couple the strikes with rapid body reorientation. Here, we examined omnidirectional strikes in flattie spiders (Selenopidae), a group of sit-and-wait ambush predators found on open surfaces. These spiders attack prey throughout their entire peripheral range using rapid strikes that consist of rapid translation and rotation toward the prey. These spiders ambush with radially oriented, long, laterigrade legs in a ready-to-fire status. Once prey is detected, the spider maneuvers toward prey using a single flexion of the legs closest to the prey, which is assisted by 0–3 extension strides by contralateral legs. The within-stance joint actions by a few legs generate a large resultant force pointing to the prey and a large turning moment. Furthermore, the turning speed is enhanced by rapid midair leg adductions, which effectively reduce the spiders’ moment of inertia during angular acceleration. Our results demonstrate a novel hunting behavior with high maneuverability that is generated with effectively controlled reconfigurations of long, laterigrade legs. These results provide insights for understanding the diversity of animal legs and developing highly maneuverable multi-legged robots.