Movement behavior of a spider on a horizontal surface

Studying the locomotive behavior of animals has the potential to inspire the design of the mechanism and gait patterns of robots ("bio-inspired robots"). The kinematics characteristics of a spider (Ornithoctonus huwena), including movement of the legs, movement of the center of mass (COM) and joint-rotation angle, were obtained from the observation of locomotion behaviors recorded by a three-dimensional locomotion observation system. Our results showed that one set of the stance phase consists of four legs, which were leg-1 and leg-3 on one side and leg-2 and leg-4 on the other side. Additionally, two sets of the stance phase comprised eight legs alternately supporting and driving the motion of the spider's body. The spider primarily increased its movement velocity by increasing stride frequency. In comparison to other insects, the spider, O. huwena, has superior movement stability. The velocity and height of COM periodically fluctuated during movement, reaching a maximum during alternation of leg phase, and falling to a minimum in the steady stance phase. The small change in deflection angle of the hind-leg was effective in driving locomotion, whereas each joint-rotation angle of the fore-leg changed irregularly during locomotion. This research will help in the design of bio-inspired robots, including the selection of gait planning and its control.