Markerless tracking suggests a tactile sensing role for forelegs of Dolomedes spiders during locomotion

Summary statement We developed a machine vision technique for markerless tracking of locomotion in the spider Dolomedes aquaticus. Gait analysis suggests that each pair of legs plays a specific role in locomotion. Abstract Because of their rigid exoskeleton with relatively simple joint mechanics, arthropods can provide useful models for studying the sensory-neural and mechanical design principles of agile animal locomotion. Gait analysis usually requires attaching markers or manually identifying reference points in video frames, which can be time consuming and inaccurate, especially with small animals. Here we describe a markerless motion capture technique and its application to gait analysis in the New Zealand semi-aquatic hunting spider, Dolomedes aquaticus. Our machine vision approach uses a model of the spider’s skeleton to infer the location of the centre of mass and the configuration of the skeleton in successive video frames. We found that stride length and frequency are correlated with running speed. Inter-limb coordination during the gait cycle suggests that different legs have specialized roles in locomotion. Phase relationships among the six hindmost legs exhibit an alternating tripod gait, as in hexapod insects. The middle two leg pairs appear to be primarily responsible for generating thrust, while the hind legs contribute more to stability. The front legs are not phase-coupled to the other legs and appear to be used as tactile probes during locomotion. Our machine vision approach has the potential to automate arthropod gait analysis, making it faster and easier. Our results indicate how specialization of limb function may contribute to locomotor efficiency and agility of a specialized hunting spider, and how arthropod design principles may contribute to developing efficient, agile legged robots.