The Southern California endemic mite Paratarsotomus macropalpis was filmed in the field on a concrete substrate and in the lab to analyze stride frequency, gait and running speed under different temperature conditions and during turning. At ground temperatures ranging from 45 to 60°C, mites ran at a mean relative speed of 192.4±2.1 body lengths (BL) s , exceeding the highest previously documented value for a land animal by 12.5%. Stride frequencies were also exceptionally high (up to 135 Hz), and increased with substrate temperature. Juveniles exhibited higher relative speeds than adults and possess proportionally longer legs, which allow for greater relative stride lengths. Although mites accelerated and decelerated rapidly during straight running (7.2±1.2 and −10.1±2.1 m s −2 , respectively), the forces involved were comparable to those found in other animals. Paratarsotomus macropalpis employs an alternating tetrapod gait during steady running. Shallow turns were accomplished by a simple asymmetry in stride length. During tight turns, mites pivoted around the tarsus of the inside third leg (L3), which thus behaved like a grappling hook. Pivot turns were characterized by a 42% decrease in turning radius and a 40% increase in angular velocity compared with non-pivot turns. The joint angle amplitudes of the inner L2 and L3 were negligible during a pivot turn. While exceptional, running speeds in P. macropalpis approximate values predicted from inter-specific scaling relationships.
The Southern California erythracarid mite species, Paratarsotomus macropalpis, was filmed using a high frame‐rate video camera in the field to analyze speed, stride frequency, acceleration and deceleration. We also recorded higher resolution lab footage of mites starting, stopping and turning to analyze gait and kinematic mechanisms. Mites running in the field on concrete substrates at high temperatures (40°C to 60°C) were shown to travel at mean relative speed of 192.4 bl s−1 (body lengths per second), exceeding the highest currently documented speed for land animals (171 bl s−1). Despite this exceptional value, it conforms broadly with predictions based on interspecific scaling. Stride frequencies were also exceptionally high (as fast as 135 Hz), and increased significantly with substrate temperature. Mites accelerate and decelerate rapidly (mean values of 7.2 ms−2 and −10.1 ms−2, respectively), although the forces involved are simiilar to those found in other running animals. Calculations show that air resistance is a minor contributor to deceleration; the forces are likely borne by a combination of muscle and cuticle ligament strain. During normal running, adjacent and opposite tarsi are 180 degrees out of phase. This gait cycle is preserved during turning, although the two front pairs of tarsi initiate acceleration cycles and duty factors increase for inner tarsi during turns. Grant Funding Source: Supported by the Howard Hughes Medical Institute
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