Drosophila uses a tripod gait across all walking speeds, and the geometry of the tripod is important for speed control.

Chanwoo Chun,Vikas Bhandawat,Tirthabir Biswas

doi:10.7554/elife.65878

Abstract

Changes in walking speed are characterized by changes in both the animal's gait and the mechanics of its interaction with the ground. Here we study these changes in walking Drosophila. We measured the fly's center of mass movement with high spatial resolution and the position of its footprints. Flies predominantly employ a modified tripod gait that only changes marginally with speed. The mechanics of a tripod gait can be approximated with a simple model - angular and radial spring-loaded inverted pendulum (ARSLIP) - which is characterized by two springs of an effective leg that become stiffer as the speed increases. Surprisingly, the change in the stiffness of the spring is mediated by the change in tripod shape rather than a change in stiffness of individual legs. The effect of tripod shape on mechanics can also explain the large variation in kinematics among insects, and ARSLIP can model these variations.

Highlights

Behavior, including locomotion, results from interactions between the nervous system, the body, and the environment (Chiel and Beer, 1997; Full and Koditschek, 1999)
We evaluated the performance of the spring-loaded inverted pendulum (SLIP) and ARSLIP models by fitting them to the fly’s center of mass (CoM) kinematics
We converted the ka and ks values to their nondimensionalized versions ga and gs: We found that, despite all the simplifying assumptions, the predicted ga and gs derived from the tripod geometry were close to the optimal ga and gs obtained from the best fit to the CoM kinematics (Figure 7C)

Summary

Introduction

Behavior, including locomotion, results from interactions between the nervous system, the body, and the environment (Chiel and Beer, 1997; Full and Koditschek, 1999). In mammals, gait transitions measured in terms of speeds relative to their size defined as Froude number (Fr) occur at specific Fr (Alexander, 1989) They walk below Fr of 0.3 while choosing other gaits at higher Fr. In contrast, insects employ a tripod gait at a wide range of Fr from 0.001 in flies (Biswas et al, 2018), Fr of 0.25 in ants (Reinhardt et al, 2009), and Fr > 1 in cockroaches. Insects do change their gaits (Wendler, 1966); when insects change gait, the gait selection in insects appears to be probabilistic, that is, different

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Conclusion