Insects running on elastic surfaces

Andrew J. Spence,Chris Mullens,Justin Seipel,Robert J. Full,Shai Revzen

doi:10.1242/jeb.042515

Andrew J. Spence, Chris Mullens + Show 3 more

Open Access

https://doi.org/10.1242/jeb.042515

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Abstract

In nature, cockroaches run rapidly over complex terrain such as leaf litter. These substrates are rarely rigid, and are frequently very compliant. Whether and how compliant surfaces change the dynamics of rapid insect locomotion has not been investigated to date largely due to experimental limitations. We tested the hypothesis that a running insect can maintain average forward speed over an extremely soft elastic surface (10 N m(-1)) equal to 2/3 of its virtual leg stiffness (15 N m(-1)). Cockroaches Blaberus discoidalis were able to maintain forward speed (mean +/- s.e.m., 37.2+/-0.6 cm s(-1) rigid surface versus 38.0+/-0.7 cm s(-1) elastic surface; repeated-measures ANOVA, P=0.45). Step frequency was unchanged (24.5+/-0.6 steps s(-1) rigid surface versus 24.7+/-0.4 steps s(-1) elastic surface; P=0.54). To uncover the mechanism, we measured the animal's centre of mass (COM) dynamics using a novel accelerometer backpack, attached very near the COM. Vertical acceleration of the COM on the elastic surface had a smaller peak-to-peak amplitude (11.50+/-0.33 m s(-2), rigid versus 7.7+/-0.14 m s(-2), elastic; P=0.04). The observed change in COM acceleration over an elastic surface required no change in effective stiffness when duty factor and ground stiffness were taken into account. Lowering of the COM towards the elastic surface caused the swing legs to land earlier, increasing the period of double support. A feedforward control model was consistent with the experimental results and provided one plausible, simple explanation of the mechanism.

Highlights

Running animals diverse in leg number and posture often negotiate complex, heterogeneous environments (Dickinson et al, 2000)
Track with compliant substrate Cockroaches with accelerometer backpacks ran across the long axis of a 48 cmϫ28 cm rectangular Plexiglas arena where the ‘floor’ of the arena was Plexiglas apart from a central rectangular section of
Cockroaches ran with mean (±s.e.m.) forward speed 37.2±0.6 cm s–1 on the rigid substrate, and mean forward speed 38.0±0.7 cm s–1 on the compliant substrate, values which were not significantly different

Summary

Introduction

Running animals diverse in leg number and posture often negotiate complex, heterogeneous environments (Dickinson et al, 2000). These environments may have complex spatial structure, be composed of materials with non-linear mechanical properties, be dynamic, or more likely show some combination of these attributes. May not, be able to maintain stability as it attempts to move through a complex, unpredictable environment at a given speed. Failure to maintain stability increases the probability of extreme yawing, pitching or rolling, and may result in a reduction in forward speed, all of which are likely to be highly detrimental to the survival of an animal that depends upon rapid running

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