Climbing in hexapods: A plain model for heavy slopes

Abstract

Usually, a climbing cockroach attaches with three legs to a substrate. According to a recent model study, pulling forces underneath the front leg are required at some critical slope angle in upward locomotion. This critical angle depends on the animal's anatomy and leg positioning. In this study, we asked especially how this critical angle can be biased by one parameter that may be controlled during climbing: the body height above the substrate. We found that the typical ratio between body height and length (0.2) adopted by cockroaches is slightly higher than the very ratio (0.15) at which the critical slope angle can be increased most strongly for a given decrease in body height. In other words, it is likely that a geometrical body design of cockroaches evolved, which enables a delicate reduction in body height perfectly suitable for preventing the danger of slipping or even falling over rearwards at steepening slopes (approaching the vertical). In that sense, our model predicts, not just for hexapods but rather for any three-point climber, that taking up a low ratio of body height to the distance between the foremost and the hindmost attachment point (very crouched posture) makes body height a good parameter for climbing control.