Meeting Abstract

27.4  Friday, Jan. 4  Division of labour between adhesion and friction pads in stick insects (Carausius morosus) LABONTE, D*; FEDERLE, W; University of Cambridge; University of Cambridge dl416@cam.ac.uk

Stick insect legs bear two types of attachment pads, tarsal “heel” pads (euplantulae) and a pre-tarsal “toe” pad (arolium). In order to investigate whether these pads are specialised for different functions, we measured friction and adhesion of single pads under varying normal and shear force loads, using a custom-built 2D force transducer. Euplantulae were found to generate negligible adhesion (peak values below 15% body weight), but large friction forces exceeding the insect’s body weight. In contrast, peak adhesion of arolia amounted to up to 80% body weight. Adhesive forces significantly increased with the applied shear force, and were independent of the normal pre-load over nearly one order of magnitude. These results suggest that stick insects use their tarsal euplantulae for generating friction forces when no adhesive force is needed (e.g. when walking upright or for legs below the centre of gravity during vertical climbing), and thereby minimize costs associated with detachment of the pads. The distal arolia, in turn, are likely used as “true” adhesive pads that maintain surface contact during vertical climbing or inverted walking. The shear stress (friction per apparent contact area) of euplantulae (but not of arolia) was dependent on normal load. This dependency may be explained by the specific surface topography of the euplantulae, which are covered by tapered microtrichia. High-magnification light microscopy and reflected-light contrast recordings of the euplantula contact area during force measurements confirmed that their real contact area increased both with normal and shear force via a larger number of microtrichia contacting the surface and/or side contact of individual microtrichia.