Functionally Different Pads on the Same Foot Allow Control of Attachment: Stick Insects Have Load-Sensitive “Heel” Pads for Friction and Shear-Sensitive “Toe” Pads for Adhesion

Abstract

Stick insects (Carausius morosus) have two distinct types of attachment pad per leg, tarsal “heel” pads (euplantulae) and a pre-tarsal “toe” pad (arolium). Here we show that these two pad types are specialised for fundamentally different functions. When standing upright, stick insects rested on their proximal euplantulae, while arolia were the only pads in surface contact when hanging upside down. Single-pad force measurements showed that the adhesion of euplantulae was extremely small, but friction forces strongly increased with normal load and coefficients of friction were 1. The pre-tarsal arolium, in contrast, generated adhesion that strongly increased with pulling forces, allowing adhesion to be activated and deactivated by shear forces, which can be produced actively, or passively as a result of the insects' sprawled posture. The shear-sensitivity of the arolium was present even when corrected for contact area, and was independent of normal preloads covering nearly an order of magnitude. Attachment of both heel and toe pads is thus activated partly by the forces that arise passively in the situations in which they are used by the insects, ensuring safe attachment. Our results suggest that stick insect euplantulae are specialised “friction pads” that produce traction when pressed against the substrate, while arolia are “true” adhesive pads that stick to the substrate when activated by pulling forces.

Two functional types of attachment pads on a single foot in the Namibia bush cricketAcanthoproctus diadematus(Orthoptera: Tettigoniidae)

“…In comparison, the nubs on the euplantulae of A. diadematus are 5 mm in length with an aspect ratio of 2-3. They are strikingly similar in shape and density to those found on the euplantulae of the euphasmatodean species Aretaon asperrimus [11], C. morosus [6,8] and Neohirasea maerens [11]. We will subsequently call…”

“…Indeed, the smooth distal euplantulae (E3) of the feet facing upward were in contact with the glass plate more often than the smooth euplantulae of the feet facing downward ( figure 3c,d). Interestingly, in both studies comparing smooth and nubby attachment structures [6,8], smooth attachment structures reached at least similar but often even better adhesive and friction forces compared to nubby structures. From these results, it might be surprising that nubby euplantulae occur at all.…”

“…Nubby euplantulae respond in their friction forces to changing load to a higher degree compared with smooth euplantulae [8].…”

“…In polyneopteran species, arolia and euplantulae have been described to be smooth, covered with protuberances or completely absent (figure 2) [2,[6][7][8][9][10][11]13,14,[20][21][22][23][24][25][26][27][28][29][30][32][33][34]36,37]. Different outgrowths of tarsal adhesive pads have been previously described: nubs, acanthae, microtrichia and setae.…”

“…Smooth arolia of the phasmid species Carausius morosus generated high adhesive forces when shear movements were applied, whereas high loads increased the friction forces of its nubby euplantulae [8]. Smooth euplantulae of Cuniculina impigra (Phasmida) generated high adhesive and friction forces on smooth surfaces, while the nubby euplantulae of C. morosus seem to be adapted to different surface roughness [6].…”