In a previous post, I briefly described the tarsi of the stick insect and the function of the tarsal claw. The tarsi have 5 segments, with the claw extending from the fifth segment. The proximal four segments have pads. The stick insect can often be seen resting on the proximal pads with its claw disengaged. How do the tarsal pads (euplantulae) provide grip?
Labonte and Federle* investigated the structure and physical properties of the stick insect leg. High magnification of the euplantulae shows that the pad is covered with dense hair-like cuticle outgrowths. As force on the pad increases, the hairlike structures deform and grip with mores surface area. When there is little pressure, only the tips contact the surface and the leg can be easily disengaged.
Think of a toothbrush. If a tooth brush is lightly resting on a surface with only the tips of the bristles engaged, the tooth brush will glide over the surface. If enough pressure is applied to the tooth brush to bend the bristles, it becomes more difficult to push as much of the forward motion is lost to movement and deformation of the bristles. When the stick insect pushes with its leg, the bristles on the euplantulae deform and resist movement. When the stick insect lifts its leg and removes pressure, the bristles readily disengage. Thus, they generate friction when the insect pushes off and release the surface to allow the leg to freely move forward.
*Labonte, David, and Walter Federle. “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.” PloS One 8.12 (2013): e81943.