In previous posts, I have mentioned that the most distal (fifth tarsal segment) segment of the walking stick leg differs from the four proximal segments. A tarsal claw arises from the fifth segment and the “pad” of the fifth segment (called the arolium) differs in structure from the pads on the previous four (euplantulae). The euplantulae are friction pads that grip when the leg is pushed down. The arolium is an adhesive pad that generates stronger grip when the leg is pulled away from the surface. This happens naturally when climbing vertically or upside down on horizontal surfaces.
A walking stick hanging upside down will only engage its arolium (see image below). The euplantulae are not engaged. If the leg slips and is pulled back toward the body, the adhesive arolium will grip the surface and prevent the stick insect from slipping or falling.
The different parts of the tarsi have complementary functions. The claw grips the surface and gives the leg traction as it pulls its body forward. The euplantulae grip the surface as the leg pushes back and the body moves forward. This allows the insect to walk rapidly in an upright posture. The arolium is used to cling to surfaces, walk and climb where the claws and the euplantulae provide minimal grip.
*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.