Scaling of stick insect adhesion

Summary

The climbing abilities of animals, like insects and tree frogs, have stimulated the curiosity of scientists for centuries. How are these animals capable of this spectacular feat with seemingly effortless execution? Scientific progress within the last few decades has revealed the functional morphology of an insect’ s sticky footpads – a soft, sponge-like pad that secretes a thin fluid film. However, the mechanisms underlying their adhesion remain elusive. Here, we explore these underlying mechanisms by simultaneously measuring adhesive force and contact geometry of the adhesive footpads of live, tethered Indian stick insects, Carausius morosus, spanning across two orders of magnitude in body mass. We find that adhesive stress (or tenacity) decreases with body mass, indicating that capillary pressure, due to the fluid they secrete from their footpads, is not solely responsible for adhesive force. However, pad tension (force per unit length of contact perimeter) is independent of body mass, indicating that pad tension dictates adhesive performance and that increasing the contact perimeter of adhesive pads can increase the adhesive forces they generate. Therefore, our insights may inform the biomimetic design of scale-independent reversible adhesives.