  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 liquid film. However, the physico-chemical 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, \textit{Carausius
morosus}, spanning more than two orders of magnitude in body mass. We find that
the adhesive force we measure is similar to previous measurements that use a
centrifuge. Our measurements afford use the opportunity to directly probe the
adhesive stress \textit{in vivo}, and use existing theory on capillary adhesion
to predict the surface tension of the secreted liquid and compare it to
previous assumptions. From our predictions, we find that the surface tension
required to generate the adhesive stresses we observed ranges between 0.68 mN/m
and 12 mN/m. The low surface tension of the liquid would enhance the wetting of
the stick insect's footpads and promote their ability to conform to various
substrates. Our insights may inform the biomimetic design of capillary-based,
reversible adhesives and motivate future studies on the capillary properties of
the secreted liquid.
