Fluid-based adhesion in insects - principles and challenges

Insects use hairy or smooth adhesive pads to stick to almost all known surfaces. Although studied for more than 300 years, the principles of insect adhesion are still not fully understood and explaining them still challenges biologists, physicists and engineers. Today we know that the contact between the insect adhesive organs and the substrate is mediated via nanometrethin films of adhesive fluid. This fluid helps to increase the pad's contact area on rough surfaces and its special rheology combines capillary adhesion with resistance against sliding. We summarize recent advances in the understanding of fluid-based insect adhesion and highlight the large number of unresolved questions, regarding the mechanism of fluid secretion, the thickness of the mediating fluid layer, and the influence of surface properties on insect adhesive forces.