Fig. 4. Schematic diagram illustrating two possible strategies of compensating small-scale surface roughness (roughness smaller than the dimensions of the pad). Good adhesive contact can either be achieved `dry' with very fine seta tips or `wet' by secretion of a fluid that can fill out substrate cavities. A fluid may not only be necessary in smooth adhesive pads (e.g. many insects and tree frogs) as shown here but also in hairy pads, when setae are relatively large and blunt as in many insects.