The distal segments of a water-walking spider are carpeted with micro-setae that form an air-trapping layer. Each seta is tapered and angled to increase apparent contact angle through the Cassie-Baxter effect. When a droplet meets this layer, it rests on the tips rather than wetting the cuticle, preserving the high contact angle measured on the leg.
Wax crystals coat the setae and add chemistry-driven repellency. They also smooth microscopic defects that would otherwise pin the contact line. High-resolution imaging shows that the wax is not continuous; small gaps allow trapped air to circulate when the leg flexes, which prevents pressure buildup that could trigger wetting.
Functionally, the setae reduce drag by limiting the wetted area during strokes. They also provide a reset mechanism: when the leg lifts, stored air rushes back to refill the plastron. Replicating this in biomimetic materials requires both the micro-scale geometry and a hydrophobic coating that survives abrasion and UV exposure.