The plastron, a thin, persistent air film, forms between overlapping setae on a spider’s tarsus. Its stability depends on pressure differentials: the Laplace pressure across each meniscus resists collapse, while dissolved gases slowly diffuse inward to replenish losses. Computations show that a 20-30 micrometer spacing with a 150-degree apparent contact angle can hold kilopascal-scale overpressure, enough to survive brief submersion or splashing from an energetic stride.
Air exchange is not uniform. SEM studies reveal microchannels where wax coverage thins; these gaps provide diffusion paths that let oxygen reach the cuticle without flooding the chamber. When the leg compresses the plastron during stance phase, stored air redistributes through these channels and then re-expands on lift-off, effectively breathing with each step.
Synthetic analogs often fail because coatings degrade or setae replicas lose taper. Maintaining the taper preserves the gradient in capillary pressure that keeps air pinned, and selecting coatings that resist UV and shear prevents wetting transitions. Monitoring plastron stability over cycles provides a simple health indicator for biomimetic devices.