Two dimensionless groups keep water-walking spiders within a safe operating window. The Weber number (We = (rho * U**2 * L) / gamma) compares inertia to surface tension. When We stays below 1, the interface resists splash and puncture. High-speed video shows leg tips moving around 0.2-0.3 m/s over a characteristic length of 1-2 mm, which yields We comfortably under 1 in freshwater; fast accelerations push the ratio toward unity and coincide with visible dimples.
The Bond number (Bo = (rho * g * L**2) / gamma) measures how gravity competes with capillary forces at the body scale. For a 3-4 mm thorax spanning multiple legs, Bo remains well below 1, meaning surface tension dominates and the spider can rely on distributed support. Larger biomimetic robots risk Bo approaching 1; they must either spread contact over more legs or lower mass to keep the interface from sagging.
Controller design can borrow directly from these bounds. Cap stride impulses once We nears unity, and tune stance width so Bo stays subcritical even in warm water where gamma drops. Embedding these thresholds into gait planners prevents the device from transitioning into a semi-planing mode where drag spikes and control authority slips away.