The behaviour is called fog-basking. On fog nights the beetle emerges before dawn, climbs to the ridge where the moving air is strongest, and adopts a steep head-down stance. Droplets condense on the elytra, coalesce, and run forwards along the body under gravity to the mouthparts. Beetles have been recorded taking up a substantial fraction of their body mass in a single session.
The surface itself matters. Study of the beetle's back has focused on how texture and wettability let small droplets grow and roll rather than evaporate, and the case has been widely cited in the design of fog-harvesting meshes and condensing surfaces.
One caution is worth flagging. The famous bumpy hydrophilic-and-hydrophobic pattern was described from a different Namib darkling beetle, Stenocara gracilipes, and the extent to which the same surface chemistry explains fog-basking in Onymacris is disputed in the literature. The behaviour is well documented; the surface mechanism is less settled than popular accounts suggest.