At night, during foggy weather, these beetles climb sand dunes and stand on their forelegs in order to capture water droplets as they run down their vertical bodies and into their mouths[1] The behaviour is called "fog-basking" and is unique to Onymacris unguicularis and Onymacris bicolor.

[1] The mechanism for this ability is still being discovered, but current research points towards the beetle’s elytra containing some hydrophobic component that can catch water droplets being blown in the wind.

The idea of harvesting atmospheric moisture from early morning dew using special materials has been widely utilized in many parts of the world.

[2][3] The species is confined to the major dune fields within the northern (Cunene, Skeleton Coast) and southern (Namib) sand seas of the desert.

[4] The species’ migratory habits correlate very highly with the observed pattern for common fog occurrence in the region.

[7] A majority of the moisture is brought in by fogs blowing from the ocean, bringing water droplets that condensate across the environment.

[8] In this handstand position, fog water condenses on the surface of the beetle’s elytra and runs down their body into their mouth.

A limitation that has plagued the ability to confirm any hydrophilic regions has been that most research has investigated the elytral properties of dead beetles.

[5][13] Onymacris unguicularis is able to regulate its body heat at temperatures much lower than those experienced during normal daytime activity.

In the events leading to copulation, male Onymacris unguicularis beetles will mount stationary or moving females and remain on top for extended periods of time.

Forelegs and middle legs are primarily used to stay clung to the female while hindlegs are left limp and are mostly allowed to drag along.

Such is the case with Onymacris unguicularis and Stenocara gracilipes, which in non-scientific literature have both been called the “fog-basking beetle.” The confusion seems to originate from a paper[11] detailing a mechanism for S. gracilipes’s ability to capture water using hydrophilic bumps and hydrophobic troughs on the beetle’s elytra during fog-basking.

Additionally, it was noted that the tilting posture being identified as “fog-basking” was most likely a common alarm response found in many species of the Tenebrionidae family.

[10] A later experiment would test the fog-basking behaviour and water-collecting efficiency between four Tenebrionidae beetles: O. unguicularis, Onymacris laeviceps, S. gracilipes and P. cribripes.

The study supported many of the points made in the response such as re-identifying the beetle as P. cribripes according to their own observations and noting no fog-basking behaviour in S.