A Desert Beetle Taught Engineers How to Drink From Fog

Dawn on the dunes of the Namib Desert, along the Atlantic coast of southern Africa. A cool, grey fog is rolling in over the sand, and on the crest of a dune a small, shiny black beetle climbs to the very top, turns to face the breeze, and slowly tips forward until its back end points at the sky. It looks like a beetle doing a headstand. It is actually having breakfast.

The Namib gets very little rain. What it does get is fog: tiny droplets of water drifting in on the wind, on around 30 days a year even in the inland desert. Several of the desert's darkling beetles have found ways to use it. Some dig little trenches or ridges in the sand to catch the fog, but two species, Onymacris unguicularis and Onymacris bicolor, use their own bodies as fog collectors.

The trick is called fog-basking. With its head down and its back tilted into the wind, the beetle lets the fog drift against its hard, domed wing covers. Tiny droplets settle there and merge into bigger ones, and when a drop grows heavy enough it slides down the slope of the beetle's back, straight towards its mouth. The beetle drinks water it has just caught out of the air.

Look closely and the backs of some Namib beetles are not smooth but covered in tiny bumps, with a waxy coating around them. In 2001 a short paper in the journal Nature described a Namib beetle whose back mixes patches that attract water with waxy patches that repel it, so that droplets gather in some places and roll off others. Engineers loved the idea at once: a surface that sorts water, gathering it into drops and then sending those drops where they are needed.

Since then, researchers have built many beetle-style surfaces. In one study from 2019, a team made a flexible sheet with tiny water-loving bumps on a water-repelling silicone base, then curved it like a beetle's back. When fog blew along it, the curved bumpy sheet collected more than 16 times as much water as a flat version. AskNature, a library of nature-inspired design, lists several more beetle-inspired ideas, from fog-catching mesh towers to materials for recovering water from cooling towers.

Here is a twist that makes the story better, not worse. When the zoologists Thomas Nørgaard and Marie Dacke measured how well different Namib beetles collect fog, in a study published in 2010, they found only small differences in how much water each species gathered for the area of its back, even though the surfaces looked quite different. Their conclusion was that for Onymacris unguicularis the headstand itself matters more than the fine structure of its shell. They also noted that other scientists had questioned whether the beetle from the 2001 paper really gathers fog this way in the wild.

So how do we know the fog-basking beetles really drink the fog? Researchers have watched them climb dune crests on foggy mornings, take up the head-down stance facing the wind, and drink the water running down to their mouths. Beetle numbers add another clue: according to AskNature, in dry spells the fog-using beetles hold on, while some beetles that cannot use fog fall to less than 1 per cent of their usual abundance.

The exact job of the bumps is still being argued over, which is how science is supposed to work. But the beetle's lesson for engineers is secure: stand in the right place, face the wind, and let gravity carry every drop home.