FOG FOR DRINK
Sea fog creeps into the desert. Water. Winds blow detritus, scraps of dry grass and seeds, into the dune fields. Food. Thus the basic requirements are satisfied for animal life at the bottom of the food chain in the Namib.
It is here that the humble beetle finds its niche. Not just one beetle, dark and round, but a horde of them. Commonly called tok-tokkies for a sound they make to attract a mate, the tenebrionid family consists of some 200 species.
They inhabit slipfaces and interdune valleys on the sheltered side of dunes where windblown detritus accumulates. Slipfaces occupy less than 1% of dune surfaces, but are alive with insects and reptiles, above all species endemic to the Namib. The attraction for all of them is a food supply. Basically, beetles and fishmoths eat detritus; lizards and geckos eat beetles and fishmoths; and snakes eat lizards and geckos.
Although they once had wings, tenebrionid beetles are flightless. In the distant past their wing cases, made to move aside to expose their wings for flight, came to be fused together. A cavity with one small opening was left at the place where they once folded their wings. As their spiracles or respiratory pores now opened into this humid cavity, whereas previously they had been exposed to the dry air outside, water loss due to evaporation was much reduced.
It was a case of evolutionary backtracking as wings are an advanced adaptation in insects. In an environment that was becoming hyper-arid tenebrionid beetles had more to gain from water retention than from the ability to fly away from their natural enemies. They now take their chances on the ground.
The "fog-basking" beetle (Onymacris unguicularis) taps the fog for drink. Although it is ordinarily diurnal, it emerges from the sand on foggy nights and climbs to the dune crest, where water condensation is greatest. Head lowered and posterior raised in a kind of handstand, it faces into the fog-bearing wind, to let moisture condense on its back and trickle down to its mouthparts.
The "fog-trapping" beetle (Lepidochora discoidalis) does it differently. After it has foraged during the first part of the night, it normally digs itself back into the sand. It only returns to the surface when fog comes, late at night or early in the morning, so as to excavate a narrow trench in the sand across the path of the fog. Somehow it knows that ridges alongside the trench will absorb all the moisture it needs to drink.
