Summer World_ A Season of Bounty - Bernd Heinrich [74]
We are not deterred by heat so much as by lack of water. In his book The Hunters or the Hunted, C. K. Brain notes that in southwestern Africa, all the Hottentot villages in the Namib Desert are scattered directly along the Kuiseb River. Here the people have dug wells from which they get their water when the river runs dry. Birds there get water from eating insects, and most insects get water from live plants. But one group of Namib beetles of the family Tenebrionidae are an exception. Some of them stay in water balance even when eating only dried plant detritus that blows around in the wind.
These beetles are ground-dwelling, usually large, and black (the melanin absorbs heat but is necessary to protect them from damage by ultraviolet light). They live on the sand surface. Those that live on the hottest sands have stiltlike legs to reduce heat input from below. Others reduce heat input from above, from the sun, by light-colored wax on their black backs. But even then there is still the problem of getting sufficient water, and there is no standing water and no rain when they are active. Although they are subjected to a desiccating environment during the daytime, at night temperatures in the Namib typically drop and wind from the Atlantic coast may sweep in with moisture-laden air. The beetles then orient themselves by standing on the sand dunes with the head straight down and the abdomen up into the air. Water condenses on the beetle’s back and flows down in droplets to its mouth.
Fig. 30. A Namib Desert tenebrionid beetle, which elevates itself above the most intense heat at the ground surface.
Fig. 31. A Namib Desert tenebrionid beetle that catches water from moist air blowing in from the Skeleton Coast by doing headstands. The water condenses on its back in tiny droplets, which then coalesce and run down to its mouth.
The beetle’s amazing behavior is cobbled together by evolution from structures and behaviors that previously had other functions. Their backs are modified wing covers (elytra) that no longer cover any wings but serve instead as physical protection for the body. But in these beetles the elytra have taken on an additional, very different, and novel function. All tenebrionid elytra are sculptured in various patterns. In these beetles they have a pattern of bumps that helps capture vapor molecules into tiny droplets. Waxy valleys between the bumps channel the water droplets so that they coalesce and roll down into the mouth. I recalled seeing similar tenebrionid beetles in our southwestern Mojave Desert, where they are sometimes derisively called “butt-head” beetles because here also they stand with their butt in the air. But in this case they do their headstands for a different purpose: defense. The headstanding exposes a gland in the tip of the abdomen from which the beetle can exude a foul liquid that may spread over the back and will repel most predators. The Namib beetle’s water-catching behavior was probably derived from similar defensive behavior that later became joined to an existing morphology.
Although I had often seen butt-head beetles in the Mojave, I was not fortunate enough to witness the dew-catching behavior of the African beetles when I was in Namibia with my graduate student James Marden to study desert insects. We stayed at the Namib Research Institute at Gobabeb next to the “shore” of the Kuiseb River. The riverbed was dry at the time, but it was the only place where we saw trees and shade. The trees’ roots tapped the groundwater, and that water fed insect fauna. We saw innumerable black tenebrionid beetles