Short Communication Respiration beneath Desert Sand: Carbon Dioxide Diffusion and Respiratory Patterns in a Tenebrionid Beetle

The massive dune sea of the southern Namib Desert is approximately 800 km in length and some 140 km wide. These dunes are among the highest in the world and, in spite of very sparse vegetation cover, they support a surprising variety of reptiles and arthropods (Koch, 1961; Louw & Seely, 1982). A feature of the arthropod fauna is the high species diversity of flightless tenebrionid beetles. These beetles do not construct burrows, but swim through the sand as if it were a fluid medium. For a description of the physical properties of dune sand see Lancaster (1981). Among the tenebrionids, Onymacris unguicularis is of special interest because it engages in fogbasking on the crests of the dunes when intermittent condensing fog occurs in the desert (Hamilton & Seely, 1976). This beetle occurs almost exclusively on the high slip-faces of the dunes, where it feeds on wind-blown plant detritus. To escape extremely high surface temperatures during midday and very low temperatures at night, O. unguicularis submerges beneath the sand to a depth of 5—10 cm for as much as 18 h per day (Seely, Mitchell & Louw, 1985). The present study assesses the respiratory consequences of such submergence by measuring the diffusion rate of carbon dioxide through a 5-cm layer of sand and the respiratory patterns, carbon dioxide production, and oxygen consumption of the beetles. Onymacris unguicularis were collected on the slip-faces of sand dunes 10 km west of Gobabeb, Namibia. They were transported by air, together with a large sample of slip-face sand, to our laboratory in Cape Town, where they were maintained on a diet of lettuce and rolled oats in a photothermal gradient. In order to measure respiratory patterns, the beetles (mean mass 0737 ± 0120g; mean±S.D.) were individually placed in a cylindrical Perspex respirometer (diameter 6 cm; total volume 136 ml). Dry medical air was led into the upper portion of the respirometer at a flow rate of 200mlmin~, while the exit port, also at the top of the respirometer, was connected via a flow meter to an infra-red CO2 analyser (Analytical Development Co., 225MK3). The electrical output from the CO2 analyser was connected to a chart recorder which allowed resolution to an accuracy of 3 p.p.m. CO2. The analyser was