The Body Temperature of Tenebrionid Beetles in the Namib Desert of Southern Africa

Abstract

1. In direct sunshine the abdomen of living Onymacris brincki, which is covered by white elytra, was 3-4 °C cooler than its thorax, while in the same conditions the abdomen and thorax of O. rugatipemtis, which is an entirely black beetle, had the same temperature. Since similar effects were observed in dead beetles, the warmer thorax was not due to muscular activity. When the white elytra of brincki were covered with carbon black, its abdominal temperature rose to equal that of rugatipenntis. Models, made of pieces of black and white elytra, showed similar effects. 2. The smaller species Stenocara eburnea (with white elytra) showed a smaller and less consistent temperature depression when compared with the similar-sized species S. depressa (which is black). Abdomens of O. rugatipennis, O. laeviceps and Physosterna cribripes (all large black beetles) were from 3-5 °C wanner than O. brincki, but showed no consistent difference among themselves. 3. When O. rugatipennis was oriented head-on to the sun in its natural environment its thorax was about 4 °C cooler than when its long axis was normal to the sun's rays. Beetles exposed to sunshine were 12-15 °C warmer than they were in the shade. When transferred from one to the other, the greater part of temperature equilibration occurred in about 2 min and equilibration was complete in about 4 min. 4. Observation suggested that beetles thermoregulate by moving between sunshine and shade. Measurements showed that a beetle exposed to alternating periods of 10 sec in sunshine and 30 sec in shade reached a steady temperature of about 33 °C, while the reverse conditions (10 sec in the shade and 30 sec in sunshine) led to a temperature of about 38 °C. In continuous alternations thoracic temperatures remained constant within about 1 °C. 5. Laboratory experiments in which the angle of incidence of the sun's rays was varied through 180° confirmed the field results and showed that white elytra lower the abdominal temperature of living and dead beetles. 6. Upper lethal temperatures for 30 min in saturated air varied from 50 °C for the desert tenebrionid Onymacris plana to 42.5 °C for the mesic species Trigonopus capicola. In general, upper lethal temperatures correlated well with habitat and habit. 7. Making certain assumptions, analysis of the heat balance of beetles during one exposure in sunshine suggests that the difference between the white elytra of brincki and the black ones of rugatipennis as regards the amount of short-wave radiation reflected is 0.227 cal cm-2 min-1, and further, that the reflectivities of the two surfaces are 79% and 38% respectively. 8. The selective advantage of white elytra probably has little to do with heat balance; the vivid contrast between white and black suggests that the pattern may be aposematic.