Respiratory Strategies in Relation to Ecology and Behaviour in Three Diurnal Namib Desert Tenebrionid Beetles.

Abstract

Simple SummaryThe Namib Desert has a large diversity of darkling beetle species. All these beetles are flightless and feed on plant detritus. The aim of this study was to investigate whether the respiration strategies used by these beetles are linked to their ecology and behaviour. Three beetle species, which are all active during the day, in direct sunshine, running across the sand surface, but found in different parts of the desert, were chosen for this study. All three beetle species used intermittent breathing. They held their breath for several minutes and released CO2 in pulses. The large beetle species, which runs rapidly on the dune slip face when air temperatures are high, used evaporative cooling to prevent over-heating. The water used in the cooling comes from their respiratory surfaces and is replenished from metabolising food and drinking water droplets left on vegetation after a fog event. The two smaller beetle species, which inhabit the gravel plains, limit the area of the respiratory surface exposed to the atmosphere, which reduces body water loss. These two beetle species are not known to drink water, and thus have a greater need to conserve their body water.The respiratory physiology of three diurnal ultraxerophilous tenebrionid beetles inhabiting either the dune slipface or gravel plain in the Namib Desert was investigated. The role of the mesothoracic spiracles and subelytral cavity in gas exchange was determined by flow-through respirometry. All three species exhibited the discontinuous gas exchange cycles with a distinct convection based flutter period and similar mass specific metabolic rates. There was variation in their respiration mechanics that related to the ecology of the species. The largest beetle species, Onymacris plana, living on the dune slipface, has a leaky subelytral cavity and used all its spiracles for gas exchange. Thus, it could use evaporative cooling from its respiratory surface. This species is a fog harvester as well as able to replenish water through metabolising fats while running rapidly. The two smaller species inhabiting the gravel plains, Metriopus depressus and Zophosis amabilis, used the mesothoracic spiracles almost exclusively for gas exchange as well as increasing the proportional length of the flutter period to reduce respiratory water loss. Neither species have been reported to drink water droplets, and thus conserving respiratory water would allow them to be active longer.