Respiratory airflow in a wingless dung beetle

The sealed subelytral cavity of many flightless beetle species is widely acknowledged to be an adaptation to water saving in arid-habitat species. However, this hypothesis relies on the acceptance of two largely untested assumptions: (i) that the movement of respiratory gases is unidirectional from anterior to posterior and (ii) that the coordinated action of the spiracles directs this flow. We tested these assumptions by simultaneously measuring CO2 and O2 exchange at the anterior mesothorax, independently of gas exchange in the posterior body, which included the subelytral cavity, of a large apterous beetle, Circellium bacchus. Flow-through respirometry revealed a marked discontinuous gas-exchange cycle (DGC) pattern from the anterior half of the body. Very little CO2 was released from the posterior body, where the DGC was not apparent. Labelled air was shown to flow forwards from the posterior to the anterior body. Individual sampling from the mesothoracic spiracles revealed that the right mesothoracic spiracle, lying outside the elytral cavity, is the primary route for respiratory gas exchange in C. bacchus at rest. This discovery necessitates a reassessment of the currently assumed role of the subelytral cavity in water conservation and is, to our knowledge, the first demonstration of forward airflow associated with the unilateral use of a single thoracic spiracle for respiration in an insect.