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Double-barrelled, ion-selective microelectrodes were used to record simultaneous ion activities and potential differences in the tubule lumen and perinephric space of the rectal complex of larvae of the desert beetles Onymacris plana and O. marginipennis. Maximal tubule lumen activities of K+, Na+ and Cl- were 3348, 114 and 3100 mM, respectively. Calculated net electrochemical potentials are consistent with passive movement of Na+, K+ and H+ from the haemolymph to the perinephric space, and active accumulation of Na+, K+ and H+ in the tubule lumen. Chloride activities are in equilibrium with the electrical potential in the tubule lumen and perinephric space.

Unlike Tenebrio, predicted maximal osmolalities in Onymacris tubules, calculated from the threshold for water vapour absorption, exceed KCl solubility. Although NaCl is more soluble and is readily available to Onymacris larvae, tubule lumen Na+ was 9.9 times less abundant, on average, than K+. The ratio of tubule lumen Na+ to K+ activities was lower in Onymacris than in Tenebrio.

We suggest that lower absorption thresholds and higher KCl activities in Onymacris tubules depend on KCl supersaturation. Morphological differences between the rectal complexes of the two genera are consistent with the superior ion-concentrating ability of Onymacris.