Water balance and osmoregulation in Stips stali, a nocturnal tenebrionid beetle from the Namib Desert

Abstract

Dehydration (6 days at 26°C) of the Namib tenebrionid beetle, Stips stali (Haag), resulted in weight loss (approx 7%), a drop in total body water, and a substantial decline in haemolymph volume (33%). All of these values returned to normal when access to water was permitted (day 6–day 8). Haemolymph osmolality, sodium, potassium and chloride concentrations increased during dehydration and decreased during rehydration, but despite marked changes in the volume of haemolymph, changes in these parameters were subject to osmoregulatory control. However, although the sodium and potassium concentrations had returned to normal at the end of the rehydration period, the level of chloride remained lower than normal. While changes in protein concentrations were as expected from haemolymph-concentration and -dilution, total lipid levels remained constant throughout dehydration. Over the short-term, therefore, Stips stali does not or cannot mobilize its lipid, and metabolic water input via this avenue appears to be negligible. Drinking (when fog water is available) probably contributes largely to total water uptake, and together with efficient water conservation, must serve to effectively maintain water balance in these insects.