Water balance and osmotic regulation in the East African tenebrionid beetle Phrynocolus petrosus

Abstract

1. When tenebrionid beetles of the species Phrynocolus petrosus undergo evaporative dehydration, the relative water content changes less than is predictable from the water loss alone. The discrepancy from the predictable change is apparently due to metabolic production of water by utilization of fat. 2. By fat metabolism the rate of metabolic production of water will amount to less than 15% of the rate of the total water loss when the beetles are kept in dry atmosphere at 20°C. Metabolic water production cannot compensate for evaporative loss unless air humidity is close to saturation. 3. As the beetles undergo dehydration of up to 10% of the body wt, they display no osmoregulation. A water loss of up to 10% of the body wt is accompanied by a strong increase in the concentration of free amino acids in the haemolymph, suggesting that the initial water loss takes place at the expense of the haemolymph volume alone. Concentrations of sodium and potassium do not increase more than expected from the increase in osmolality. 4. As dehydration proceeds beyond 10% of the body wt, efficient osmoregulatory mechanisms are activated. These mechanisms seem to involve the regulation of the haemolymph concentration of free amino acids, but not of sodium and potassium.