The water conserving physiological compromise of desert insects

Insects living in arid tropical areas may spend long periods without access to free water, and at the end of the dry season they may be severely dehydrated. To survive under such conditions insects have developed a highly restrictive water economy, and tenebrionid beetles from arid tropical areas may lose water at a rate which is a hundred-fold lower than those of insects from humid habitats.In most insects the dominant route of evaporative water loss is across the cuticle. In dry habitat tenebrionid beetles cuticular water permeability has been reduced so much that the water loss accompanying the exchange of respiratory gases across the spiracles has become the major water loss component. A further significant beetles seem to have utilized this opportunity in that they have metabolic rates which are markedly lower than those of most other insects.The low metabolism must imply a corresponding reduction in cellular production of ATP, which is the energy source for cellular ionic pumps. Cellular extrusion of sodium is estimated to consume a substantial fraction of the ATP. Reduced ATP production will therefore also cause a reduced cellular sodium pumping and thus a reduced energy gradient of sodium across cell membranes. This in turn reduces the sodium coupled cellular accumulation of amino acids which requires energy from the sodium gradient. This gives rise to the relatively low extracellular concentrations of sodium and high concentrations of amino acids displayed by these insects. In most animals extracellular amino acid concentrations of this magnitude would have led to a substantial urinary loss of amino acids. However, since desert insects possess an exceptionally efficient rectal system for reabsorption of water and solutes from the urine, a large quantity of amino acids can be returned to the haemolymph from the urine in these animals. Thus, the unique capacity of desert tenebrionids to reabsorb water and solutes from their urine appears to be an important condition also for the low transpiratory water loss of these insects.