THE WATER CONSERVING PHYSIOLOGICAL COMPROMISE OF DESERT INSECTS

Insects living in arid tropical areas may spend long periods without a ccess to free water, and at the end of the dry season they may be seve rely dehydrated. To survive under such conditions insects have develop ed a highly restrictive water economy, and tenebrionid beetles from ar id tropical areas may lose water at a rate which is a hundred-fold low er than those of insects from humid habitats. In most insects the domi nant route of evaporative water loss is across the cuticle. In dry hab itat 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 i n that they have metabolic rates which are markedly lower than those o f most other insects. The low metabolism must imply a corresponding re duction in cellular production of ATP, which is the energy source for cellular ionic pumps. Cellular extrusion of sodium is estimated to con sume a substantial fraction of the ATP. Reduced ATP production will th erefore also cause a reduced cellular sodium pumping and thus a reduce d energy gradient of sodium across cell membranes. This in turn reduce s the sodium coupled cellular accumulation of amino acids which requir es 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 substa ntial urinary loss of amino acids. However, since desert insects posse ss an exceptionally efficient rectal system for reabsorption of water and solutes from the urine, a large quantity of amino acids can be ret urned to the haemolymph from the urine in these animals. Thus, the uni que capacity of desert tenebrionids to reabsorb water and solutes from their urine appears to be an important condition also for the low tra nspiratory water loss of these insects.