Osmotic regulation in adult Drosophila melanogaster during dehydration and rehydration.

We have examined the osmoregulatory capacities of laboratory populations of the insect Drosophila melanogaster by measuring hemolymph osmotic concentration during desiccation and upon recovery from a bout of desiccation. Recovery treatments entailed allowing the flies access to distilled water, a saline solution or a saline+sucrose solution after a desiccation bout shown to reduce hemolymph volume by approximately 60%. Prior to desiccation, the hemolymph osmotic concentration was 353+/-11 mOsm. We found that Drosophila display strict osmotic regulation under prolonged conditions of dehydration. Osmotic regulation continued during recovery from desiccation, regardless of the fluid provided. This result is evidence that this insect does not require an external source of osmolytes or energy to regulate its hemolymph osmotic concentration or to restore hemolymph volume, which is reduced during desiccation. We also examined populations that have been selected for over 250 generations for enhanced desiccation resistance to identify physiological characters that have evolved in response to the selection regime. The selected lines displayed a reduced pre-desiccation hemolymph osmotic concentration (315+/-7 mOsm) and a marginally improved capacity for osmoregulation.