Evolution of water conservation mechanisms in Drosophila

Flies of the genus Drosophila inhabit a wide range of habitats, from the tropics to deserts to boreal forests. The primary physiological mechanism allowing Drosophila and other insects to survive in arid habitats is a reduction in rates of water loss. To understand mechanisms of water retention in greater detail, we investigated the three main routes by which Drosophila lose water: excretion, cuticular transpiration and respiratory loss through the spiracles. Excretory losses comprised <6% of total water flux and did not differ between xeric (cactophilic) and mesic species. No consistent relationship was observed between water-loss rates and the composition, physical properties or amounts of cuticular hydrocarbons, suggesting that cuticular transpiration did not differ among species from different habitats. Metabolic rates and water-loss rates were highly correlated. Cactophilic Drosophila were less active, and female cactophiles had lower metabolic rates than female mesic species of the same size. They were also more likely to exhibit a pattern of cyclic CO₂ release that may help to conserve water. We conclude that lower overall rates of water loss are achieved primarily by reduction of respiratory losses.