Water acquisition and partitioning in Drosophila melanogaster

insects, water can be gained through consumption, absorption of atmospheric water vapor and production of water during metabolism (Hadley, 1994). Simultaneously, water is lost through the processes of excretion, cuticular and respiratory transpiration and in bodily secretions. When living in desiccating environments, insects must be able to maintain water balance by restricting water loss (Edney, 1977). Water conservation is especially challenging for Drosophila melanogaster, in which there is no absorption of water from the atmosphere and net water loss occurs even in atmospheres exhibiting greater than 90 % relative humidity (Arlian and Eckstrand, 1975). Investigators studying physiological responses in populations of Drosophila melanogaster that have undergone laboratory selection for enhanced desiccation-tolerance have proposed that resistance to dehydration is increased in adults primarily by a reduction in the rate of cuticular water loss and by an increase in body water content (Hoffman and Parsons, 1993; Gibbs et al., 1997; Djawdan et al., 1998). Some additional features, whose functional significances are currently unclear, appear to play a role in the evolution of desiccation-resistance in Drosophila melanogaster. These include changes in the respiratory pattern and behavior during dehydration as well as modifications of the rate of development and of the carbohydrate-to-lipid ratio of storage products (Williams et al., 1998; Williams and Bradley, 1998; Chippindale et al., 1998; Djawdan et al., 1998). On the basis of the above studies, a model has been proposed describing the processes by which Drosophila melanogaster responds to desiccation (Gibbs et al., 1997; Chippindale et al., 1998; Bradley et al., 1999). At 4 days post-eclosion, desiccation-resistant adults are larger than control adults because of an increased water content. This increase in stored 3323 The Journal of Experimental Biology 204, 3323–3331 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 JEB3523