Comparative Biochemistry and Physiology Part A 133 (2002) 781–789 1095-6433/02/$ - see front matter  2002 Elsevier Science Inc. All rights reserved. PII: S1095-6433 Ž 02 . 00208-8 Review Water balance in desert Drosophila: lessons from non-charismatic microfauna  Allen G. Gibbs* Department of Ecology and Evolutionary Biology and Center for Insect Science, 1041 E. Lowell Street, University of Arizona, Tucson, AZ 85721, USA Received 13 January 2002; received in revised form 7 March 2002; accepted 24 March 2002 Abstract Water stress is a particularly important problem for insects and other small organisms in arid environments. Cactophilic fruit flies in the genus Drosophila have invaded deserts on numerous occasions, including multiple independent invasions of North American deserts. Because the evolutionary history of this genus is so well studied, we can investigate the mechanisms of adaptation in a rigorous phylogenetic context. As expected, desert fruit flies lose water less rapidly than their mesic congeners. They are also able to tolerate the loss of a greater percentage of body water, but this difference is mainly due to phylogenetic history, and does not represent an adaptation specifically to desert habitats. A laboratory analogue of desert Drosophila is provided by populations of D. melanogaster that have been subjected to selection for desiccation resistance. Selected populations resemble desert species in that they lose water slowly, relative to control populations, and are not more tolerant of dehydration stress. They differ, however, in having much higher water contents and different behavioral responses to desiccating conditions. Our comparisons of laboratory and natural populations reveal that not all possible adaptive mechanisms evolve in stressful environments. Different physiological and behavioral strategies may evolve depending upon the particular options available in the environment.  2002 Elsevier Science Inc. All rights reserved. Keywords: Activity; Desert; Desiccation; Discontinuous ventilation; Drosophila; Laboratory selection; Metabolism; Water stress 1. Introduction The desert is a hot, dry place. Physiologists have devoted great effort to understand how desert organisms survive and thrive in such an extreme environment. Larger animals have two important size-related advantages. Their surface area:volume ratio is lower, so they are less prone to evaporative  This paper was originally presented at ‘Chobe 2001’; The Second International Conference of Comparative Physiology and Biochemistry in Africa, Chobe National Park, Botswana – August 18–24, 2001. Hosted by the Chobe Safari Lodge and the Mowana Safari Lodge, Kasane; and organised by Natural Events Congress Organizing (information@natural- events.com). *Tel.: q1-520-626-8455; fax: q1-520-621-9190. E-mail address: agibbs@arl.arizona.edu (A.G. Gibbs). water loss from their surface, and their size pro- vides thermal inertia that can reduce the danger of overheating. A third advantage is their greater mobility, generally, so that many desert mammals and birds simply leave the area when conditions become too severe. Insects and other arthropods do not have these advantages, yet they are the most diverse and successful desert animal taxa. Studies of desert arthropods have emphasized their small size, but most of the species examined (e.g. tenebrionid beetles, scorpions, grasshoppers) are actually quite large in relation to typical arthropods. Smaller, less well studied arthropods face even greater physio- logical challenges than these ‘charismatic megafauna’.