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The desert fruit fly Drosophila mojavensis experiences environmental conditions of high temperature and low humidity, To understand the physiological mechanisms allowing these small insects to survive in such stressful conditions, we studied the effects of thermal acclimation on cuticular lipids and rates of water loss of adult D. mojavensis. Mean hydrocarbon chain length increased at higher temperatures, but cuticular lipid melting temperature (T-m) did not, Lipid quantity doubled in the first 14 days of adult life, but was unaffected by acclimation temperature. Despite these changes in cuticular properties, organismal rates of water loss were unaffected by either acclimation temperature or age, Owing to the smaller body size of warm-acclimated flies, D. mojavensis reared for 14 days at 33 degrees C lost water more rapidly on a mass-specific basis than flies acclimated to 25 degrees C or 17 degrees C. Thus, apparently adaptive changes in cuticular lipids do not necessarily result in reduced rates of water loss, Avoidance of high temperatures and desiccating conditions is more likely to contribute to survival in nature than changes in water balance mediated by surface lipids.