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Desiccation resistance in insects has long been thought to covary with environmental water availability, and to involve changes in both cuticular and respiratory transpiration. Here, we adopt a large-scale approach to address both issues. Water loss rate and precipitation are positively related at global scales. A significant proportion (68%) of the interspecific variation in water loss rate is explained at the genus level or above. The relationship between metabolic rate and water loss rate differs substantially between mesic and xeric species. While these variables covary as a consequence of their independent covariation with body mass in mesic species, this is not the case in xeric species. In the latter, there is a strong relationship between the residuals of the water loss rate-body mass and metabolic rate-body mass relationships, and water loss rate is much reduced. Moreover, because metabolic rate does not differ significantly between xeric and mesic species of a similar size, respiratory transpiration constitutes a greater proportion of total water loss in xeric than in mesic species of a similar size. This implies that respiratory transpiration and the extent to which it can be modified must be of considerable importance in xeric insect species, although finer scale studies suggest otherwise. (C) 2001 Elsevier Science Ltd. All rights reserved.