Thermodynamics of cuticular transpiration

Water conservation is a significant physiological problem for many insects, particularly as temperature increases. Early experimental work supported the concept of a transition temperature, above which water-loss rates increase rapidly as temperature increases. The transition phenomenon was hypothesized to result from melting of epicuticular lipids, the main barrier to cuticular transpiration. This explanation has been challenged on theoretical grounds, leading to thermodynamic analyses of cuticular transpiration based on reaction rate theory. These studies have not directly addressed the mechanistic basis of the transition temperature. Models developed in the context of cell membrane transport provide potential explanations that can be tested experimentally. These models include changes in the activation entropy for diffusion through the cuticular lipids, increased solubility of water in melted lipids, and lateral heterogeneity of the cuticle.

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► Transpiration of water through the insect cuticle is non-linear.
► Simple thermodynamic models cannot account for non-linear behavior.
► A model in which water molecules preferentially partition into melted cuticular waxes is outlined.
► This model is consistent with experimental studies.