Mechanisms of water vapor absorption in insects

Vapor absorption is the mechanism by which specially adapted terrestrial arthropods expend energy to escape from conventional equilibria, dictated by the difference between water activity of the body fluids (a w = 0.99) and the environment. vapor absorbers have an identifiable “condensing” or “carrying” fluid compartment with significantly lower activity. The sites of vapor uptake may be limited to mouth or rectal regions because the mechanisms have probably evolved by modifying existing water transport systems. Morphometric analysis of condensing structures have helped understand the vapor uptake mechanism in Arenivaga and tenebrionid larvae. Activity lowering of condensing fluid is accomplished either by solute‐independent capillary condensation or by solute transport. Linear absorption kinetics, shown by the better‐known vapor absorbers, is achieved by flow‐modulated fluid or ion transport. Comparative studies may help identify anomalous uptake mechanisms worth further study and predict where other examples of the phenomenon may be found.