Active water vapour absorption in Porcellio scaber is associated with the endogenous production of strongly hyperosmotic fluid in the pleoventral chamber (pleoventralraum; PV). Pre-desiccated animals show increased pleon fluid secretion within 1–2 min of transfer to suprathreshold humidities (>87 % relative humidity). The conspicuous increase in fluid volume is accompanied by a rise in osmolality from isosmotic (approximately 750mosmolkg−1) up to as much as 8.2osmolkg−1. Vapour absorption is marked by the onset of metachronal pleopodal ventilation and a subsequent decline in fluid osmolality as uptake fluid approaches equilibrium with ambient water activities. The effects of sealing of the PV by the margins of the depressed pleopods, along with the observation that animals ventilate even when PV fluid activities are somewhat below ambient, suggest that resultant pressure increases may elevate humidity within the PV and thereby augment vapour uptake. Ion-selective microelectrode measurements of Na+, K+, Ca2+ and Cl concentrations in nanolitre samples of isolated pleon fluid identify Na+ and Cl as the major osmolytes. Possible preadaptions favouring evolution of vapour absorption in the Oniscidea are discussed

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