Effect of relative humidity (RH) and water vapour on the 18O composition of the haemolymph of P. americana. (a) With drinking water at 50 versus 98% RH, while breathing enriched vapour at −18‰ from local UK tap water; (b) with drinking water at 50 versus 98% RH, while breathing depleted vapour at −22‰ from Vienna tap water; (c) without drinking water at 50 versus 98% RH, while breathing enriched vapour at −18‰ from local UK tap water; and (d) without drinking water at 50 versus 98% RH, while breathing depleted vapour at −22‰ from Vienna tap water (filled circles, observed; open circles, predicted).