The use of water activity for comparing aqueous and vapour phases is recommended (Edney 1977;
Wharton 1985). Relative humidity RH is 100 times av, the activity of water in air. The term av can be directly compared to the haemolymph concentration as aw, and the difference av — aw between the air and the insect determines whether the insect will gain water from the surrounding air or lose water to it. Insect haemolymph has aw values in the 0.995 range, while atmospheric av is almost always much lower, so the gradient for water movement is almost invariably outwards. Note that doubling of the haemolymph osmolality from 300 to 600 mOsmolkg—1 would decrease aw from 0.995 to 0.99 and thus can have little effect on the water activity gradient between air and insect (Willmer 1980).
The water content of insects varies widely, from 40 to 90 per cent of wet mass (Edney 1977; Hadley 1994a). It is lower in insects with high fat reserves or heavy cuticles, and varies between species, instars, individuals, and over time in the same individual (Wharton 1985). Because of the inverse relationship between water content and lipid content, water content does not necessarily give a good indication of the state of hydration of an insect. Dormant insects tend to have lower water contents because of fat accumulation (Danks 2000). Water content is best expressed as mgH2Omg—1 dry mass rather than the commonly used percent water. The relationship between percent water content and the absolute water content is an exponential function (Coutchie and Crowe 1979a). For example, a small change in the percent water, from 66 to 80 per cent, represents a doubling of the absolute water content of the animal (from 2 to 4 mgH2Omg—1 dry mass).
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