In a physical chemical sense, terrestrial insects and acarines are open aqueous systems separated from the atmosphere by an integument and integumental deriva tives. These cover the external surfaces and line the tracheae, foregut, and hindgut. The midgut, although not in direct contact with the air, is the primary site of entry into the body fluids of imbibed or ingested water (101). The water content of terrestrial arthropods is different among species and stages within species, and from time to time in an individual. In terms of percentage of fresh weight, the range is from the middle 40s to the mid-90s (88). If the mean molecular weight of molecules other than water is 500, then the mole fraction of water will be about 0.99 in an animal with a midrange 70% water content in terms of fresh weight. In such circumstances, 99 of every 100 molecules will be water molecules (Il l ). Even the cuticle that separates the epidermal cells from the air has a mean water content of 43% with a range of 15-70% (89). Solutes in the body water are responsible for the osmotic pressure of this water. Among insects, osmotic pressures as low as 250 mosmol and as high as 700 mosmol have been measured (42). We use activity to express the concentration of water in cells, hemolymph, and other body structures and fluids. The activity of water, aw, to two significant figures, that encompasses the values expressed above for insects in milliosmoles, and that is reported for acarines (SO, 109), is 0.99. aw is the dimensionless ratio of the chemical potential of the aqueous material in question over that of pure water at the same temperature and pressure. The chemical