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Relative humidity (%)

Figure 4.10 Water exchange by transpiration and passive sorption in worker ants, Formica polyctena, during the first hour of exposure to different vapour activities at 29°C.

Note: Exchange rates in mg h_1 for an ant weighing 10.8 mg, with a water content of 77%.

Source : Reprinted from Comparative Biochemistry and Physiology A, 83, Coenen-Stass, 141-147, © 1986, with permission from Elsevier.

required to differentiate between fluxes (Arlian 1979; Coenen-Stass 1986). Its contribution to water gain can be high when atmospheric av is high (Devine 1978). Fig. 4.10 demonstrates clearly that net water loss, the difference between transpiration and sorption rates, decreases at high vapour activities. This is because the sorption rate increases while transpiration remains esssentially constant (Coenen-Stass 1986). Fortunately for many laboratory studies of transpiration rates, low ambient av results in insignificant levels of passive sorption. In many terrestrial environments, however, passive sorption can be important in the water economy of insects, especially during fasting.

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