20 40 60
Figure 4.3 Simultaneous recording of water loss (upper trace) and CO2 emission (lower trace) in the grasshopper Taeniopoda eques at 25°C.
Note: Peaks in water loss represent respiratory water loss (RWL); the balance is presumed to be cuticular water loss (CWL).
Source: Physiological Zoology, Quinlan and Hadley, 66, 628-642. © 1993 by The University of Chicago. All rights reserved 0031-935x/ 93/6604-92108$02.00
usually much less, than 20 per cent of total water loss (Chown 2002). For example, simultaneous isotopic measurement of water and CO2 losses in Periplaneta showed that tracheal water loss averaged only 3.8 per cent of total water loss (Noble-Nesbitt et al. 1995): this value is much lower than the 13 per cent measured by Machin et al. (1991) using a continuous gravimetric method. Only in species from more arid environments, in which cuticular permeability is reduced, can we expect respiratory transpiration to constitute a greater proportion of total water loss (Zachariassen et al. 1987; Zachariassen 1991a; Quinlan and Hadley 1993; Addo-Bediako et al. 2001). Hadley (1994b) pointed out that, in the majority of species examined, respiratory water loss comprised such a small portion of total water loss that changes in its relative contribution would have little effect on the water status of the insects.
There are certain problems with the suggestion that because respiratory water loss constitutes a small proportion of total water loss it is unlikely to be important (Chown 2002). First, the null expectation has never been stated. That is, it is not at all clear what the expected relative contributions of
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