Conflicting needs of obtaining oxygen and preventing water loss are met in insects by internal tracheal surfaces for respiratory exchange, opening to the exterior via recessed and occludable spiracles (Chapter 3). The role of respiratory water loss in the water economy of insects is currently an active research area, reviewed most recently by Chown (2002). Measurements of respiratory and cuticular water loss have always been closely connected (see above). Earlier studies estimated respiratory transpiration by difference, comparing water losses of untreated insects and those with sealed spiracles (Ahearn 1970; Edney 1977; Loveridge 1980). The solution to the problem of cuticle damage due to spiracle sealing came from allowing unrestrained and resting insects to seal their own spiracles during discontinuous gas exchange (DGC, see also Chapter 3) (Kestler 1985). Evaporative losses can be measured by continuous weighing during such experiments (Kestler 1985; Machin et al. 1991), but are preferably measured electronically in conjunction with flow-through respirometry, usually involving CO2 analysis because of its greater sensitivity (Lighton 1991b). A sample trace of simultaneous water loss and CO2 emission in the lubber grasshopper Taeniopoda eques (Orthoptera, Acrididae) is shown in Fig. 4.3. Water loss during the interburst periods is assumed to represent cuticular water loss alone, whereas water loss during the burst periods represents total evaporative losses. Recently, Gibbs and Johnson (2004) described a new method of partitioning cuticular and respiratory water losses: when water loss rates are plotted against CO2 production, a positive relationship is obtained, and the intercept represents cuticular transpiration.
Respiratory versus cuticular water loss Published data on the relative contributions of cuticular and respiratory transpiration to total water loss vary widely, both because of technical considerations and because of the strong influence of factors such as temperature and activity on metabolic rate (for discussion see Hadley 1994a,b). In general, spiracular control ensures that respiratory losses in resting insects are relatively minor: spiracular transpiration represents less, and
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