Respiratory water loss in insects.

The contribution of respiratory transpiration to overall water loss in insects is contentious. Misgivings concerning the importance of this route of water loss have arisen largely as a consequence of work on discontinuous gas exchange cycles (DGC). Most studies have found that respiratory water loss constitutes only a small proportion of total water loss. Thus, it has been argued that modulation of metabolic rate and/or the components of the DGC is unlikely to constitute a fitness benefit. In contrast to these intraspecific studies, interspecific comparative data suggest that, at least in xeric species, respiratory transpiration is an important component of water loss. However, these arguments are confounded by several factors. In DGC-based studies, these include multiple effects of the experimental treatments, the absence of a null expectation for the contribution of respiratory to total water loss, and problems with the use of proportions as a way of assessing the importance of respiratory water loss. The interspecific studies are confounded by the likely significance of influences other than water conservation on metabolic rate, the absence of analyses of phylogenetic independent contrasts, and little information on behavioral differences between species. Future work should be based on a strong inference approach and designed in such a way that these problems can be resolved. Moreover, in the case of the DGC it should be recognized that several factors are likely to influence this gas exchange pattern, and that they probably act in concert, especially during dormancy.