Most of the recent work has been concerned with the exploration of particular mechanisms, and there is now both a need and an opportunity to take a wider view and attempt to synthesise such knowledge into an understanding of water balance in whole animals in natural situations.
Water availability and temperature are the two most important abiotic variables influencing the distribution and abundance of insects, although they are continuously modified by two other major climatic factors, solar radiation and wind. The physiological effects of climate are usually related to water and thermal balance, but these effects occur on a fine scale. Small size is commonly accepted as disadvantageous for insects in terrestrial environments because of the small storage capacity for water and relatively large surface area for losing it; however, it also enables them to escape to microhabitats and take advantage of favourable microclimates (Willmer 1982).
Much water balance work has been undertaken in desert regions (Addo-Bediako et al. 2001) and has been concerned with demonstrating superior desiccation resistance in species from arid environments, so it is not surprising that the emphasis has been predominantly on insects conserving water. However, high water turnover can occur for various reasons, some of which may be combined in one species: diet (animal or plant fluids), flight (especially when associated with large body size and endothermy), metamorphosis (especially water loss at adult eclosion) and aquatic habitats. The emphasis on adult insects observed by Edney (1977) is still largely true: when Addo-Bediako et al. (2001) surveyed the anglophone literature on insect water balance as far back as 1928, they found that >70 per cent of the studies concerned adult insects. However, eggs and early instar larvae can be expected to have the worst surface area:volume ratio problems (Woods and Singer 2001), and eggs and pupae are frequently the dormant stages in which dehydration is likely to be prolonged and access to water is extremely limited (Yoder and Denlinger 1991; Danks 2000).
The physiological mechanisms involved in water balance and osmoregulation of individual insects were thoroughly reviewed in Edney's (1977) monograph and its successor by Hadley (1994a), and also by Wharton (1985) with a rather different emphasis, on the kinetics of water exchange. In the standard approach to water balance physiology, the overall flux of water through an insect is partitioned into different avenues of water loss and recovery, which vary greatly in both absolute and relative terms. A recent study on water-stressed caterpillars (Woods and Harrison 2001) illustrates this hierarchical nature of organismal physiology: fitness-related performance traits represent the aggregate outcome of numerous, more mechanistic physiological traits. The performance trait in question is the growth rate of the caterpillars, which is depressed during water shortage, and the dominant mechanistic traits in this particular case involve modulation of faecal and evaporative water losses.
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