During the active portion of an insect's life cycle desiccation stress is probably frequent, but shortlived. In addition, the active stages (larvae and adults) have regular access both to free water and to water in their food. In contrast, during dormancy (diapause, quiescence, aestivation), dehydration is likely to be prolonged and access to water and energy resources extremely limited (Lighton and Duncan 1995; Danks 2000). At this time, there is likely to be strong selection both for low metabolic rate, to conserve energy resources, and for any mechanisms that might reduce water loss. Insects overwintering in temperate regions are also subject to desiccating conditions, and some show remarkable resistance to desiccation even when individuals are removed from their plant galls
(Ramlev and Lee 2000; Williams et al. 2002). This is not, however, due to the seasonal accumulation of cryoprotectant polyols and sugars, which increases haemolymph osmolality (Williams et al. 2002), but in fact has little effect on the water activity gradient between air and insect. The role of reduced water content in overwintering strategies is discussed in Section 5.3.2. Danks (2000) has reviewed the mechanisms of desiccation avoidance and tolerance in dormant insects (his table 1 is a useful summary): one of the reasons for dormancy in the first place is water stress.
Effects of desiccation on eggs and even smaller first instar larvae were recently compared for two species of Lepidoptera, Grammia geneura (Arctiiidae) and M. sexta (Sphingidae) (Woods and Singer 2001), with an emphasis not on phylogenetic differences but rather on the physiological diversity associated with different oviposition tactics. Female M. sexta lay eggs on the underside of leaves of the food plant, while female G. geneura deposit theirs in leaf litter or at the base of grasses. Neonates of G. geneura, although much smaller, cope with starvation and desiccation very effectively by reducing their metabolic rate and becoming inactive until food is available. Setae covering larval G. geneura create a boundary layer and further decrease water loss. Rates of water loss by eggs of both species did not show the expected inverse relation to initial egg mass; nor did large larval size within a species confer resistance to desiccation. Comparing individuals of different body size is not straightforward: third instar Pieris brassicae (Lepidoptera, Pieridae) lose water more rapidly than fifth instars, but cuticular permeability might decrease with age (Willmer 1980).
Ants have successfully colonized a wide variety of habitats and vary greatly in body size both within and between species, but several studies show that large size is less of an advantage than might be expected. Desiccation resistance in a large assortment of arboreal and terrestrial ants increased with body size (as dry mass0.55), but not as quickly as expected from the surface area to volume relationship (dry mass067) (Hood and Tschinkel 1990). Habitat accounted for much more of the variation in desiccation resistance, with arboreal ants surviving eight times longer than
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