Figure 7.6 Metabolic rate (mean ± SE) and water loss rate of frozen and supercooled goldenrod gall fly larvae at -5°C showing significant reductions in both rates in frozen insects.

Source: Irwin and Lee. Journal of Experimental Zoology 292, © 2002. Reprinted by permission of Wiley-Liss, Inc., a subsidiary of John Wiley & Sons, Inc.

been co-opted to promote survival of subzero temperatures (Ring and Danks 1994). Indeed, the building blocks for both freeze intolerance and freezing tolerance are present in extant pathways of insect metabolism, and the biochemical responses may simply represent an upregulation of pathways already extant for protection against desiccation and other stresses (Pullin 1996; Sinclair et al. 2003c). This has been nicely illustrated for the springtail, Folsomia Candida, where drought acclimation confers cold tolerance as a consequence of an increase in heat shock proteins and an increase in the molar percent of membrane fatty acids with a mid-chain double bond. The latter is thought to confer both drought and cold resistance (Bayley et al. 2001).

The cold hardiness strategy adopted by an insect can also influence its desiccation resistance and likely survival of prolonged starvation. Because the vapour pressure difference between ice and a supercooled insect is much higher than that between ice and a frozen one (Lundheim and Zachariassen 1993), freezing tolerance confers considerable desiccation resistance on the insects that adopt this strategy. Conversely, freeze intolerant insects have to have a greater desiccation resistance than those that can survive freezing (Klok and Chown 1998b). Moreover, frozen insects tend to have a much lower metabolic rate than those that are supercooled. Therefore, freezing tolerance might also facilitate prolonged starvation resistance (Irwin and Lee 2002) (Fig. 7.6).

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