Info

for normalizing highly skewed macroecological data (Williamson and Gaston 1999), might also prove to be useful for allowing comparisons of SCPs between two or more samples where SCP frequency distributions are not bimodal but retain a significant skew.

The role of water

Like the ingestion of some kinds of food particles, drinking also has a significant effect on SCP, often causing a wholesale elevation of the SCP (Block 1996). Given that there is a relationship between water volume and the likelihood that it will supercool (Lee and Costanzo 1998), this is perhaps not surprising. However, it does suggest that altering water content might contribute substantially to the avoidance of freezing. Indeed, it is now widely appreciated that a reduction in water content is characteristic of several freeze-intolerant species, and that this decline in water content causes a substantial lowering of the SCP (Zachariassen 1991b). Such a lowering of water content is typical of both freeze-intolerant and freezing tolerant species (Ring and Danks 1994), and may serve not only to lower SCPs, because of an increase in the concentration of cryoprotectants, but also to reduce mechanical damage caused by ice formation and to limit the extent of water lost by desiccation during extended cold periods. Extreme desiccation resistance in Embryonopsis halticella larvae may be responsible for their very low SCP, which is substantially lower than the environmental temperatures they are likely to experience (Klok and Chown 1998b).

Lundheim and Zachariassen (1993) have shown that the cold hardiness strategy adopted by an insect is likely to have a considerable influence on its ability to resist desiccation. Their rationale is straightforward. In insects that overwinter where they are exposed to ice, the haemolymph of frozen individuals will be concentrated by the formation of ice until the vapour pressure of the liquid fraction is equivalent to that of ice at the same temperature. In consequence, no water loss will take place. Insects that do not freeze, but that supercool, will not be in vapour pressure equilibrium with the ice, and will continue to lose water. Thus, freezing tolerant insects should be more desiccation resistant than freeze-intolerant ones, and this is often found to be the case (Ring 1982; Lundheim and Zachariassen 1993).

The similarity of the responses of insects to both cold and dry conditions is being increasingly recognized (Ring and Danks 1994; Block 1996). The basic biochemical mechanisms (accumulation of sugars and/or polyols and thermal hysteresis proteins) are common to both freezing tolerant and freeze-intolerant strategies (Section 5.3.4), though serving different functions in each. Similarly, the accumulation of low molecular weight compounds has been implicated in the absorption of atmospheric water by Collembola (Bayley and Holmstrup 1999), and in the protection of cells against osmotic damage during extreme dehydration (Danks 2000). Thermal hysteresis proteins are also well known from the desiccation tolerant, but generally not cold hardy beetle Tenebrio molitor (Walker et al. 2001). Although similarities in responses, such as the production of heat shock proteins, may be more indicative of a generalized stress response because levels of expression differ (Goto et al. 1998), or because there is little cross-tolerance (Tammariello et al. 1999), it does seem likely that the response to cold hardiness may represent upregulation of pathways already extant for protection against desiccation (Storey and Storey 1996). Similarity in injuries caused by low temperature, especially ice formation, and by desiccation provide additional support for this idea.

Was this article helpful?

0 0

Post a comment