Figure 7.1 Representative data for thermolimit respirometry experiments on the tenebrionid beetle, Gonocephalum simplex. (a) 21% and (b) 10% oxygen.
Note: The upper trace shows VCO2, while the lower trace indicates activity in arbitrary units, where both negative and positive values indicate activity. Source: Klok et al. (2004).
In other words, the critical thermal maximum is not set by a failure of oxygen delivery, but more likely as a consequence of cellular level damage (see also Barclay and Robertson 2000). This is perhaps not surprising given the highly efficient tracheal system that is responsible for gas exchange in insects (Chapter 3).
By contrast, it appears that insufficient aerobic capacity of mitochondria at low temperature might well be important in setting lower critical limits. In Pringleophaga marioni (Lepidoptera, Tineidae) caterpillars, there is a precipitous decline in metabolic rate at the critical thermal minimum (Sinclair et al. 2004). Moreover, in both honeybees and Drosophila, decreasing temperature results in a steady decline in the resting potential of flight muscle neurons, and critical thermal minimum appears to be the temperature at which the Na+/ K+-ATPase pump can no longer maintain nerve cell polarization to a level where action potentials could be produced (Hosler et al. 2000). Thus, lack of energy owing to insufficient aerobic capacity might well set lower limits in insects. In P. marioni caterpillars, cells continue to respire, indicated by no difference in metabolic rate in caterpillars that die from freezing and those that survive (this is a freezing tolerant species), but water loss increases rapidly in the former. This indicates that control at the organismal level, rather than at the cellular level, is lost (Sinclair et al. 2004). This is in keeping with Portner's (2001) hypothesis.
Therefore, although the physiological events needed to attain thermal tolerance in terrestrial insects probably require aerobic conditions, the high and low-temperature responses differ, and are probably not linked by alterations in mitochondrial density. Such profound differences between marine and terrestrial species should not be surprising, given the physical characteristics of the marine and terrestrial environments. For example, temperature change can take place much more rapidly in the terrestrial environment than in the marine situation because air has a lower heat capacity than water. Furthermore, latitudinal variation in temperature variability differs considerably between marine environments, which show low variation at high latitudes, and terrestrial systems, which show most variability at high latitudes. These physical and biological differences between the marine and terrestrial realms may have profound influences on the distribution of diversity within them (Chown et al. 2000).
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