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Figure 4.1 Arrhenius plot of the effect of temperature on water loss from a grasshopper (Melanoplus sanguinipes).

Note: The plot is biphasic, with a break point at 42°C, and the same break point is obtained when the data are plotted without correction for vapour pressure deficit. Source: Gibbs (1998).

Study of the biophysical properties of cuticular lipids is difficult, because of the small amounts available and the complexity of the lipid mixtures. However, in recent years, improved technology has been available in the form of Fourier transform infrared spectroscopy (FTIR), used extensively by Gibbs and colleagues to investigate phase behaviour in 5-50 mg samples of surface lipids. For example, lipids extracted from the exuviae of individual grasshoppers showed progressively higher levels of n-alkanes and higher melting points during acclimation to higher temperatures (Gibbs and Mousseau 1994). Lipid melting points and transition temperatures are highly correlated in the same individuals (Fig. 4.2). In general, data from model membranes, in vitro preparations and intact insects tend to support the lipid melting model for cuticle permeability. However, there are still inconsistencies in the relationships between water loss and the chemical and physical properties of cuticular lipids (Gibbs 2002b). For example, apparently adaptive changes in cuticular lipids of Drosophila mojavensis during thermal acclimation do not result in reduced water loss (Gibbs et al. 1998).

Dramatic regional differences in permeability are seen in the special case of evaporative cooling in desert cicadas (see also Section 6.3.3). Diceroprocta apache (Homoptera, Cicadidae) is active at

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