Cuticular water loss

Figure 6.2 Correlation between cuticular water-loss and total water-loss in female ants, Pogonomyrmex barbatus. Differences in cuticular transpiration account for 97% of the variation in total water-loss. Different symbols indicate differences in mating status. From Johnson and Gibbs (2004) reproduced with permission.

Finally, even when HC composition and cuticular transpiration are correlated, causation cannot be assumed. For example, higher cuticular water-loss rates in the desert ant, Pogonomyrmex barbatus, are correlated with a decrease in abundance of an n-alkane and an increase in a methylalkane (Figure 6.2 Johnson and Gibbs, 2004). This is exactly what one would expect if lipid melting points affect cuticular permeability, but this increase is also accompanied by a change in mating status. Mated, de-alate queens that have founded... 

Rourke, B. C. (2000). Geographic and altitudinal variation in respiratory and cuticular water loss from the lesser migratory grasshopper, Melanoplus sanguinipes. 

The other long understood and, indeed, fundamental function of insect cuticular lipids is to restrict water loss to prevent a lethal rate of desiccation (Hadley, 1984 Noble-Nesbitt, 1991 Nelson and Blomquist, 1995). Conservation of water is a primary challenge faced by terrestrial animals with high surface area to volume ratio such as insects. The anti-desiccatory function of the cuticular waxes is crucial in meeting this need, and makes them a focused target for insect control. 

Is cuticular permeability important Cuticular and respiratory water-loss... 

Consideration of the waterproofing function of cuticular lipids first requires an assessment of cuticular transpiration relative to the overall water budget. The fact that organismal water-loss rates increase greatly when surface lipids are removed does not necessarily mean that increased cuticular permeability is responsible. Insects can lose water by transpiration through the cuticle, by evaporation from the tracheal system through open spiracles, and by... 

We have, to some extent, set up a straw man in this section, but it is important to recognize that most studies have not distinguished cuticular transpiration from other components of the overall water budget. This is probably not a serious problem for work with inactive insects it may be in other cases. The permeability of the cuticle to water is clearly an important aspect of insect water balance, but rigorous analysis requires a good quantitative understanding of cuticular and other routes for water-loss. Below, we first discuss the role... 

Figure 6.1 Discontinuous gas exchange in a grasshopper, Trimerotropis pallidipennis. Note that total water-loss more than doubles when the spiracles open to allow gas exchange. In this case, cuticular transpiration is easily estimated by measuring water-loss when C02 release is negligible.

Correlations between cuticular lipids and water-loss rates... 

The composition of cuticular lipids varies at all levels of organization in insects, from among species to within individuals. The amount of cuticular lipid can also vary substantially. For example, wax blooms of desert tenebrionid beetles are associated with reduced water-loss (Hadley, 1994). High densities of wax may also serve to reduce heat load by reflecting solar radiation (Hadley, 1994) or to deter predators (Eigenbrode and Espelie, 1995) thus, it cannot be assumed that water conservation is the primary function of wax... 

The majority of publications on cuticular lipids involve analyses of lipid composition. Which compounds are present, and what is their function Correlations between lipid composition and water-loss have provided indirect tests of the phase transition hypothesis, under the assumption that changes in lipid composition predictably affect lipid properties. In this section, we summarize available information on how specific structural changes affect the physical properties of pure surface lipids, as well as how different lipids interact with each other. 

We know the most about cuticular hydrocarbons, because they are abundant and because it is relatively easy to isolate and identify them. They are also the most hydrophobic lipid components, and so should provide the best barrier to water-loss. -Alkanes isolated from insect cuticles typically have chain lengths of 20-40 carbons. These can be modified by insertion of cis double bonds, or addition of one or more methyl groups. Relatively polar surface lipids include alcohols, aldehydes, ketones and wax esters (see Chapter 9). Given this diversity, is it possible to predict lipid phase behavior (and, by extension, waterproofing characteristics) from composition alone If so, a large body of literature would become instantly interpretable in the context of water balance. Unfortunately, this is not the case. 

Figure 6.5 Arrhenius plot of water-loss from a live grasshopper, Melanoplus sanguinipes. Cuticular lipids extracted from this individual melted at 42°C. Data from Rourke and Gibbs (1999).

Parkash, R., Kalra, B. and Sharma, V. (2008a). Changes in cuticular lipids, water loss and desiccation resistance in a tropical drosophilid - analysis of within population variation. Fly, 2,187-197. 

Hadley, N.F. and Savill, A. (1989). Water loss in three subspecies of the New Zealand tiger beetle Neocicindela perhispida correlation with cuticular hydrocarbons. Comp. Biochem. Physiol., 94B, 749-753. 

The surfaces of all higher plants are covered by a layer of cuticular waxes. These are composed mainly of long-chain aliphatic components but also of cyclic compounds. The primary role of the waxes is to prevent uncontrolled water loss. The chemical composition of plant cuticular waxes can affect the resistance of plants to herbivores and herbivore behaviour. Cuticular waxes and their separate components enhance or deter insect oviposition, movement or feeding. 

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