Cuticular Composition

Furthermore, many insects, particularly the adults, are protected against the entry of contact insecticides by thick and sclerotized cuticle (Ebeling, 1974). For example, newly molted American cockroach adults picked up three to four times more malathion than those that were darkly tanned (Matsumura, 1959). 

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The surface layers of solids usually differ from the deeper zones of the same specimen in their chemical composition, their degree of lattice perfection (e.g., the frequency of dislocations), their state of stress, and so on. This renders unpalatable the notion of a surface tension in solids, but suggests the existence of a kind of surface energy, unknown in liquids, which it was proposed to designate as cuticular energy. 

Solids possess an energy unknown in typical liquids. This cuticular energy exists because the surface region of innumerable solids has a chemical composition, a frequency of lattice defects, and so on, different from those in the bulk. 

Small solid particles obtained by cooling of vapors, by grinding, or many other methods, usually have a less perfect lattice and more impurity than have bigger crystals of nominally identical composition. Hence, the cuticular energy of the former exceeds that of the latter. 

Terrestrial BMOs have also been widely used for monitoring environmental contaminants. In particular, the lipid-like waxy cuticle layer of various types of plant leaves has been used to monitor residues of HOCs in the atmosphere. However, some of the problems associated with aquatic BMOs apply to terrestrial BMOs as well. For example, Bohme et al. (1999) found that the concentrations of HOCs with log KoaS < 9 (i.e., those compounds that should have attained equilibrium) varied by as much as 37-fold in plant species, after normalization of residue concentrations to levels in ryegrass (Lolium spp.). These authors suggested that differences in cuticular wax composition (quality) were responsible for this deviation from equilibrium partition theory. Other characteristics of plant leaves may affect the amount of kinetically-limited and particle-bound HOCs sampled by plant leaves but to a lesser extent (i.e., <4-fold), these include age, surface area, topography of the surface, and leaf orientation. 

Wagner D., Tissot M. and Gordon D. (2001) Task-related environment alters the cuticular hydrocarbon composition of harvester ants. J. Chem. Ecol. 27, 1805-1819. 

Baker, G., Pepper, J.H., Johnson, L.H. and Hastings, E. (1960). Estimation of the composition of the cuticular wax of the Mormon cricket, Anabrus simplex Hald. 

Dronnet S., Lohou, C., Christides, J.-P. and Bagneres, A.-G. (2006). Cuticular hydrocarbon composition reflects genetic relationship among colonies of the introduced termite Reticulitermes santonensis Feytaud. J. Chem. Ecol., 32, 1027-1042. 

It is generally accepted that insects synthesize a majority of their cuticular hydrocarbons (Nelson and Blomquist, 1995), although studies have shown that dietary hydrocarbons are incorporated into cuticular lipids (Blomquist and Jackson, 1973a). However, for most species it appears that dietary lipid accounts for very small amounts of insect cuticular hydrocarbon. Some inquilines, which use cuticular hydrocarbons in chemical mimicry, synthesize hydrocarbons with a composition very similar to those of their host termites (Howard et al., 1980 see also Chapter 14). A number of studies with widely diverse insect species have established that the major site of hydrocarbon biosynthesis occurs in the cells... 

Hadley, N.F. (1981). Cuticular lipids of terrestrial plants and arthropods a comparison of their structure, composition, and waterproofing function. Biol. Rev., 56, 23 47. 

Kaib, M., Eisermann, B., Schoeters, E., Billen, J., Franke, S. and Francke, W. (2000). Task-related variation of postpharyngeal and cuticular hydrocarbon compositions in the ant Myrmicaria eumenoides. J. Comp. Physiol. A, 186,939-948. 

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. 

Transpiration through the cuticle involves more than just the single step of diffusion through the epicuticular lipid layer. Molecules of water must leave the tissues adjacent to the cuticle, diffuse through the cuticle itself, enter the lipid layer, diffuse across the lipids, and enter the gas phase outside the animal. Each step is likely to be affected by temperature to a different extent. Lipid composition and physical properties can also differ from one region of the cuticle to the next, so that the biophysical details of cuticular transpiration may not be homogeneous across the entire animal. Thus, transpiration at the organismal level involves multiple steps, and parallel routes for water flux. 

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... 

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