Cuticular waxes of insects

The ability of insects to withstand desiccation was recognized in the 1930s to be due to the epicuticular layer of the cuticle. Wigglesworth (1933) described a complex fatty or waxy substance in the upper layers of the cuticle which he called cuticulin . The presence of hydrocarbons in this wax of insects was suggested by Chibnall et al. (1934) and Blount et al. (1937), and over the next few decades the importance of hydrocarbons in the cuticular wax of insects was established (Baker et al., 1963 and references therein). The first relatively complete chemical analyses of the hydrocarbons from any insect, the American cockroach, Periplaneta americana (Baker et al., 1963), occurred after the development of gas-liquid chromatography (GLC). The three major components of the hydrocarbons of this insect, //-pen taco sane, 3-methylpentacosane and (Z,Z)-6,9-heptacosadiene, represent the three major classes of hydrocarbons on insects, n-alkanes, methyl-branched alkanes and alkenes. Baker and co-workers (1963) were able to identify n-pentacosane by its elution time on GLC to a standard and its inclusion in a 5-angstrom molecular sieve. 3-Methylpentacosane... 

The cuticular waxes of insect species may contain the following chemical classes hydrocarbons, fatty acids, alcohols, triacylglycerols and wax esters (Golgbiowski et al., 2011 Nelson Blomquist, 1995). The waxes of some species also contain aldehydes, ketones, esters and sterols. The wax compositions of insects can vary depending on stage, sex, age, and their position in the colony hierarchy. Cuticular waxes can also vary within species as a response to living conditions such as temperature, dryness and available food. The major function of insect waxes is protection against desiccation, but they also prevent microbial infections, affect the adsorption of chemicals and play a role in chemical communication... 

Blomquist, G.J., Soliday, C.L., Byers, B.A., Brakke, J. W. and Jackson, L.L. (1972). Cuticular lipids of insects V. Cuticular wax esters of secondary alcohols from the grasshoppers Melanoplus packardii and Melanoplus sanguinipes. Lipids, 7, 356-362. 

As work on lipid chemistry has developed, a tremendous number of HMWHCs of one t5fpe or another have been found in high abundance in the lipids of various plants, animals, microorganisms and insect waxes (Nelson Blomquist 1995). High molecular weight esters are components of cuticular waxes of higher plants (C36-C52 Cranwell Volkman 1981). Lipids and insoluble fractions from several freshwater... 

The differences in hydrocarbon patterns in surface waxes and in the components of interior tissue are illustrated by analysis of pupae of Manduca (tobacco hornworm). n-Alkanes only comprised ca 3% of the hydrocarbon fraction of the cuticular wax, the balance being unsaturated compounds. In contrast, internal tissues (fat bodies, muscle, gut) contained the same carbon spectrum (C21 to C41) as in the wax but now branched alkanes made up the bulk of the hydrocarbon fraction ca 80%), followed by n-alkanes (9%) with the residue being unsaturated compounds The proportion of n-alkanes in the hydrocarbon fraction from cuticular wax of a Bombyx silkworm fell from 95 to 35% on passage from the larval to the pupal stage " and similar results have been found for Trichoplusia (cabbage looper) and Drosophilia (fruitfly) species. However, it is likely that the cuticular wax has a more stable composition over the adult life of most insects and is only synthesized at (low) rates sufficient to replace that lost by wear and tear. The site of synthesis has been demonstrated to be in the cuticle in a cockroach species no hydrocarbon synthesis occurred in preparations from fat bodies . 

Waxes are biosynthesized by plants (e.g., leaf cuticular coatings) and insects (e.g., beeswax). Their chemical constituents vary with plant or animal type, but are mainly esters made from long-chain alcohols (C22-C34) and fatty acids with even carbon numbers dominant (Fig. 7.11). They may also contain alkanes, secondary alcohols, and ketones. The majority of wax components are fully saturated. The ester in waxes is more resistant to hydrolysis than the ester in triacylglycerols, which makes waxes less vulnerable to degradation, and therefore more likely to survive archaeologically. 

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. 

Lange, C., Basselier, J.-J., Bagneres, A.-G., Escoubas, P., Lemaire, M., Lenoir, A., Clement, J.-L., Bonavita-Cougourdan, A., Trabalon, M. and Campan, M. (1989). Strategy for the analysis of cuticular hydrocarbon waxes from insects using gas chromatography with electron impact and chemical ionization. Biomed. Environ. Mass. Spectrom., 18, 787-800. 

The external cuticle of insects is covered by a waxy layer composed of mixtures of hydro-phobic lipids that include long-chain alkanes, alkenes, wax esters, fatty acids, alcohols, aldehydes, and sterols. The primary purpose of this layer is to maintain water balance and prevent desiccation, as described in Chapter 6, but many of the cuticular lipid components have important secondary roles as intraspecific contact chemical signals (pheromones). These roles include species and sex recognition during reproductive interactions, and nestmate recognition and other colony organization functions in social insects. Thus, these compounds are essential mediators of insect behaviors. Cuticular compounds are also exploited by parasitoids and predators as interspecific contact cues (kairomones) to aid in host location. 

First, almost all studies of insect cuticular lipids have used gas chromatography (GC) to analyze lipid extracts, using standard GC conditions that only allow compounds under C40 to be detected. More specialized GC equipment that can extend this range to >C60, in particular, columns that can withstand high temperatures (>400°C) are now available. However, they have not yet found routine use for cuticular lipid analysis, despite recent studies that have clearly demonstrated that cuticular lipids do indeed contain hydrocarbons, waxes, and other compounds with molecular weights above 500 daltons (Cvacka et ai, 2006). Thus, to date, many studies may only have examined subsets of the cuticular components. 

The arthropods, especially the insects, exhibit probably the greatest ability to synthesize and utilize alkanes of any class of the Animal Kingdom. Their external surfaces are covered with cuticular waxes that provide a barrier which is impervious to water and prevents invasion by micro-organisms as well as providing general protection. This barrier may also effect (both positively and negatively) the penetration of insecticides. The waxes contain a wealth of hydrocarbons with n-alkanes sometimes predominating. Over one-hundred hydrocarbons have also been isolated and characterized from internal... 

More vigorous synthesis is likely in glandular tissue where hydrocarbons are lost as pheromonal and defensive secretions. It is generally considered that the composition of cuticular lipids is not correlated with the diet of insects, although the latter may have some ecological significance as indicated by recent work on insect-insect predation. Nevertheless, it has frequently been observed that different insect species and their parasites which live in association have similar cuticular waxes that often also resemble the wax from the food plant... 

Gas Chromatographic Analysis of Plant and Insect Surface Compounds Cuticular Waxes and Terpenoids... 

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. 

Most plants have trichomes on their aerial surfaces. The trichomes may be simple hairs or more specialized glandular trichomes, whose main function may be the production and accumulation of chemicals such as essential oils. The vast majority of these consists of monoterpenoids, sesquiterpenoids and diterpenoids with a high vapour pressure. They may be absorbed on the cuticular wax layer. The trichome secretions are closely related to plant-insect or plant-microbe interactions. Terpenoids can attract, rep>el or initiate defence reactions in insects. Apart from their ecological roles, plant terpenoids are widely used in the pharmaceutical and fragrance industries. The properties of essential oils are correlated with their qualitative and quantitative compositions. 

The surfaces of insects are also covered by a layer of wax. Insect cuticular waxes are also involved in various types of chemical communication between individuals of a species and reduce the penetration of chemicals and toxins as well as infectious microorganisms. Analyses and identification of insect waxes is the first step towards developing methods of insect control. 

Improvements in analytical techniques have led to the characterization of plant and insect surface compounds and provided new insights in chemical ecology. Moreover, an enormous number of plant terpenoid analyses are associated with their pharmaceutical and fragrance applications. Qualitative and quantitative analyses of cuticular waxes and terpenoids are usually achieved by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). Peak identification is based primarily on retention times, retention indices and comparison of recorded spectra with an MS library. This review will describe gas... 

Cuticular waxes are commonly present on the surfaces of higher plant and insect species, but they often differ from one another in composition. Generally speaking, cuticular waxes consist of complex mixtures of long-chain nonpolar compounds, with the molecular weights of individual components ranging from about 200 to 600 Da, but from time to time exceeding 1000 Da... 

Wax extracts are separated into classes of waxes by TLC, LC or HPLC Table 1 gives examples of the methods for separating insect cuticular waxes. 

Table 1. Examples of the isolation and separation of insect cuticular waxes...

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