Production and Physiological Role of Cuticular Hydrocarbons

Hydrocarbons are found on the epicuticle of almost all insects. It is however not entirely sure where the hydrocarbons are produced and or where they are stored within the insect. In flies cuticular hydrocarbon production is believed to be under direct control of ecdysteroid hormones that are indirectly influenced by the juvenile hormone (Trabalon et al. 1994; Wicker and Jallon 1995). Genetic feminization studies in Drosophila melanogaster have shown that cuticular hydrocarbon production u

Extract from Dufour gland species A

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'Extract from Dufour gland species B

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Fig. 10.6 Chromatographic analysis of the contents of the Dufour glands from two bumblebee species. The top part shows the chromatograms (mirrored), the bottom part the mass spectra of peak A (from bumblebee species A) and B (from bumblebee species B) respectively

occurs in the sub-cuticular abdominal cells (oenocytes) of adult flies (Ferveur et al. 1997) and since these cells also produce cuticular hydrocarbons in mosquitos, locusts and cockroaches (Schal et al. 1998) they, along with the epidermal cells, are probably the site of cuticular hydrocarbon production in most insects. The hydrocarbons are then transported from the oenocytes by a high-density haemolymph lipoprotein to various target tissues, including the ovaries (since insect eggs also require protection from desiccation), and pheromone or cuticular hydrocarbon-emitting glands (Gobin et al. 2003). It is possible that hydrocarbons are produced in several different glands depending on the function of produced hydrocarbon in insects (Soroker and Hefetz 2000; Thompson et al. 1981). Undecane for example is frequently found in the Dufour gland of Formicine ants, and is thought to have a function as an alarm pheromone (Fujiwara-Tsujii et al. 2006).

The principle role of cuticular hydrocarbons is controlling the trans-cuticular water flux in insects due to their high surface: volume ratio. Although cuticular hydrocarbons account for about 0.1% of total mass of a typical insect, their presence can reduce the insect's permeability to water by up to 1,300% (Edney 1977). Hydrocarbons are very non-polar molecules and hence prevent water from passing through a layer of hydrocarbons. Due to their structure alkanes can be nicely layered and hence they are more suitable for waterproofing then alkenes or methyl branched alkanes. The hydrophobic-ity of cuticular hydrocarbons also prevents the wetting of insects, since a single drop of water would increase the body weight of an insect enormously. Having evolved a wide range of cuticular hydrocarbons to protect against dehydration, the insects had the potential to develop them as signalling molecules. Communicating with chemicals is a fundamental process, which drives speciation and evolution of social structures, in a similar way that communicating by sound, such as language or song, has been a key factor in the evolution of vertebrates. Howard and Blomquist (2005) identified up to six different areas of signalling where cuticular hydrocarbons may play an important role, these are: species and gender recognition, nest-mate recognition, task-specific cues, dominance and fertility cues, chemical mimicry, and acting as primer phero-mones. Hydrocarbons have also been shown to be used as deterrents in ants, although only one example exists of this (Martin et al. 2007),

The rise in the number of studies on cuticular hydrocarbons has been driven by the increased availability of gas chromatography coupled to mass spectrometry (GC-MS). Despite this, the number of cuticular hydrocarbons that have been shown to have a pheromonal effect remains relatively low, especially in relation to the number of compounds identified. Direct evidence is lacking (Dani 2006) in all but a few cases that are highlighted below. In the next paragraph the four main classes of hydrocarbons will be discussed in relation to their physiological role within insects.

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