Phenols

Phenols and quinones are both found widely among insects, but so far have been found mainly in Coleoptera, Orthoptera (crickets and locusts), Isoptera (termites) and Dictyoptera (cockroaches). Phenols provide protection upwards against predators and downwards against microorganisms. They can be formed through a variety of biosynthetic routes. Some phenols have already been encountered among acetogenins (Chapter 4). They can also be formed from phenylpyruvic acid as in the formation of tyrosine from phenylalanine (Figure 8.3). Phenol itself is widely scattered, but not frequently encountered, from millipedes (diplo-pods) and opilionids (daddy-longlegs, or harvestmen) to grasshoppers and beetles. Beetles are the most frequent users of phenols, often mixed with other compounds. o-Cresol and m-cresol are common in beetle defensive glands and /?-ethylphenol is found in the glands of the cockroach Periplaneta americana and is probably responsible for that insect's characteristic odour. Both o-cresol and p-cresol are known in the defensive secretion of a grasshopper Romalea microptera. Phenols like salicylaldehyde that are obtained from plants on which insects feed are considered in Chapter 10. The large metapleural glands of the ant Crematogaster deformis contain a mixture of m-substituted phenols and resorcinols shown in Figure 8.10. The presence of mellein (see Figure 4.3) with the phenols suggests they are all of polyketide origin.

Simple phenols are found occasionally in the defensive secretion of millipedes and more commonly in opilionids. Three species of millipede have been shown to produce phenol and 2-methoxyphenol from tyrosine, but phenylalanine is not incorporated, which is an indication they are unable to hydroxylate phenylalanine. Other alkylphenols {e.g. 2,3-dimethylphenol, guaiacol or O-methylcatechol, and 5-ethyl-2-methylphenol, of unknown origin) are present in some opilionids. The centipede Scolopendra subspinipes multilans (Plate 5) secretes 8-hydroxyisocoumarin or centipedin (Figure 8.10), which has an antibiotic effect. In biosynthetic studies [14C]acetic acid was efficiently incorporated, indicating it is probably produced through a polyketide like mellein.

Two species of ticks (Amblyomma americanum (Plate 14) and A. macu-latum) were found to use the unusual 2,6-dichlorophenol (Figure 8.10) as

mellein oh

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