SummaryAdults of forty-five aposematic insect species from four orders, and/or, in eight cases, their early stages, were examined for the presence of 2-methoxy-3-alkylpyrazines by coupled gas chromatography/mass spectrometry. The survey also included eight cryptic and related species and six unrelated mimics, twelve selected hostplants of those herbivores that had proved positive for such pyrazines, and nine well known toxic plants. Positive results were as follows: aposematic adult species, 26 (58 %); obscurely coloured species, 1 (12.5%); mimics, 2 (33%); host-plants, 5 (42%); well-known toxic plants, 4 (44%). Among the taxonomic groups, these pyrazines were found in the lepidopterän families and subfamilies Papilionidae, Danainae, Heliconiinae, Acraeinae, Zygaenidae and Arctiidae, and among the Coleoptera, in the Lycidae, Cantharidae, Endomychidae, Coccinellidae, Oedemeridae and Meloidae. Single cases of the occurrence of these pyrazines were also detected in the Hemiptera (Cercopidae) and Orthoptera (Pyrgomorphidae). Mimics were found with and without the pyrazines. Among the plants they were detected in Aristolochiaceae, Asclepiadaceae, Asteraceae, Papaveraceae, Passifloraceae, Solanaceae and Urticaceae. Some aposematie and phytophagous insects apparently sequester pyrazines from their hostplants whereas others, particularly the predatory species, manufacture these substances de novo. Alerting signals in general are discussed.

Between-tree variation in the terpenoid composition in the foliage of six species of Eucalyptus was investigated in relation to its effects on herbivory by Christmas beetles (Anoplognathus spp.). All six eucalypt species showed considerable intraspecific variation in terpenoid composition; the cineole content ranged from 13% to 78% of the total oil in different E. melliodora trees, from 0% to 67% in E. conica, 3% to 79% in E. sideroxylon, 1% to 76% in E. camaldulensis, 3% to 60% in E. rubida and 20% to 79% in E. blakelyi. Levels of defoliation by Christmas beetles of nine E. melliodora and eight E. conica trees were quantified from frass traps placed under each tree, and were used to confirm the reliability of visual ratings of defoliation. Defoliation was assessed visually for all six species and was found to be unrelated to the total amount of terpenoids in the foliage of each tree, but strongly associated with the percentage of cineole in the terpenoid mixture. Levels of most terpenoid components were significantly inter-correlated, so it was not possible to determine which components directly affected defoliation. The dominant Christmas beetle at all sites was A. montanus.

2-Methoxy-3-alkylpyrazines have been identified as odour components in Danaus plxippus, agaena lonicerae and an Australian Amata species, by means of coupled gas chromatography/mass spectrometry, with selected ion monitoring. Wide variability in pyrazine content was observed with D . p1ex;PPus and this was correlated with similar variability in the larval food-plants, Asclepias spp., which are presumed to be the odour sources for this butterfly. The importance of pyrazines in the biology of aposematic insects and their role as ingredients of a generalized warning system are discussed.

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