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Pyrrolizidine alkaloids (PA) have been identified in six species of Arctiidae reared on Senecio and Crotalaria. These include senecionine, seneciphylline, integerrimine, jacobine, jacozine, jacoline, jaconine and a metabolite (C,,H,,NO,) from Senecio, and monocrotaline, trichodesmine and crispatine from Crotalaria.The all-red aberration of Tyriajacobaeae (var. conyij contained much less of the metabolite than normal examples of this species. Female Spilosoma lutea reared on the same plants of S. Jhcobaea contained markedly more jacobine and jacoline than the males.Host plant relationships and secondaly plant substances are discussed. It is suggested that the Arctiid moths' own deterrent secretions, directed against vertebrate predators, pre-adapts them for reeding on foliage likewise protected against large herbivores by toxic secondary plant substances such as cardenolides and pyrrolizidine alkaloids. These latter substances are more toxic to vertebrate than to insect herbivores, and their dual function of deterrent and insect aphrodisiac puts a premium on their sequestration and storage once a species has achieved the initial steps, and occupied the plant niche concerned. It is further suggested that the polyphagous habits of the Arctiidae result in a more equitable distribution of the secondary plant substances within the Miillerian complex concerned, thus providing a generalized warning message for the potential vertebrate predator.KEY WORDS:-arctiid moths -host plant relationships -pyrrolizidine alkaloid storage.

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