On the basis of experiments with Manduca sexta, the tobacco horn-worm, we suggest that the evolution of an aposematic poisonous insect, from the more common, harmless, cryptic type, may simply involve a change to a related food plant containing different toxic properties from those of its usual host. If, however, circumstances favour a switch to certain toxic host plants, a cryptic insect is frequently destined to become warningly coloured 1,5. It is generally agreed that the cryptic life-style is more ‘successful’ than either of the other two, which are relatively rare, and it is difficult to envisage the evolutionary steps necessary to enable a species to change to the more hazardous warning life-style. There are cryptic species, which metabolise or rapidly excrete the toxic substances present or avoid their ingestion by selective feeding aposematic or warningly coloured species which store plant toxins in their tissues unchanged or slightly modified 4 and aposematic species which superficially resemble or mimic toxic species-without actually storing poisonous plant products-or those warningly-coloured non-storers which secrete their own toxins.

INSECTS feeding on toxic plants demonstrate three fairly distinct types of life-style 1–3.

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