Toxicity induced in the tobacco horn-worm (Manduca sexta L.) (Sphingidae, Lepidoptera)

Abstract

INSECTS feeding on toxic plants demonstrate three fairly distinct types of life-style^1–3. 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 modified⁴; 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. 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. If, however, circumstances favour a switch to certain toxic host plants, a cryptic insect is frequently destined to become warningly coloured^1,5. 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. Storage and the acquisition of toxicity and warning colour could follow this crucial switch.