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The phagostimulatory response of some diabroticite cucumber beetles toward triterpene cucurbitacins is used as a model in support of an alternative hypothesis explaining the evolution of pharmacophagous feeding behavior in insects. Whereas the use of noxious compounds from nonhost sources for purposes other than nutrition or host-plant recognition (pharmacophagy) has historically been explained in terms of the ancestral host hypothesis, we suggest that the less than perfect specificity of the binding properties of some peripheral receptors provides an opportunity for novel compounds sharing the configuration and polarity of target molecules to elicit a feeding response by coincidence rather than adaptive design.