This paper shows that metal concentrations that are non-lethal for mayfly larvae can become lethal when those larvae undergo metamorphosis to become winged adults. This points to an intriguing mechanism that may explain field patterns in which insect emergence is a more sensitive indicator of stream pollution than larval densities (Schmidt et al. 2013). In short, metamorphosis is a stressful event, and when combined with other stressors, metamorphosis can magnify their effects. The result is a disconnect between responses in the water (larvae) and responses in the air (emerged adults).