Aposematism in semi-aquatic caterpillars: warning coloration fails to prevent predation by fish

Abstract Animals display conspicuous coloration to warn predators about their unpalatability. Although many terrestrial caterpillars (Lepidoptera) adopt this strategy, it has never been tested in the few semi-aquatic species. Here, we investigated whether the black-and-orange coloration in the later instars of the semi-aquatic caterpillar Paracles klagesi (Erebidae) could function as a warning display to potential predators. Specifically, we tested whether: (i) a predator (fish, Astronotus ocellatus) has an innate aversion to the orange coloration; (ii) caterpillars are unpleasant; and (iii) predators will learn faster to avoid black-and-orange caterpillars compared with black caterpillars. We found that the fish attacked both types, black and black-and-orange caterpillars, equally often. After attacking, the fish displayed aversive behaviours, demonstrating that caterpillars are unpleasant to fish. In short, fish did not display innate colour avoidance towards black-and-orange patterns, learning to avoid all caterpillars regardless of colour. Warning displays can be multimodal; therefore, fish might use other cues, such as shape, texture, and possibly chemical signals, to recognize and avoid P. klagesi caterpillars. We conclude that orange coloration does not affect fish avoidance learning, and we discuss additional hypotheses suggesting that the dynamics of warning colour in freshwater animals operate in a different way compared with terrestrial environments.