The effect of signal appearance and distance on detection risk in an aposematic butterfly larva (Parnassius apollo)

Aposematic coloration commonly involves patterns with contrasting colours. The early larva of Parnassius apollo is uniformly black, but the later instars develop two rows of dorsal orange spots. We tested the effect of these orange markings on the conspicuousness of the last-instar larva, by manipulating larval coloration in photographs from a natural habitat and measuring how fast human subjects could discover the larva on a touch screen. In the first experiment we compared the detectability of the natural phenotype with that of manipulated uniformly black and uniformly orange variants at different distances. In the second experiment with another set of photographs we added manipulations involving enlarged and reduced spot sizes. Generally, detection time increased with distance, and decreased with the proportion of orange in the coloration. The uniformly black larvae were surprisingly hard to detect even at the closest distances, suggesting that the young black instar has an efficient camouflage. Furthermore, even a small amount of orange colour increased the conspicuousness of the larva considerably, indicating that the orange markings are costly in terms of detectability. Importantly, as the increase in detection time over distances was larger for the natural coloration than for the orange coloration, we suggest that the natural coloration may involve a distance-dependent switch from conspicuousness to camouflage with increasing distance. Thus, even though the orange markings most probably have a signalling function, the coloration is not maximized for either crypsis or conspicuousness.