Stress-mediated covariance between nano-structural architecture and ultraviolet butterfly coloration

1. Structural coloration is a striking component of sexual ornamentation, and may function as a signal of mate quality. Although the proximate optical mechanisms are often well defined, we know much less about the morphological basis for intraspecific variation in structural colour. 2. Males of the butterfly Colias eurytheme L. possess a thin-film interference array on their dorsal wing scales that generates a bright and iridescent ultraviolet (UV) signal. This signal is used in mate choice. 3. Using scanning electron microscopy, we investigated the covariance between nanostructural architecture and UV reflectance in samples that were variously subject to either nutrient stress (using a larval host-plant manipulation), or thermal stress (using transient heat and cold shocks during the pupal period). We employed these two stressors to mimic natural stressful processes and to accentuate the variance in UV signal characteristics. 4. Two primary structure–reflectance relationships were apparent. First, UV brightness increased with the density of scale ridges that bear the interference reflectors. Second, the breadth of above-wing angles for viewing the UV covaried with a measure of thin-film angular orientation. These relationships were, however, either limited to, or stronger among, males of the nutrient stress sample. 5. Our results are consistent with a causal effect of developmental stress on nanostructural architecture and henceforth UV reflectance, but also suggest that the proximate basis for signal variation may be intimately linked to the nature of prevailing stressors.