No evidence for thermal melanism in Canadian butterflies despite long-term lightening of wing colour

Climate change can alter the morphology of ectotherms, yet the extent to which warming shapes colour and body-size traits in insects remains unclear. The thermal melanism hypothesis predicts that lighter-coloured individuals should be favoured in warmer environments because paleness reduces solar heat absorption. Body size may further modify this relationship, as larger individuals gain and lose heat more slowly. Using 991 museum specimens of 10 butterfly species collected across Canada over 54 years, we examined how wing colour paleness and wing length vary across spatial and temporal temperature gradients. We found no relationship between paleness and spatial-temporal variation in summer temperature, and wing length did not modify these patterns. Thus, our results offer no support for the thermal melanism hypothesis at broad geographic scales. In contrast, wing length increased across the spatial-temporal temperature gradient but showed no long-term change. Despite this finding, butterflies became paler over time, indicating that morphological change is occurring independently of macroclimatic warming. Instead, this trend may reflect other environmental pressures, such as land-use change or shifts in larval nutrition. Together, our findings demonstrate that different morphological traits respond differently to climatic change and that spatial-temporal trait-environment relationships may not reliably predict long-term insect responses to climatic warming.