Heated debate: Is the seasonal polyphenism of the map butterfly Araschnia levana driven by thermoregulation?

The seasonal forms of the temperate butterfly Araschnia levana (Nymphalidae: Nymphalinae) differ in mass, wing area, wing loading (i.e., the ratio of mass to wing area), and colouration. Spring individuals are predominantly orange with higher wing loading while summer individuals are black with a white stripe and have lower wing loading. However, it remains unclear if and how these seasonal differences affect heating and cooling dynamics. We compared thermal responses of seasonal forms, focusing on the roles of morphology and colouration. Further, we assessed whether live butterflies heat and cool differently from dead individuals to detect the presence of physiological thermoregulation. Morphology and colouration influenced the thermal dynamics of the thorax and wings as expected from heat-transfer principles, but we found no evidence of physiological thermoregulation on the thorax. Based on aligned temperature curves, seasonal forms of A. levana showed similar thermal dynamics driven by counteracting differences in morphology and colouration: lower wing loading and darker colouration in the summer form compared to spring individuals. After accounting for significant morphological differences between forms, the thorax of spring individuals heated and cooled faster than that of summer ones, despite higher wing loading and lighter wings. This trend suggests form-specific optimisation of thermal performance, likely as a response to temperate climates. Thermal responses differ between forms in ways not directly explained by the polyphenism itself, potentially reflecting a broader trait of multivoltine ectotherms to cope with seasonal temperature changes.