Butterflies play a crucial role in understanding the spread of life due to their complex thermal adaptations. The cooling capacity provided by wing scales is a dominant factor associated with the ambient temperature of their habitats. However, it remains unclear how the wing scale structure of butterflies varies to regulate cooling capacity and participate in shaping distribution patterns.

Based on data acquired from quantitative measurements, rank sum and ANOVA tests were used to test whether the structure and cooling capacity of wing scales responded to butterfly distribution. Virtual simulations and Spearman tests were used to confirm the correlation between the structure and cooling capacity of wing scales. The response was first resolved using three representative species with gradient differences in distribution, and then macroscopically validated using 99 species within their phylogenetic framework, with a presampling control to exclude potential effects of taxonomic position, body size, and migratory behaviour. Both optical and thermal properties were used to measure the cooling capacity. Thermal data generated from specimens in different states were used to exclude the effects of other thermoregulatory pathways.

The results show that the cooling capacity of butterfly wings decreases and becomes more homogeneous as the temperature of their habitat decreases. The decrease in cooling capacity is due to the decrease in maximum emissivity, while the homogenisation is due to both the decrease in maximum emissivity and the increase in minimum emissivity. Variation in cooling capacity is due to changes in the structure of wing scale, which homogenises as the habitat becomes colder. As butterflies generally spread from the tropics to temperate zones, it is inferred that the generation of a low overall cooling capacity through structural homogenisation of wing scales has supported the dispersal of butterflies.

For the first time, we provide cascading evidence for the links between butterfly distribution, thermal adaptation, and functional morphology. We also highlight the role of structural homogenisation on the poikilothermic body surface in the adaptive process for dispersal. Further investigation using genetic information would be beneficial to resolve the mechanism behind thermal adaptation at a deeper level.