Morphological and physiological differences between diurnal and nocturnal Lepidoptera

Trait-environment relationships drive species distribution, life histories, and biological responses to climate change, but research typically focuses on variations in temperature and solar radiation on a broad geographical scale. However, in the same location, environmental conditions can vary substantially during the day, with diel fluctuations often matching or exceeding those across latitudinal gradients. Compared to their diurnal counterparts, nocturnal Lepidoptera are exposed to cooler temperatures during activity. Therefore, nocturnal species may benefit from being larger and lighter coloured, as larger bodies reduce heat loss, while the absence of UVB radiation at night alleviates the selective pressure for costly melanin production. Simultaneously, nocturnal species are more energetically constrained, because heat cannot be absorbed through basking but must be maintained metabolically (e.g. pre-flight warm-up). We investigate whether nocturnal moths, (a) are larger in body size and lighter coloured than diurnal butterflies and moths, and (b) show a weaker increase in wing area relative to body size than diurnal species (1361 species). We further investigate whether nocturnal species (c) offset relatively higher energetic costs of activity at night by feeding on nectar or reducing the number of generations per year (242 species). Nocturnal species were larger and lighter coloured than diurnals. These differences were phylogenetically independent, suggesting that diel activity has an immediate effect on these traits. Unexpectedly, nocturnal species did not compensate for the energetic cost of nocturnal flight by offsetting a high wingbeat frequency via nectar feeding. However, a larger body area was generally associated with fewer generations per year, whereas only diurnal moths with a higher wingbeat frequency were capable of producing more generations. Overall, our results suggest that diel activity is an important factor that influences the energy budget of Lepidoptera by shaping the expression of morphological traits, whereas the activity-dependent relationship between wingbeat frequency and voltinism in moths emphasise broadly different life-history strategies even between closely related species.