2.3 | Environmental data
Based on the predictions of the thermal melanism hypothesis and Bergmann’s rule, we considered annual mean temperature as a predictor of geographical patterns in the colour lightness and body size of the assemblages of odonates. In addition, annual precipitation (AP) was included as a predictor because, in insects, lower surface-area to volume ratios of larger species are hypothesised to be advantageous under dry climate due to the associated reduction of water loss through the cuticle (Kühsel et al., 2017; Remmert, 1981) and because of the protective function of melanin against pathogens under humid climates (e.g., Delhey, 2019; Rapoport, 1969; Stelbrink et al., 2019). Note that a principal component analysis based on the correlation of all 19 commonly used bioclimatic variables indicated that each of the two selected variables strongly contributed to the main annual trend in temperature and precipitation (Table S1). We considered only annual mean temperature and annual precipitation, rather than all 19 commonly used bioclimatic variables, to facilitate interpretations of their effects and comparability with other studies (e.g., Pinkert et al., 2017; Zeuss et al., 2017). Climate variables used in the analysis were extracted from climate data with a resolution of 2.5 arcminutes (retrieved from www.chelsa-climate.org; Karger et al., 2017, 2018), based on the geographical coordinates of the assemblages included in our analyses (with a buffer radius of approximately 1 km).