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).