2.2 | Trait data
Following the most common approach used in the analysis of morphological traits based on digital images (Zeuss et al., 2014, 2017; Pinkert et al., 2017), we calculated the average colour lightness and body volume of species using drawings of European Odonata (Dijkstra & Lewington, 2006). To prepare images for the analysis, the body (head, abdomen, and thorax) in scanned drawings of species’ dorsal body surfaces (24-bits, sRGB, 1200 dpi resolution) was cropped out and saved to separate files using functions of Adobe Photoshop CS2. Based on these images the average colour of the pixels of an image across the red, green and blue channels was calculated as an estimate of the colour lightness of a species (8-bit grey values ranging from 0: absolute black to 255: pure white). In addition, these images were scaled with the magnification factor provided in Dijkstra and Lewington (2006) and used to calculate body volume in cm3 (π × [½ length of pixel row]2 × pixel edge length) as an estimate of the body size of a species based on the assumption that odonates generally have a cylindrical body form. The calculations were performed using functions of the R-package png(Urbanek, 2013). Body volume instead of linear size measures, such as wing length, head width and body length, was used because as a three-dimensional measure it allows for a more realistic estimate of the body mass of a species (Kühsel et al., 2017). Note that previous studies showed that the colour lightness and body volume estimates are correlated between drawings from different sources and between males and females (Pinkert et al., 2017, Zeuss et al., 2017). Subsequently, the average colour lightness and body volume were calculated across the species of each local assemblage. Previous studies found that the colour lightness and body size, as well as the habitat preference of European odonates, carries a phylogenetic signal (Zeuss et al. 2014; Letsch et al., 2016; Pinkert et al., 2017). Based on a recent phylogeny of the European odonates (Pinkert et al., 2018), we, therefore, partitioned the total variance of the colour lightness and body volume into a phylogenetic component and a species-specific component using Lynch’s comparative method (Lynch, 1991), implemented in the R-package ape (Paradis et al., 2004). The phylogenetic component represents the variation in colour lightness and body volume predicted by the phylogenetic relationships of the species, whereas the species-specific component is the difference of the observed trait estimate from the phylogenetically predicted part. The advantage of this method is that it allows assessing the effect of phylogenetic signals in traits (i.e., P - component) that is often neglected as a source of bias, in addition to the model results that have been corrected for phylogenetic autocorrelation (i.e., S - component).