Image analysis
To remove non-linearity of the camera response, we converted all raw photos to linearised TIFF format images using Dcraw (Coffin 2008). Then we rescaled each colour channel to ensure that the 99 % reflectance standard had the corresponding value. After this process, RGB values of each pixel (ranging from 0 – 255 for each colour channel) scale linearly with reflectance of 0 - 100 %. We used the B channel for ultraviolet photos, R, G, and B channels for visible photos, and R and B channels for near-infrared photos because the camera sensors have sensitivity in the corresponding spectral range.
For all images, we measured mean channel values of three body regions: thorax-abdomen (both of which are important for thermoregulation due to haemolymph circulation between them (Rawlins 1980); we refer to thorax for brevity), basal wings, and entire wings. Thus, we measured six different body regions for each specimen: dorsal thorax (DT), dorsal basal wings (DB), dorsal entire wings (DE), ventral thorax (VT), ventral basal wings (VB), and ventral entire wings (VE). Because butterfly wings were symmetrical, we measured wing regions only from one side (left). For each region, we averaged the pixel values of all measured channels to get a mean reflectance value. Then we calculated the mean reflectance of each body part over 1) VIS (320 – 700 nm; animal-visible range), 2) NIR (700 – 1050 nm; animal-invisible near-infrared range), and 3) VIS-NIR (320 – 1050 nm) ranges. For example, VIS reflectance was calculated by (60u + 360v )/(60 + 360), where u andv are the mean reflectance of ultraviolet and visible-range images respectively, and the numbers 60 and 360 are wavebands of the spectral images. We measured the entire wing area for each species (averaged among specimens) and used this as a size index for each species.