Photography
We photographed 684 butterfly specimens from 343 species held in the
Lepidoptera collections at the Natural History Museum in London (NHMUK).
This includes species from all six families, covering approximately 70
% of all species found in Europe. The distribution of analysed species
ranged from 34° to 70° N and 10° W to 44° E. The mean temperature over a
species’ distribution varied from -2.7 to 18 °C, and the mean
precipitation varied from 353 to 1544 mm per year. For each species, we
selected and photographed two specimens whenever available and chose
specimens with well-preserved wings and body. For the polymorphic
species, we photographed each morph once. We focused on capturing inter-
rather than intra-specific variation because we were primarily
interested in evolutionary factors that have shaped broad-scale
interspecific ecogeographic patterns. We note that most specimens were
collected before 1980; however a previous study found no effect of
specimen age on visible and near-infrared reflectance in butterflies
(Munro et al. 2019) and any potential degradation will contribute
variation within species but is unlikely to mask interspecific variation
or affect biological conclusions at the scale of our analysis.
Photography was done in a dim room using two light bulbs simultaneously:
an LED bulb (True-light LED 12W E27, Frankfurt, Germany CRI index 98,
spectral power distribution for this bulb is provided in Fig. S1) and a
3,000K tungsten-halogen lamp (150W, Long Life Lamp Company, Harrow, UK).
The tungsten-halogen lamp emitted all UV, visible, and NIR light. We set
up these two bulbs approximately 60 cm above the photographic spot. A
full-spectrum converted DSLR camera (Nikon D7000 converted by Lifepixel,
Mukilteo, WA, USA; maximum) was set directly above the photographic
area. We positioned the camera slightly below the bulbs to avoid light
reaching directly to the camera lens (Jenoptik UV-VIS-IR 60 mm 1:4 APO
Macro lens; transmission waveband is between 290 to 1500 nm). We used
lens filters (Baader, Mammendorf, Germany) to capture ultraviolet
(U-Venus filter), visible (UV/IR cut filter), and near-infrared
wavelengths ranges (IR-Pass filter). These filters completely blocked
wavelengths outside the transmission range and enabled us to photograph
each specimen in three spectral ranges: ultraviolet (320 – 380 nm),
visible (400 – 680 nm), and near-infrared (670 – 1050 nm; 1050 nm is
the maximum sensitivity of the camera sensors provided by the
manufacturer) ranges, respectively. Our analysed spectral range (320 –
1050 nm) captures approximately 80% of the energy in solar irradiation.
We changed lens filters with a minimum disturbance to the camera body
using a combination of magnetic lens adapters and filter holders
(Manfrotto, Cassola, Italy).
To fix each specimen, we used a square paper box floored with styrofoam
(17 x 10 x 5 cm). We placed each specimen at the centre with a 99%
reflectance standard (WS-1-SL, Labsphere, NH, USA) on the upper-right
corner of the box. Camera settings were constant (ISO 400, F8) except
for the shutter speed which varied depending on the type of filters (1.3
s for ultraviolet, 1/500 s for visible, 1/800 s for near-infrared
photos). This produced 4,104 images saved in raw format (see Fig. S2 for
sample images).