Data and original code for: Polarization and reflectance are linked to climate, size and mechanistic constraints in a group of scarab beetles

Beetles exhibit an extraordinary diversity of brilliant and colourful appearances and optical effects invisible to humans. Their underlying mechanisms have received some attention, but we know little about the ecological variables driving their evolution. Here we investigated environmental correlates of reflectivity and circular polarization in a group of optically diverse beetles (Scarabaeidae–Rutelinae). We quantified the optical properties of 261 specimens representing 46 species using spectrophotometry and calibrated photographs. Then, we examined associations between these properties and environmental variables such as temperature, humidity, and vegetation cover, controlling for body size and phylogenetic relatedness. Our results showed larger beetles have higher visible reflectivity in drier environments. Unexpectedly, near-infrared (NIR) reflectivity was not correlated with ecological variables. However, we found a correlation between humidity and polarization (chiral nanostructures). We identified trade-offs between optical properties: beetles without polarization-associated nanostructures had higher NIR reflectivity. By contrast, visible reflectivity was negatively correlated with the accumulation of pigments such as melanin. Our study highlights the value of a macroecological approach for testing alternative hypotheses to explain the diversity of optical effects in beetles and to understand the link between structure and function.

Methods
This data set was collected from 261 specimens of Australian scarabs representing 53 morphs of 46 species and 9 genera of the subfamily Rutelinae and one species of the subfamily Melolonthinae. 

Reflectivity and transmissivity were collected with spectroscopy methods. Absorptivity was calculated based on these two variables

Circular polarization was studied using calibrated photographs of the beetle specimens in three filters, visible light, left-handed polarized visible light, and right-handed polarized visible light.

The length of the beetles was calculated from the calibrated photographs and used as an indication of size. 

Climate data was extracted from ALA and BioClim. The original code is attached. 

Phylogenetic data was reconstructed from published COI data and additional sequences available in NCBI

Processing:
The linearization and equalization of the photographs were done with a custom-made protocol in MatLab.

Most of the processing of the data was done in R Statistical Software [v4.1.2]. All the packages used are detailed in the annotated code.