Passive thermoregulation is an important strategy to prevent overheating in thermally challenging environments. Can the diversity of optical properties found in Christmas beetles (Rutelinae) be an advantage to keep cool? We measured changes in temperature of the elytra of 26 species of Christmas beetles, exclusively due to direct radiation from a solar simulator in visible (VIS: 400–700 nm) and near-infrared (NIR: 700–1700 nm) wavebands. Then, we evaluated if the optical properties of elytra could predict their steady state temperature and heating rates while controlling for size. We found that higher absorptivity increases the heating rate and final steady state of the beetle elytra in a biologically significant range (3 to 5°C). There was substantial variation in the absorptivity of Christmas beetle elytra; this variation was achieved by different combinations of reflectivity and transmissivity in both VIS and NIR. The size was an important factor in predicting the change in temperature of the elytra after 5 min (steady state) but not the maximum heating rate. Lastly, we show that the presence of the elytra covering the body of the beetle can reduce the heating of the body itself. We propose that beetle elytra can act as a semi-insulating layer to enable passive thermoregulation through high reflectivity of elytra, resulting in low absorptivity of solar radiation. Alternatively, if beetle elytra absorb a high proportion of solar radiation, they may reduce heat transfer from the elytra to the body through behavioural or physiological mechanisms.

In this experiment, we correlate the heating of the beetle elytra and body with its optical properties. Thus, our data set includes files of the spectral reflectance and transmittance of 26 species of Christmas beetles measured with a dual spectrometer in the range between 400 to 1700 nm. We combined the raw spectra with the theoretical irradiance data from the sun and the irradiance of the solar simulator used in our experiments (from the manufacturer) to calculate reflectivity, transmissivity, and absorptivity. Files and original code provided. On the other hand, the temperature of the beetle body and beetle elytra were recorded with thermocouples in a controlled environment. We provide the raw data of the temperature for each experiment consisting of a cycle of 3 rounds of illumination for 5 minutes and 10 minutes of darkness in between, where each illumination was in a different waveband: total spectrum of the solar simulator, only NIR light (700 to 1400 nm) and only visible light (400 to 700 nm). Finally, we also provide two files of the processed consolidated data: one for the single elytron experiments and one for the full body experiments. In these files, we listed the optical properties, change in temperature (both maximum heating rate and total change after 5 minutes), and size for each sample. These are the ones used in our statistical analysis. Code for the data processing and final analysis is also included.

All the raw files are in format .csv which should be readable by multiple software including office, R, and text processors. R and R studio are needed to open the .Rmd original code. We also include a .html interactive version of the code for easy implementation.Funding provided by: Australian Research CouncilCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100000923Award Number: DP190102203, FT180100216