Pretty cool beetles: Can manipulation of visible and near-infrared sunlight prevent overheating?

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.