Christmas beetles may use their hard wings to keep cool

By acting as shields or light traps, the beetles’ hard outer wings may have a role in thermoregulation.

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When Christmas beetles are exposed to direct sunlight, their hard outer wings – known as elytra – could help to limit an increase in their body temperature, University of Melbourne researchers have discovered. The elytra of different species of Christmas beetles vary widely in colour. The beetles may use these colours to avoid overheating, either by shielding the beetle from sunlight or by trapping the sunlight so it can be moved away from the body.

The research was conducted by PhD student Laura Ospina-Rozo, Dr Jegadesan Subbiah and Professor Devi Stuart-Fox, along with a colleague from the Carnegie Museum of Natural History in the USA.

For some small animals, such as insects, body temperature depends directly on the environment. Even small changes in temperature can affect an animal’s fitness – prompting it to shelter from heat rather than feed or mate.

In Australia, Christmas beetles are most active in December and January, a period with low humidity, strong sunlight and high temperatures. Most beetles open their elytra only during flight, so the hard coverings are the largest area of the insects that is exposed to sunlight. When closed, the elytra protect the secondary wings, leaving an air gap between the outermost layer and the body.

Although previous studies had looked at how elytra reflect light, little was known about how they might absorb or transmit it. The researchers examined the elytra of 28 species of Christmas beetles in a range of colours – including black, green, iridescent red, pearlescent white and gold – to see if their differences in appearance are linked to differences in heating.

The researchers illuminated one elytron from each species and used a spectrometer to measure how much of the light was reflected by the elytron or transmitted through it. The light that is neither reflected nor transmitted is absorbed by the elytra. The team also used a solar simulator to recreate heating of the beetles under the sun.

With this set-up, the researchers were able to accurately detect the small but significant differences in heating between the elytra of different colours. They also showed that the body of the beetle heats up less when it is covered by the elytra than when it is exposed.

They found that how much light the elytra absorb varies widely between species and is achieved through different combinations of reflectivity and transmittance. The researchers propose that beetles could keep cool using one of two opposing methods. Whereas some species use high reflectivity to shield their bodies from heat, others use high absorption and low transmittance to trap heat in their elytra, where it can be directed away from the body by recirculating the air immediately underneath the elytra.

Next steps

The researchers are now investigating whether Christmas beetles that live in warmer environments (with higher temperatures, more solar radiation and scarce vegetation cover) have lighter or darker elytra than species living elsewhere. This will help them to determine if most species prefer the same strategy for thermoregulation or if each species uses a different strategy depending on its particular habitat. The team also hopes to correlate its findings with other curious optical tricks produced by Christmas beetles’ elytra, such as light polarisation.

Banner photo: Laura Ospina-Rozo

Funding

Australian Research Council Discovery Project (DP190102203) to Professor Devi Stuart-Fox

Australian Research Council Future Fellowship (FT180100216) to Professor Devi Stuart-Fox

Publication

Ospina-Rozo L et al (2022) Pretty cool beetles: Can manipulation of visible and near-infrared sunlight prevent overheating? Integrative Organismal Biology 4(1): obac036. doi: 10.1093/iob/obac036

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