How the Christmas Beetle Plans to Stay Cool During This Hot Holiday Season

Melbourne University researchers have discovered that the colourful and Australian Christmas beetle may use its hard outer wings or elytra to keep cool under direct sunlight, but not in the expected way. Image supplied by Laura Ospina-Rozo/ University of Melbourne

Researchers from Melbourne University have discovered how Australia’s Christmas beetles moderate their body temperature through the use of their colourful hard-shell.

There are approximately 36 species of Christmas beetle in Australia and the colour of different species’ “elytra” or hard shell can vary widely. The body temperature of some small animals like insects is directly dependent on their environment and even small changes can impact their fitness.

Christmas beetles are most active from December through to January, which in Australia is the summer period, featuring high temperatures, strong sunlight, and low humidity. In this climate, poor heat regulation could force these beetles to search for shelter instead of a mate, resulting in extinction.

In research performed by Melbourne University scientists and the Carnegie Museum of Natural History, the elytra of Christmas beetles was found to be a potential form of protection against sunlight.

The elytra of most beetles are only open during flight because they protect the secondary wings and normally comprise the largest area exposed to sunlight.    

Researchers suggest the beetles may use their shells to not only protect them from sunlight through reflection but also by trapping heat.

Melbourne University researchers suggest that Christmas beetles may use their elytra to not only shield their body from sunlight through reflection, but also trap the heat away from their body through transmittance and absorption. Image supplied by Laura Ospina-Rozo/University of Melbourne

The Structure of the Elytra

In an email to The Epoch Times, one of the researchers conducting the study, PhD student Laura Ospina-Rozo said that the elytra can vary in both colour and structure.

“Indeed, most of the palette and variation in appearance in Christmas beetles’ elytra, is created by the nanostructures in their hard shells,” said Ospina-Rozo

She said that there was an air-gap between the elytra and the body.

“The air gap, also known as sub-elytral cavity, is formed between the body and the elytra. The second pair of wings are commonly found folded inside this cavity,” she said.

“Their properties could also alter the heat transfer from the elytra towards the body.”

Paul Cooper, the editor-in-chief of the Australian Journal of Zoology and lecturer at the Australian National University, told The Epoch Times that different species’ elytra structures vary slightly.

Cooper said most Christmas beetles had similar round-shaped elytra.

“If someone was going to study this, they'd need to do a more thorough study on the morphological characteristics of the elytra and how the wings are folded underneath,” he said.

“Because the wings of beetles actually have a little hinge, and they fold under the elytra and the elytra protect the wings from damage.”

He said that the protection provided by the elytra is an important component when Christmas beetles are moving around on a plant as opposed to when they’re flying.

How it Keeps the Bugs Cool?

Although previous studies have looked at the elytra’s capacity to reflect light, not much was known about how elytra may absorb and transmit it.

They study examined 28 species of Christmas beetles, which had elytra in a range of different colours, including black, pearlescent white and gold, iridescent red, and green.

The researchers illuminated one elytron from each species with a solar simulator, to recreate the sun, and measured how much light was reflected off or transmitted through the wing, using a spectrometer.

The light that was shone through the elytron, but was neither reflected nor transmitted, was absorbed by the hard wings.

This experiment demonstrated that there are small but significant heating differences between elytra of varying colours, and that the body heats up less when covered by the elytra. The researchers also discovered that the amount of light absorbed by the elytra varies broadly between different species and that absorption occurs through a combination of reflection and transmittance.

The scientists proposed that Christmas beetles can keep cool using two different methods: high reflectivity or high absorption and low transmittance to trap heat away from their body in the elytra. They suggested that the trapped heat is then directed away through air circulation within the hard shell.

Researchers are now investigating Christmas beetles living in warmer climates, which are exposed to more solar radiation, higher temperatures, and smaller amounts of vegetation cover. They intend to determine whether most species of the beetle use the same technique for thermoregulation or individual species develop varying techniques in accordance with their habitat.

They also hope to correlate the findings of this study with other intriguing optic tricks of the Christmas beetle, such as light polarisation.

What About the Colour of the Shell?

Ospina-Rozo said that from an optics or materials perspective the elytra’s colour is directly related to how much light it can reflect or transmit.

“Our aim was to prove that the differences in colour are enough to cause a difference in temperature,” Ospina-Rozo said.

“Some species may take advantage of this phenomenon for thermoregulation, but others may not.”

“Additional ecological and behavioural data remains to be collected to determine which species have evolved their colours for thermoregulation, camouflage or any other biological advantages,” she said, noting that colour may be used for multiple purposes for some species.

Meanwhile, Cooper said that colours could simply be a case of helping beetles recognise their species for reproduction.

“They do use pheromones to ensure that males recognize the females, but there can be mistakes made,” he said.

“That source of coloration is kind of one of these tricky things, people like to think that why we see colors are important, but it may be different things for the insects themselves.”

ANU contract lecturer, Paul Cooper said that Christmas beetles do take up colours from their food, but the beetles can only reproduce with beetles of the same species, so recognition is one of the important aspects. Image supplied by Laura Ospina-Rozo/University of Melbourne