Glowing insects like fireflies, fire beetles and worms are a romantic, magical sight – but the fossil record of their light-producing adaptations is sparse.
A beetle has now shone some of its light on this conundrum. It was unsuspectingly trapped and exquisitely preserved in golden amber in a dinosaur-dominant rainforest ecosystem about 100 million years ago.
“With over 3500 described species, beetles are the most diverse light-producing organisms on land, but when and how their pyrotechnic abilities came about has long remained a mystery,” says Erik Tihelka, from the University of Bristol, UK, co-author of a study published in the journal Proceedings of the Royal Society B.
The newly discovered specimen reveals that “a radiation of bioluminescent beetles occurred by the mid-Cretaceous” and had already begun to diversify then.
Light production is thought to have evolved in beetles’ vulnerable, soft larvae as a defence mechanism and to have been taken up by adults for other functions, such as communicating and attracting mates. Some females even use it as a predatory lure – to attract clueless males to eat.
The new beetle fossil was discovered by senior author Chenyang Cai, from the Chinese Academy of Sciences, in Myanmar’s northern Kachin State, bordering China, which has a treasure trove of Burmese amber fossils.
When searching through the specimens, one caught his eye – a medium-sized male beetle with unusual, 12-segmented branching antennae.
Closer inspection using phylogenetic analysis revealed a small whitish area on the beetle’s abdomen that looks dark and hairless under an epifluorescent microscope. Its position, shape and structure corresponded to the light organ seen in fireflies and glow-worm beetles, confirming that the males could produce light.
An extinct relative of the firefly, the newly named Cretophengodes azari sp. nov is most similar to modern light-producing beetle families Rhagophthalmidae and Phengodidae, which produce an array of colours from green to red.
But its distinct features have earned it a place in its own new family, Cretophengodidae, adding another branch to the rich Elateroidea superfamily tree that has around 24,000 species described so far.
The age of the fossil coincides with the diversification of insect-eating animals such as frogs and bird-like dinosaurs, suggesting the elateroid beetle used luminescence to ward off its hungry predators.
It’s possible “that early bird-like dinosaurs could have pressured beetles into developing this novel anti-predatory strategy,” says Tihelka. “Digging animals that fed on the forest floor such as early mammals may have also played a role.
“Glowing fireflies dancing on warm summer nights have been regarded as messengers of love and a symbol of elegance for centuries,” he adds. “I find it fascinating that we may owe the glow of fireflies to an ancient predator-prey arms race with the dinosaurs.”