A fossil in amber sheds light on the evolution of bioluminescent insects

An insect preserved in amber.

An insect preserved in amber.

Caught in amber for about 100 million years, an exceptionally well-preserved light-producing beetle recently found sheds light on the diversification of bioluminescent beetles in the Cretaceous period and provides the link fossil lost among the living relatives of fireflies, according to researchers in the journal 'Proceedings of the Royal Society B'.

With more than 3,500 described species, light-producing beetles are the most diverse bioluminescent land animals. Fireflies, fire beetles, firefly beetles, and their relatives use light to ward off predators, attract mates, and some females even use it to lure unsuspecting males to eat. Historically, despite its diversity, the evolution of bioluminescence in beetles has been poorly understood.

An exceptionally well preserved fossil

"Most light-producing beetles are soft-bodied and quite small, so they have a very poor fossil record. However, this new fossil, found in amber from northern Myanmar, is exceptionally well preserved, even the lumen organ in his abdomen is intact, "explains Dr Chenyang Cai, a researcher at the University of Bristol and associate professor at NIGPAS.

The presence of a lumen organ in the male's abdomen provides direct evidence that the adults of 'Cretophengodes' were capable of producing light, about 100 million years ago.

"The newly discovered fossil, preserved with real fidelity in amber, represents an extinct relative of fireflies and the living families 'Rhagophthalmidae' and 'Phengodidae'," notes Yan-Da Li of the Nanjing Institute of Geology and Paleontology (NIGP ) and Peking University in China.

most of light-producing beetles belong to the giant superfamily Elateroidea with some 24,000 known species and thousands more waiting to be described. The discovery of this beetle provides the missing fossil link between living families, and in doing so helps scientists understand how these beetles evolved and how they should be classified.

"Elateroidea is one of the most heterogeneous groups of beetles and that has always been very difficult for entomologists to manage, particularly since important anatomical innovations often evolved independently in unrelated groups. The discovery of a new family of extinct elateroid beetles is significant because it helps shed light on the evolution of these fascinating beetles, "explains Erik Tihelka, from the Faculty of Earth Sciences.

"We believe that light production initially evolved in the soft and vulnerable larvae of the beetle as a defensive mechanism to protect themselves from predators. The fossil shows that in the Cretaceous, light production was also absorbed by adults. It could have been co-opted to serve other functions like pairing, "says Robin Kundrata, an expert on elateroid beetles at Palacky University in the Czech Republic.

Light-producing beetles often have unusual adaptations. One of the most surprising is that females often look nothing like their male counterparts and instead retain many larval characteristics into adulthood.

"A good example of this is the trilobite beetle, where the females don't look like beetles at all and instead superficially resemble trilobites. This means that females are often overlooked when collecting in the field. We want to focus on these unusual beetles when searching the fossil record for years to come, "says Yan-Da Li.


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