100-million-year-old beetle reveals fireflies once hunted by smell, not flash

A hundred million years ago, a small beetle wandered across the bark of a tree in what is now northern Myanmar and stepped into a bead of resin. The resin sealed around it, hardened into amber, and held the creature whole through all the ages since. When paleontologists finally cracked that amber open, they found something that stopped them: antennae unlike anything a beetle carries today.

The beetle, named Icaroramus perisi, had antennae that branched in a pattern no living insect matches. Each of the middle eight segments sprouted two separate pairs of side branches—one set longer and slightly swollen at the tip, coated in coarse hairs, the other shorter and thinner with only fine hairs. The researchers call this arrangement quadriheteroramose, a term that means four branches of two different types sprouting from a single segment. Yanda Li, a doctoral student at the University of Bristol, led the team that made the discovery alongside Professor Chenyang Cai of the Nanjing Institute of Geology and Palaeontology. The species name honors paleoentomologist David Peris, while Icarus—the mythological figure who flew too close to the sun—inspired the genus.

To understand what those antennae meant, you have to understand what antennae do. They are an insect's nose. Fine hairs called sensilla line them, each one holding nerve cells that detect airborne chemicals. More branches mean more surface area, and more surface area means more sensilla, which sharpens the sense of smell dramatically. In many insects, males carry the most elaborate antennae because they face the harder task: tracking the faint pheromone trails that females leave behind. The specimen preserved in amber was male, and in many firefly relatives, females never develop wings at all. They keep a larval shape into adulthood while winged males fly out to hunt for them. That division of labor pushes males to evolve better and better scent detectors. The branched antennae on Icaroramus suggest the beetle hunted for mates primarily by smell, with the two branch types possibly tuned to different classes of chemicals—a strategy moths use today.

But the fossil held another surprise. On the underside of its abdomen sat a light organ, a structure that in modern fireflies can warn off predators, attract mates, lure prey, or illuminate a path. That pairing—elaborate scent-detection antennae combined with a working light—is strange. Modern fireflies rarely carry both, because the two traits usually align with different mating strategies. Species that flash for mates tend to have plain antennae and large eyes. Species with elaborate antennae usually court by scent and abandon flashing altogether. The researchers doubt that Icaroramus used its glow mainly for courtship. They argue instead that the light worked as a warning to predators, a defensive role that likely came before fireflies ever thought to use bioluminescence for romance.

If that reading is correct, it rewrites the early history of firefly evolution. The first firefly relatives probably courted by scent, their heavy antennae doing the real work of finding mates. The glow may have started as a larval trait—many firefly larvae glow to warn predators—and only later did adults borrow the light for courtship. Icaroramus perisi hints at a halfway stage, a moment when an ancestor beetle might have begun folding its glow into the mating ritual without dropping pheromones as its primary signal. Some living fireflies today run both channels at once, flashing and releasing scent during mating, a double system that may have deep roots in the Cretaceous.

No beetle alive today carries antennae like Icaroramus. The researchers read the design as a brief experiment, one that natural selection abandoned or that later conditions made impossible. A few modern relatives grow a stray third branch on each segment, but those are short, blunt bumps—nothing like the long, fully formed sensory tools that Icaroramus carried. At just 0.4 inches long, this beetle packed antennae no descendant has matched in the hundred million years since it died. The team places it in an extinct family called Cretophengodidae, though they acknowledge the fit remains uncertain. The study appears in the Proceedings of the Royal Society B, a finding that opens a window onto a time when fireflies were still experimenting with how to find each other in the dark.