Think of the firefly abdomen like a black box of bioluminescence.
For around 60 years, scientists have known what basic ingredients go into the box—things like oxygen, calcium, magnesium, and a naturally occurring chemical called luciferin.
And they've known what comes out of the box—photons, or light, in the form of the yellow, green, orange, and even blue flickers you see dancing across your backyard on summer nights.
But until recently, the actual chemical reactions that produce the firefly's light have been shrouded in mystery. And scientists like Bruce Branchini at Connecticut College love a good mystery. (Also see "Fireflies Are 'Cannibals'—And More Surprising Facts About the Summertime Insect.")
"The way enzymes and proteins can convert chemical energy into light is a very basic phenomenon," he says, "and we wanted to know how that biochemical process worked."
In new research, Branchini and his colleagues did just that: They found an extra oxygen electron that's responsible for the beetles' summertime glow.
The discovery, published recently in the Journal of the American Chemical Society, provides the most detailed picture yet of the chemistry involved in firefly bioluminescence. (See other pictures of glowing animals.)
Apples, Oranges, and Fireflies
The conventional explanation of how a firefly turns its backside into a bioluminescent beacon has always troubled Branchini and other chemists. For starters, it shouldn't work.
Specifically, two of the ingredients mentioned above—oxygen and luciferin—aren't likely to react to each other in the way they would need to in order to produce light.
Understanding why this is gets complicated fast, but a simple explanation is that apples tend to only create chemical reactions with apples, while oranges tend to only create chemical reactions with oranges. In other words, oxygen and luciferin are like apples and oranges.
Branchini's experiments showed the oxygen involved in the firefly's glow comes in a special form called a superoxide anion.
"Superoxide anion is a form of molecular oxygen that contains an extra electron," says Branchini. (Watch video: "The Science of Summer.")
This extra electron gives the oxygen properties of both a metaphorical apple and a metaphorical orange. This means that the molecule would, in fact, be able to cause a chemical reaction with the luciferin like scientists have suspected.
He adds that these superoxide anions could be the way bioluminescence works across nature, from plankton to deep-sea fish.
Lightning (Bug) In A Bottle
"To me, chemically, this is the only way it makes sense," says Stephen Miller, a chemical biologist at the University of Massachusetts Medical School who also studies luciferin and its potential uses for human health.
Miller, who was unaffiliated with the study, says it's important to keep studying luciferin and bioluminescence because of their potential applications for medicine.
For instance, earlier this year, Miller was part of a team that used luciferin to detect specific enzymes in the brains of living rats, which could someday offer doctors another window into the human brain.
Firefly luciferin is already proving to be a useful tool in imaging human tumors and developing cancer-fighting drugs, says lead author Branchini.
Ultimately, though, "we just want to know how nature works," he says. "The applications may or may not follow."