Solving the Mystery of How Fireflies Glow


By Samba Lampich

The intermittent glow of fireflies at dusk usually signals the height of the summer season. To us, the twinkling bugs are magical, but there is a serious chemical reaction happening in their bodies.

How Do Fireflies Glow?

For decades, scientists have had a rough idea of how fireflies glow, but never the whole picture. Research shows that fireflies glow when a substrate called luciferin, an enzyme called luciferase, and oxygen combine, producing a chemical reaction that causes their abdomens to light up. This emission of light is called bioluminescence. What remained unknown was what happened in the last step to cause the bioluminescence.

The Hidden Ingredient

Dr. Bruce Branchini, a chemist from Connecticut College, and his colleagues from Yale conducted a study that was able to confirm and further identify how the biochemistry in fireflies works. They replicated the fireflies’ chemistry in the lab and revealed their findings in a report in the Journal of the American Chemical Society. They found that a charged particle called superoxide ion was critical to the bioluminescent reaction.

Dr. Branchini used EPR (electron paramagnetic resonance) spectroscopy, which uses magnetic fields and microwaves. ERP was able to trap the fast bioluminescent reaction long enough to see what was happening. If the superoxide ion was actually present, it would react with another substance in such a way as to confirm its presence.

Superoxide ion is toxic to animals, but because the reaction in fireflies is contained and happens so fast (300 milliseconds from start to finish) it doesn’t hurt the bug.

Implications of the Discovery

The findings that superoxide plays a key role in bioluminescence could lead to new biomedical procedures, including monitoring tumors by tagging them. Instead of using MRI or taking tissue samples to monitor a tumor, doctors could make it more visible by lighting it up, a procedure that would be much less invasive.

Why Do Fireflies Glow?

From the colorful plumage of a peacock to the fancy dancing of guppies, all animals have evolved ways to attract a mate. Male fireflies use a flashy way to catch the eye of their mate by generating a glowing pattern at a particular wavelength. When a female of the same species identifies a pattern she is interested in, she responds by flashing back in a particular way.

But fireflies also glow to deter would-be predators. The same chemicals that make them glow, lucibufagins, taste terrible. So when the firefly glows, it’s a way of reminding predators that they taste bad and wouldn’t make a good meal.

A more sinister reason for the glow is to help the firefly become a predator. The female of some species lure males with deceptive flashes. When the unsuspecting male approaches her, she grabs him and eats him.

Classroom Discussion

  • What other living organisms glow? Why do they glow?
  • What other applications are there for bioluminescence?


  • Bioluminescence
  • Enzyme
  • Biochemistry
  • Superoxide