Scientists have debunked a scientific myth that was over a hundred years old

A new study has shown that the generally accepted estimate of firefly brightness was greatly overestimated. It all began with an experiment in 1912, and for decades the erroneous result was simply reproduced, converted into new units of measurement, but almost no one went back to the original data.

The study has been published in the *American Journal of Physics*.

How the error arose

In 1912, the American physicist William Koblenz was the first to attempt to measure the brightness of firefly flashes.

To do this, he compared their glow… to that of a candle. At the time, this was a perfectly modern method.

Later, the results were repeatedly converted into new, more precise units of light measurement. Over time, the scientific literature came to accept the figure that a firefly emits approximately one milliwatt of visible light.

This figure remained virtually unchallenged for over a hundred years.

A scientist decided to double-check everything

Physicist David Silver re-evaluated the brightness of fireflies using four independent methods at once.

He calculated how many photons might be produced during the biochemical luminescence reaction, measured the brightness of wild fireflies using a modern lux meter, carefully studied the original 1912 publication, and re-analysed later studies.

All the methods yielded virtually the same result.

Fireflies turned out to be much dimmer

It turned out that the conventional estimate had been overestimated by a factor of 100 to 10,000.

According to the author’s calculations, fireflies emit approximately 100 million to 100 billion photons per flash, whereas the established figure corresponded to 10–100 trillion photons.

In other words, the insects glow much more faintly than had been thought for many decades.

Why this matters

At first glance, it might seem that this is merely a matter of refining the figures.

However, the study highlights a more significant issue.

Sometimes old scientific data becomes so firmly entrenched in textbooks and articles that it comes to be regarded as an indisputable fact. Yet the original measurements may go unchecked for a long time.

According to the study’s author, the story of the fireflies is a good example of why even widely accepted scientific findings are worth reviewing from time to time.

What does this change?

For biologists, the discovery means that fireflies’ mating signals may be significantly weaker than previously thought. This will help to study their behaviour and visual characteristics more accurately.

But the main conclusion applies not only to fireflies.

The study serves as a reminder that science is constantly testing itself, and even data that has been considered accurate for decades sometimes requires a fresh perspective.

Background

Fireflies glow as a result of a chemical reaction between the substance luciferin and the enzyme luciferase. This is one of the best-known examples of bioluminescence in nature.

The absolute brightness of their flashes was first measured in 1912. Since then, the original estimate has been cited countless times and converted into modern units of measurement, with virtually no re-examination.

Source

David H. Silver. How bright is a firefly? Resolving a century of overestimation. American Journal of Physics (2026).