I've always found fireflys to be magical little creatures. Some of the best summer nights were spent outside just watching them fly around blinking their little lights. 

But while I never really gave much thought to why their tiny behinds glow, Clyde Sorenson did. And it led him to a career as a professor of entomology at North Carolina State University.

What are fireflies? 

Despite their name, fireflies are not flies or bugs. They’re actually bioluminescent beetles. This means they naturally produce light through a distinct chemical reaction. 

Their bodies contain a special organic compound called luciferin. This compound combines with oxygen and an enzyme known as luciferase in the insect’s lower abdomen, creating a biochemical reaction that produces light that ranges in color from yellow and green to rare blue hues. 

Fireflies thrive in warm, humid regions, particularly around forests, fields, and wetlands. In the U.S., they mostly live east of the Rocky Mountains, though there are pockets of them in Utah, New Mexico, and other Western states. Over 2,000 recognized species of fireflies exist worldwide, with more than 170 species throughout North America.  

Why do fireflies glow?

“It depends on what life stage you’re talking about,” says Sorenson. For instance, firefly larvae flash to warn would-be predators to stay away. “Their bright glow signals to others that they taste bad,” says Sorenson. This defensive mechanism is known as an aposematic signal. 

And fireflies (and many bioluminescent organisms) do taste bad. This is primarily thanks to a group of toxic steroids called lucibufagins. Lucibufagins can even be lethal to small lizards. Through a bit of trial and error, potential predators learn early on to avoid such illuminated prey. 

But adult fireflies also flash to communicate with and attract members of their species. The ancestors of modern fireflies likely glowed only as larvae. But over tens of millions of years, the adult insects co-opted (borrowed and adapted) this light-producing ability into an intricate mating ritual.  

Fireflies are extremely efficient 

What’s especially cool about fireflies is that they convert nearly 100 percent of their chemical energy into light. This means that they lose almost no energy to waste heat, a resource heavy byproduct of everything from power plants to incandescent light bulbs. 

“Basically, fireflies have figured out how to make light very energy efficiently,” says Sorenson. “The more I learn about these creatures, the more fascinating they become to me.”