Dear Asher,

When I was a kitten, there were tons of fireflies in my grandparents’ yard. My litter mates and I loved to gently catch them and let them go.

I talked with my friend Richard Zack about how and why fireflies light up. He’s an insect scientist at Washington State University.

Those glowing insects are a kind of beetle. But we call them fireflies or lightning bugs. Their glow is a form of bioluminescence. That’s when a chemical reaction inside a living thing makes it light up.

Zack told me to think about a glow stick. If you shake it, you can tell it has liquid inside it. There’s also a small glass capsule filled with another liquid inside. When you crack the glow stick, the glass breaks, and the two liquids mix. That causes a chemical reaction—and the glow stick lights up.

A firefly’s chemical reaction happens inside a special light organ in its abdomen. The stuff inside the light organ—a molecule called luciferin and a protein called luciferase—don’t glow on their own. But when the firefly’s body lets oxygen into the light organ, a chemical reaction happens and the firefly’s abdomen glows.

A firefly beetle with black elytra with yellow edges and a black and red head sits on a green leafA firefly stands on a person's finger spreading its elytra and hindwings and showing its abdomen

Zack told me the big reason fireflies light up is to find mates. You’ve probably noticed that a firefly’s light doesn’t stay on. It flicks on and off. Different kinds of fireflies use different patterns of lighting up and turning off to find each other.

“If you’re watching fireflies, the ones you see flying around and flickering are mostly males,” Zack said. “If you look down at the ground, you will see flickering females. So, what happens is the male is out there giving off its species’ flick, flick, flick pattern. Then a female of the same species will respond with a different pattern. Ideally the right male finds the right female, and they mate and everybody’s happy.”

A firefly’s glow is also a warning. It lets predators know that the beetle is poisonous and not a good snack.

But some fireflies don’t glow. Those are the fireflies we have in the western United States. They either don’t light up at all or glow so faintly that you can’t see them. They use chemical signals to find their mates instead.

For some unfortunate male fireflies, a female firefly’s glow is the worst kind of invitation. Different kinds of fireflies have different eating habits as adults. Some never eat at all. Some eat nectar or pollen. But the fireflies from the group Photuris are predators. They’re also aggressive mimics.

Female fireflies from this group hang out on the ground looking for the flickering patterns of male fireflies from another group. They respond by mimicking the glow pattern the male firefly expects to see from a potential mate. He gets closer and closer—and then the female eats him. As a bonus, that gives the Photuris firefly the poison she needs to protect her from predators since this group doesn’t make it on their own.

I’m pretty sure she gives the meal a glowing review.


Dr. Universe