Why do fireflies glow? It’s more than butt goo.
Popular Science...
Growing up, Clyde Sorenson loved catching bugs, especially fireflies. But even as a kid, the budding entomologist saw that there was a lot more to these soft-bodied insects than most people realized. “I noticed that I had a pretty significant diversity of fireflies in my own yard,” says Sorenson, “so I started trying to understand that diversity.”
Sorenson, now a professor of entomology at North Carolina State University, has spent many years studying these magnificent creatures (he even helped uncover a distinct population of fireflies in North Carolina’s Blue Ridge Mountains). One thing that has always continued to fascinate him is just why these little creatures illuminate. It’s a question that many of us ponder. The answer, according to Sorenson, is surprisingly complex.
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.
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The most common firefly in the eastern United States is Photinus pyralis, also known as the eastern firefly or big dipper firefly.
“It’s the species that’s typically active when lots of folks are still out poking around,” says Sorenson. They’re also a species that’s easy to catch, since eastern fireflies tend to fly slowly and low to the ground.
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.
“In most North American fireflies, the males have a specific flash pattern,” says Sorenson. “If they fly around at the right time of night,” which is typically around dusk, “and make the right signal, a female might see the signal and flash back at him.”
The two insects will approach one another, and “maybe some kind of pheromonal communication” takes place, a type of chemical signal that says they’re both ready for action. “Then if everybody’s happy they mate, and nobody flashes anything for a while.”
This complex courtship is known as the “firefly mating dance.” Though the call-and-response system is common among species, the pattern that each species uses is different.
For instance, when the male eastern firefly flashes its J-shaped pattern about every five seconds or so, the female will respond with a half-second flash. The Elkmont area in Great Smoky Mountains National Park and Congaree National Park in South Carolina are both home to synchronous species of fireflies. These wondrous beetles coordinate their flashing patterns to create vast displays of synchronized light while mating. It often occurs in densities of hundreds—if not thousands—at a time.
Something to note: Roughly a quarter of the world’s firefly species don’t produce light as adults, and those that do “look really, really similar,” says Sorenson. However, “If you’re familiar with the various flash-patterns, you can figure out the species.”
Meet the femme fatales of fireflies
While firefly mating dances are fun to watch, not all of them are innocent. “Femme fatale” fireflies are predatory beetles (mainly of the genus Photuris) that adopt the distinct flashing patterns of other firefly species to survive. These particular lightning bugs don’t produce lucibufagins, those unappetizing steroids that keep predators away.
Instead, they deceive male fireflies from other genera by mimicking the flashing patterns of Photinus and Pyractomena females. Once they lure the males in with their trickery, they eat them for dinner, absorbing their glowing toxins with the meal.
“The femme fatales then use those [glowing] chemicals to protect their eggs,” says Sorenson, “and to some degree to protect themselves from predators.”
A firefly’s life cycle
Unlike femme fatales, most fireflies do the bulk of their eating as larvae. “As adults, their main job is to find each other and mate and make sure there are more fireflies for next year,” says Sorenson.
While the idea of starving your mature self sounds pretty grim, adulthood is a relatively brief period among fireflies. These beetles spend the majority of their life cycle—up to one or two years—as larvae, buried within the soil, inside rotting wood, or in leaf litter. Here, they get their nutrients through soft-bodied insects like slugs and snails.
Afterwards, they go through a complete metamorphosis, shedding their larval body to grow adult features like wings and legs. This stage lasts another few weeks, and then it’s time to become adults. Once they’re fully grown, fireflies have sex, lay eggs, and live large for anywhere from five days to a month. Then the cycle starts over.
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.”
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