by Mary Hoff
Some of Anna Newton's fondest childhood memories are lit with twinkling lights.
"It was one of the joys of the summer nights, to have the fireflies flashing around you," Newton recalls. "If you could focus on one, you could watch the pattern and see its movements. Then the light goes off, and it kicks on somewhere else and you could see it was the same individual."
Now a naturalist at Tamarack Nature Center in White Bear Township, Newton is far from alone in enjoying the beetles also known as lightning bugs. "Everybody loves fireflies," says Sara Lewis, an evolutionary ecologist from Tufts University who has spent the past quarter century studying them. "They're one of those insects that even the most entomophobic adults really love. It's a very cross-cultural thing too—in Japanese culture fireflies are practically revered. There's something very magical about them."
Fireflies. Generations of kids have collected them in jars and fallen asleep to the pulse of their bright flashes on the nightstand. Even among grownups, these enlightened beetles spark curiosity and ignite imagination. How do they do that? And why?
Worldwide, on every continent except Antarctica, some 2,000 species of fireflies light up summer evening skies. More than a dozen species have been recorded in Minnesota, from a wilderness outfitter's outpost near Canada to an apple orchard on the Iowa border.
"They usually appear mid-June, then reach a peak around the Fourth of July," says phenologist Jim Gilbert, who has been tracking seasonal patterns in nature since 1967. "I made phone calls to friends around the Fourth of July last year to get a reading on the distribution. I can safely say fireflies were out pretty much all over the state in the right habitat—wetlands, wet ditches, grassy tall spots, old fields, the edge of forest, sometimes lawn near natural areas. In the right habitat, they were out."
Members of the insect family Lampyridae (from a Greek word meaning "to shine"), these bright beetles begin life as sand-grain-sized eggs, often lightly glowing, deposited in damp soil at the base of grass blades or moss in midsummer. After three to four weeks, tiny light-producing larvae emerge.
The larvae, known as glowworms, burrow into cool, moist soil. The glowworm eats snails, earthworms, and other soft-bodied animals. It bites the prey and injects it with poison and digestive juices that disable and begin to dissolve it. Slurping up the prey's nutritious juices, the glowworm grows strong and fat.
Like larvae of many other insects, glowworms overwinter near the surface of the soil, sheltered from subzero temperatures by insulating layers of snow. When the snow melts and the ground warms, the glowworm builds a domed structure out of soil and transforms into a pupa. Inside the tough pupal case, its soft larval tissues dissolve and reassemble into the form of a beetle. Everything—including the organs that made the larva glow—is taken apart.
"They're reinventing their body," Lewis says. "The adult is manufactured out of entirely new stuff."
After three weeks or so of major remodeling, an adult firefly emerges with soft wing covers (elytra) and translucent flight wings. It has the antennae, head, thorax, and abdomen of a typical insect. Except for one thing: Along with the standard insect body parts, most adult fireflies sport an organ called a lantern at the end of the abdomen.
The secret to a firefly's glow lies deep within the cells of its lantern. Tiny bags within the cells that make up the lantern contain two chemicals called luciferase and luciferin. On their own, luciferase and luciferin are unremarkable. But when combined with each other, along with oxygen drawn in through tiny holes in the abdomen and an energy-releasing molecule called ATP, the product emits a bright glow without the heat we normally associate with light.
What turns the glow on and off? That's still somewhat of a mystery. But scientists think nitric oxide, a gas produced in response to a signal from the insect's nervous system, plays a role in telling the cells to mix the two compounds and produce the steady glow or a cryptic blink.
In Minnesota, the most common kind of fireflies belong to the genus Photinus. These fireflies produce a yellow flash in the early evening, right after sunset. The next most common type, Photuris, is found in lowland marshes and is active later in the evening, giving off a greenish or greenish-yellow glow.
Magical as it might seem, a firefly's display is more than just show. It is a tool fireflies use to communicate.
What do adult fireflies have to talk about? In the early ancestors of today's fireflies, says firefly researcher Christopher Cratsley of Fitchburg State University in Massachusetts, illumination likely served as a warning signal to predators. Most fireflies taste bad, and the light was a way of sending the message through the dark, "If you try to eat me, you'll regret it." But through evolutionary time, the focus shifted to communicating with other fireflies. Today, adult fireflies' light lets prospective mates know who and where they are.
In some species, the stationary female glows and males zoom in. In others, the female is attracted to the male's flashes. But the most common pattern among North American fireflies is this: A male in flight flashes a signal with a color, timing, and perhaps trajectory that advertises both his presence and his species.
"Females tend to be resting on vegetation. It's the males that are cruising for females," says entomologist Bruce Giebink. "They send out the initial signal, then the female replies." Once they've noticed each other, the two begin to converse in the language of light. And so it goes, until they meet and mate.
Scientists suspect that traits of the flash tell the insects something about each other and how fit their offspring might be. Studying one species, scientists have discovered that the faster the blink, the more attractive to the female.
"The characteristics of the firefly flash matter in some interesting ways for the other firefly that's receiving it," says Cratsley. "It goes beyond, 'I'm here,' [and] 'Hey, I'm over here!' "
In recent years fireflies have drawn attention for reasons beyond their innate intrigue. In some places, sightings seem to hold steady from year to year, or even increase. In other places, populations appear to be on the decline.
"We hear a lot of people say they don't see fireflies as they used to when they were kids," says Don Salvatore, a science educator at the Boston Museum of Science. "My guess is that a lot of it is habitat destruction—fewer meadows and fields around, where you're likely to see more types of fireflies than forest and back yards and so on." Pesticides and light pollution could also be factors, Salvatore says, "But until we have enough data, it's hard to say for sure."
Wanting to find answers, Salvatore started a program called Firefly Watch. People from all over the United States—including Minnesota—sign up to go out one night a week during firefly season. Volunteers record weather conditions and count the number and color of flashes they see in 10 seconds in their back yard or another spot of their choice. They send this information to the Firefly Watch website, which shares it with scientists.
"It's a long-term project and one that we think will over time yield us some valuable information," Cratsley says. "We have a lot of reasons to believe fireflies' populations are being diminished, because a lot of our development policies and practices can destroy firefly habitats.
"Depending on where you are, you may be able to see multiple species of fireflies in your area," he says. "As someone doing Firefly Watch, you can get a sense of that diversity because species flash differently from each other."
Salvatore's hope: As we become more aware of fireflies and the affect humans can have on them, all of us will be motivated to do our part to ensure these amazing insects will be around to captivate the minds and hearts of future generations.