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When the hikers emerged from the woods and spread out on the dark open field behind Eastman Nature Center in Maple Grove, there were several long moments of palpable disappointment.

It was just past 9 p.m. During their trek through the woods, the sky had faded from twilight to dusk, leaving only a thin band of light on the distant horizon. The hikers were bewildered. Here they were at the climax of the journey, the part they'd all been looking forward to, and the prairie grass, swaying in the deepening shadows, appeared lifeless.

Then, slowly, the field began to sparkle.

"They're starting to come out!" someone cried.

"There's one right in front of you!" someone else said.

A pinpoint of light rose into the air here, another flashed there, then a few at once, then a dozen more, their numbers gradually increasing until fireflies were darting and dipping all around, as if the insects had been waiting for their guests to arrive so they could get the party started.

And fireflies know how to party.

The charismatic bugs are the focus of firefly hikes, held annually at Eastman since 2018, a summer tradition that can provide cherished memories for those who experience it.

"I think the only word was magical," said Pam Heilman of Bloomington. In addition to the hike last month, she took a naturalist-guided firefly hike several years ago at Hyland Lake Park Reserve in Bloomington, which like Eastman is part of the Three Rivers Park District. "I don't think I've ever seen anything like that in nature before."

What happened to them?

Many people remember seeing lightning bugs in childhood and wonder why they're not around anymore, said Miranda Jones, a naturalist at Eastman who began the evening with a presentation about fireflies. (Jones and two other experts talk about fireflies on episodes 54, 55 and 56 of a Three Rivers podcast called "The Wandering Naturalist," and still more information can be found on this Three Rivers blogpost.)

If you grew up in, say, a developing Twin Cities suburb where there were once lots of fireflies but aren't anymore, look around and notice what else is missing. Did you, as a kid, play in grassy fields or patches of forest? Are those areas still there, or have they been replaced by streets and houses?

Pesticides and light pollution, which obscures their flashes, have also contributed to fireflies' dwindling numbers, Jones said. And from 1960 until the mid-1990s, the Sigma Chemical Co. harvested about 3 million wild fireflies every year for the chemicals that produce their light, paying a penny a bug to collectors around the country. That practice is now banned.

Fireflies are still comfortable in suburban outskirts, Jones said, as well as in forest clearings and the edges of streams. They like areas with water sources.

"They really like logs and damp soil," Jones said.

Fireflies are beetles, whose soft bodies are protected by hard wing shells and flat shields over their heads. "They can actually poke their little heads out and look around," she said.

There are 2,000 firefly species in the world, 120 in the United States and six in Minnesota. They come in slightly different colors and flash their lights in different patterns. Their glow is used to attract mates and ward off predators.

Some are oxymoronically called "dark fireflies" — they attract mates using pheromones rather than lights. Others, in addition to multisyllabic scientific names, go by fanciful monikers including "Black-Bordered Elf," "Woodland Lucy" and "Big Dipper." There are femme fatales who attract males of other species ... and devour them.

"Their lives are incredibly complex," said Jones, describing the stages.

Firefly lifespans begin with a few weeks as eggs, so small — "Dippin' Dot size," she said — their glow can't be detected with the naked eye.

Then for up to two years they're larva. "Very, very strange, mysterious-looking creatures, almost like a cross between a centipede and an armadillo," Jones said. Lights on their back repel predators. For most species, this stage is the only one in which they eat much, feasting on their favorite foods — snails, earthworms and nectar — after injecting them with toxin.

They spend a couple of weeks as a pupa, or chrysalis, as their bodies transform, then emerge as adults who focus on finding mates. Male fireflies "take to the air," Jones said, whereas "the female, if she finds someone attractive, sends a signal." When a male sees the signal, he approaches her and, well, they get intimate.

Then, in the merciless pattern typical of insect life cycles, they die. Males go immediately. Females lay the couple's eggs first.

Flashy captives

After that initial bewilderment, the hike was a success. Participants had been issued nets and jars to — temporarily — capture lightning bugs so they could be examined up close. There were enough insects around that anyone who tried hard enough would be successful.

Aria Warner, 8, of Falcon Heights, managed to catch seven fireflies (she caught an eighth with her hands in the grass, but gave it to a girl who had spotted it first). She mused briefly on the idea of keeping them as a lamp.

But before leaving the field, participants opened their jars and freed their glittering captives. And the fireflies returned to cruising the great grassy nature center, each hoping to find a partner and experience a few rapturous moments before their earthly journey would come to a close.

Firefly hikes will return next June at Eastman, said Three Rivers spokesman Tom Knisely. Still ahead this year, among many other outdoor activities, is a dragonfly festival July 16 at Eastman. All Three Rivers nature centers will be hosting butterfly programs in late August and early September as monarchs begin their annual migration to Mexico.