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Some Firefly Species Await a Night That Never Comes

As dusk deepens the shadows at the edge of the forest, a small beacon illuminates the darkness. Soon the dusk is full of floating lights, each winking a message in a peculiar semaphore: “Man seeks woman for short-term commitment.” This courtship takes place on summer evenings around the world among beetles of the family Lampyridae, commonly known as fireflies.

However, the darkness in which fireflies have always chased their liaisons has been broken by the glare of artificial light. The love affair of people with enlightenment has led too many of the Earth’s habitable surfaces suffer from light pollution at night. In recent years, scientists who study fireflies have heard of people fearing the insects are in decline, said Avalon Owens, an entomologist at Tufts University.

“There is a sense of doom. They don’t seem to be in places where they used to be,” she said.

So little is known about how fireflies live that it’s difficult to assess whether they are endangered — and if so, why, said Dr. Owens. But in a study published Wednesday in the journal Royal Society Open Science, she and Sara Lewis, a professor of biology at Tufts University, shone some light on how fireflies respond to artificial lighting. Experiments in forests and fields, as well as in the lab, showed that while some North American fireflies mate with wild abandon regardless of the lighting, others failed to complete any successful mating under the glare of the lights.

Fireflies seem to rely primarily on flashes of light to find each other, meaning light pollution can threaten their ability to see mates. In the four common species the study examines, the females hide on the ground and observe as males roam the air. When a woman responds to a man’s flashing her own flashes, the two engage in a dialogue that can end in an encounter and eventually mate. In previous workdiscovered Dr. Owens and Dr. Lewis that shining light on female fireflies of the species Photinus obscurellus made them less responsive to the calls of the males.

In a forest west of Boston, the scientists played the role of female fireflies, responding to Photinus greeni males with green LED lights. The lights were either in darkness or lit, as if by a street lamp. The scientists found that more than 96 percent of the men preferred darkness. Then, in lab experiments with P. obscurellus, they noted that while dim light did little to hinder successful mating, in brighter light none of the firefly pairs mate. The insects found each other and some even crawled over each other, but something prevented them from continuing.

“This is very important because we’ve all wasted our time counting flashes, and it doesn’t matter if they’re literally next to each other and not mating,” Dr. Owens himself. “It’s pretty worrying.”

She speculates that the fireflies interpret the light as day and wait to mate in weaker conditions – in fact, waiting for a night that never comes.

It was in a field in Tionesta, Pennsylvania, that Dr. Owens saw something that hampered the mischief of the lab experiments. Bruce Parkhurst, a firefly enthusiast who lives in the area, warned her about introducing bright outdoor lighting into a visitor center, so Dr. Owens and her colleagues studied the behavior of local fireflies in the adjacent field.

Over the course of many July nights, they captured and marked females of two species – P. pyralis and P. marginellusand placed them in parts of the field on a spectrum from brightly lit to completely dark. Females in bright areas tended to show up later and further into the shade, suggesting that if the insects found the light uncomfortable, they would simply move into darkness. But even where the light was nearly blinding to the researchers, fireflies of both species somehow found each other and mated successfully.

“They just mate left, right and center,” said Dr. Owens. “They don’t care at all. To stand there in the field and see it is crazy.”

In a group as large and diverse as fireflies — more than 2,000 species worldwide — adaptation to different levels of darkness may mean different responses to light pollution, the researchers suspect. Of the four species in the study, P. obscurellus, the insect that never spawned in bright light is also least active at dusk and prefers deep night. What doesn’t bother one group at all can destroy another.

Could there be a version of artificial lighting that is friendly to all fireflies – a wavelength of light that works for humans and light-sensitive insects? dr. Owens has been pursuing the idea for a while, but a universally harmless option has remained elusive.

The best solution may be something simpler and more radical: more awareness of outdoor lighting and more sparing use of it. While the study suggests that fireflies can flee light pollution to refuges of darkness, if there’s no dark place left for them, the nighttime symphony of tiny lights could become a thing of the past.

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