Each spring and fall, large dragonflies called green darners fly across North America. A green darner can travel as far as 900 miles on its two-inch wings, fluttering from Canada to the Gulf of Mexico. They are some of the most abundant dragonflies on the continent — but few people notice this mass migration.

Unlike monarch butterflies and songbirds, whose long-distance migrations draw crowds, green darners are inconspicuous travelers. These dragonflies rarely migrate in huge swarms. Radiant up close — emerald green at the thorax, splashes of blue on the abdomen — a three-inch-long dragonfly becomes a speck at a hundred yards. Even dragonfly experts have had little luck observing this cross-continental journey. In one study, published in 2006, a research team in New Jersey glued tiny radio transmitters to 14 green darners; the scientists shadowed the insects by Cessna but could follow them for only about 80 miles from the scientists' base at Princeton University.

Research published this week in Biology Letters is the first to describe dragonfly migration in detail. The authors did not bother with tracking live insects. Instead, they extracted chemicals from dragonfly wings that link an animal to its birthplace. Using 850 wing samples, from museums and private collections in the United States, Canada, Mexico and five Caribbean countries, the scientists constructed 140 years of dragonfly migratory history.

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Colin Studds, an animal ecologist at the University of Maryland Baltimore County and an author of this study, had tracked bird migration by chemical signature. This technique exploits a property of the element hydrogen. In nature, hydrogen comes in three forms, called isotopes, with different numbers of neutrons. The International Atomic Energy Agency began tracking hydrogen and oxygen isotopes in the 1950s while looking for nuclear contamination in rain. In the 1980s, biologists realized they could use this isotope data to their own ends.

Heavier forms of hydrogen concentrate more densely in the water in Southern states, where warmer weather leads to more evaporation. Dragonflies, which live in ponds as wriggling larvae, absorb the native isotopes of hydrogen and incorporate them into their chitin. That’s the stuff that, when larvae grow into adults, makes up their wings.

The ratio of hydrogen isotopes revealed the approximate location of the dragonflies' birth-ponds. It’s not a super precise tracking system, Studds said, but it’s enough to tell if a dragonfly was born in Florida, Maryland or Maine. A dragonfly caught in northern latitudes might look like a local bug, but its wing isotopes reveal whether it is a migrant.

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A multigenerational scene emerged from this data. “We’ve got three generations, one north, one south and one that’s only hanging out in Florida,” Studds said.

In the early spring, the first generations exit the shelter of their Southern ponds and fly north, an average of 400 miles on the wing. There they lay their eggs and die. The second generation hatches in the north and, by September, has flown south. There they lay eggs and die, too. Because the South is the most comfortable place for a dragonfly late in the year, the third generation won’t migrate at all. They settle in for winter in Florida or the Caribbean. Life is good — until spring rolls around, when the cycle of eggs, death, birth repeats.

The new research “is a really good bit,” said Michael May, a dragonfly expert at Rutgers University who was not a member of this research team. “We had a general idea of what was going on,” he said, but until now there was no way to follow the course of the whole migration. (The Cessna approach, flown by one of May’s collaborators, can’t cover enough territory.)

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Dragonflies fly the routes that their great-grandparents, long-deceased, once did. They do so without any communication passed from older generations. So how do they know where and when to go? Studds could not say for sure, but he had a few ideas, considering the environmental cues that trigger bird migration.

Dragonflies are sensitive to temperature. Data from citizen scientists, who recorded the first adults of the season, suggest that green darners do not leave the safety of their ponds until temperatures reach 48 degrees. Perhaps there’s another temperature signal for the animals to migrate. If so, “climate warming could really disrupt the presence of this migration,” Studds said. The insects might also migrate based on changes in the amount of daylight in a day.

May has seen a migrating swarm of green darners only once in his 40-plus-year career as an entomologist. On a barrier island in St. Augustine, Fla., the insects streamed over dunes and surf, as rapidly as 100 animals a minute. They came for three days, mostly in the morning, in a band a half-mile wide. He estimates 500,000 flew by.

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“They are pretty amazing creatures,” May said. He paused. “You could say that about any creature,” he said, with just a touch less conviction.

Correction: A previous version of this story incorrectly stated the journal that published this study. It is Biology Letters, not Current Biology.

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