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It was a moonless night in May 2016, and the Cruzeiro do Sul, a Brazilian Navy ship, was plying the South Atlantic Ocean. It was shuttling scientists back to mainland Brazil from a research station on Ilha da Trindade and gathering oceanographic data from over a seamount as it passed by. When the operators prepared to put some science equipment over the ship’s side, they fired up its 10,000-watt deck lights to illuminate the workspace.
That’s when the swarm descended.
“It was sort of startling,” says Ruy Alves, a botanist at Brazil’s Museu Nacional who was on board the ship. The streaks of insects flying toward the lights looked like, he says, “shooting stars.”
“Most of them banged into the side of the hull and fell into the ocean,” Alves recalls. But some of the concussed insects fell onto the deck. Alves and fellow scientists scrambled to gather them using envelopes and sheets of paper torn from his notebook. They managed to collect 17.
The team’s quick thinking resulted in a paper documenting this surprise sample of a very poorly understood demographic: insects that migrate over hundreds of kilometers of open ocean.
Though most insects migrate, scientific knowledge of these journeys is scant and migrations over oceans are especially mysterious. Until the past 10 years or so, scientists assumed insects were too small to fly very far or against the wind, and certainly not over oceans. But new data on insect migration has slowly emerged, showing that they have more stamina than scientists first gave them credit for.
Immediately after getting back to Brazil, Alves got in touch with Angelo Pinto, an entomologist at the Federal University of Paraná in Brazil, to help identify the 17 insects. They included a stink bug, three different species of moths, and one large dragonfly. Only the stink bug and dragonfly were previously known to fly over large stretches of water.
At the time of Alves’s insect collection, the Cruzeiro do Sul was 764 kilometers from Trindade and 389 kilometers from mainland Brazil. Some of the moths might have hitched a ride from Trindade, the authors report, but the other insects were native only to mainland Brazil and so likely came from there, perhaps blown out to sea while migrating along the coast. Why they took a route so far over water remains a mystery.
“It’s not a controlled experiment,” says Pinto. “But [these insects] open a very small window to investigate the migratory behavior of insects.”
Myles Menz, an entomologist in Germany who studies insect migration, says insects, like other animals, are compelled to migrate by weather, food availability, and other factors. Sometimes that forces them across obstacles like mountains or oceans. “It’s a lot more risky to move over water than over land, but for some insects, they don’t really have a choice,” he says. Dragonflies, he says, are known ocean-crossers: the globe skimmer dragonfly, for instance, migrates from India to East Africa, leapfrogging across the Indian Ocean with a layover in the Maldives.
While many insects use sunlight to tell direction, some fly at night. “A lot of migrants are nocturnal insects,” Menz says. These bugs may orient by moonlight or starlight, or use magnetism. Others use winds to help carry them. The silver Y moth, for instance, migrates between the British Isles and North Africa on seasonal prevailing winds.
Menz says the dearth of data on migrating insects is due to logistical challenges. “Insects are generally pretty hard to follow because they are very little,” he says. But it can be done. Researchers in China used a searchlight trap to collect globe skimmer dragonflies migrating at night across the Bohai Sea. Radar can track large swarms. And Menz is tracking moths using tiny radio transmitters.
The Cruzeiro do Sul bonanza comes with a sad note. After identifying the insects, Alves and his colleagues stored the specimens in the logical place: Brazil’s national museum in Rio de Janeiro. Most of the specimens were lost in the massive 2018 fire that gutted the building. Fortunately, Alves had been studying the moths in his office, so these specimens were spared. Now, he says, they will become the first new moths in the museum’s new moth and butterfly collection.