A new champion has been found for epic migrations, at least if distance is graded for body mass. It appears the record goes to a species of dragonfly whose migratory patterns are barely understood. The evidence comes not from observations of the insects in flight, but their genetics.
A seabird, the Arctic tern, has been thought the migratory champion, voyaging 71,000 kilometers (44,000 miles) despite weighing just 100 grams (4 ounces).
Pantala flavescens, the wandering glider, on the other hand, is just 4.5 centimeters (1.8 inches) long, with absurdly delicate wings. We don't yet know how far this dragonfly travels, but Dr. Jessica Ware of the Rutgers University-Newark has found evidence of astonishing reach. When she examined the genetics of specimens caught in places as diverse as Texas, Japan, India, and South America, Ward found very little genetic variation between these populations.
"If North American Pantala only bred with North American Pantala, and Japanese Pantala only bred with Japanese Pantala," Ware said in a statement, "we would expect to see that in genetic results that differed from each other. Because we don't see that, it suggests the mixing of genes across vast geographic expanses."
According to Ware, the secret to Pantala's migratory capacity lies in its body architecture. "These dragonflies have adaptations such as increased surface areas on their wings that enable them to use the wind to carry them. They stroke, stroke, stroke and then glide for long periods, expending minimal amounts of energy as they do so," Ware said.
When you're about to fly 100,000,000 times your body length, it's important to stop and smell the flowers. Hans Christiansson/Shutterstock
Dragonfly migrations from Asia to Africa have been reported, crossing large sections of the Indian Ocean in the process. Graduate student Daniel Troast, who coauthored a paper in PLOS ONE with Ware on Pantala genetics, said, "They're going from India where it's dry season to Africa where it's moist season, and apparently they do it once a year."
Huge numbers die in the process, but Ware said Pantala need moisture to reproduce, so finding it is essential for the species' survival.
Ware and Troast suggested the dragonflies may stop over on small islands, provided they can find pools of water. There they may lay eggs, with the next generation taking up the journey weeks later. Nevertheless, the authors acknowledge that at the moment we are largely guessing as to how Pantala make their way around the planet – any excess baggage such as radio transmitters to offer measurements would be lethal.
Crossing the Indian Ocean is one thing, the Atlantic or Pacific quite another. The flight from India to Africa is about half that between North America and Africa. For a tropical species, unable to travel via Siberia, the Pacific is an even more daunting barrier, yet somehow North America's Pantala have managed to mix their genes.
"Monarch butterflies migrating back and forth across North America were thought to be the longest migrating insects," said Troast, "But Pantala completely destroys any migrating record they would have." Monarchs have been calculated to fly 4,000 kilometers (2,500 miles), but Ware suspects these dragonflies nearly double that.