Swarms of the Common Green Darner dragonfly will be visible near Chicago through the end of the month. Chicago is the latest pit stop on the voyage of dragonflies — part of their multigenerational, mass migration — across North America that happens every Spring and Fall.

Stand close enough, and the common green darner’s namesake emerges: its metallic green thorax gives way to a shimmering blue abdomen. The green darner averages about three inches in length — as large as some songbirds.

Melissa Sanchez Hererra, an entomologist studying dragonflies at the University of Alabama, said the dragonflies will travel more than 900 miles.

“They’re going to start to cross the lake,” Sanchez Herrera said. “Because some of these are actually coming from Canada.”

When you think of migration, birds, bats and monarchs might come to mind. Herrera said that of the nearly 500 species of dragonfly, a miniscule fraction of North American species embark on long migrations, as well. The green darner typically travels down to the Gulf of Mexico, but Sachez-Hererra adds that some dragonfly species, like the Wandering Glider, have even turned up as far south as Colombia.

The dragonflies must traverse these long distances out of necessity. Sanchez Hererra said that dragonflies aggregate the way they have near Lake Michigan, typically, for one of two reasons: eating or migrating.

The dragonflies must find an aquatic habitat to lay their larvae, where they will develop over the winter and spring until they emerge when the surrounding water temperature is suitable.

“But you want to be in a place where your larvae will not freeze,” Sanchez Hererra said. “So that’s basically the reason why they’re like looking and moving all around.”

Weather radars detect dragonflies

Last week, Jacob Drucker, a PhD student studying ornithology at the University of Chicago and the Field Museum, noticed the massive concentrations of dragonflies and began tracking the swarms with weather radars.

“The same weather radars that are designed to look at precipitation, rain, snow, hail, etc,” Drucker said. “They’re made for detecting objects in the sky, and so they also detect birds and insects.”

Displayed in greens and reds, Drucker pulled two different maps showing the massive clusters of dragonflies moving across the lake and into Illinois. The radar was able to show how winds would push the dragonflies back towards the lake, only for them to return back to land.

“When I was seeing high density dragonflies on the radar the other day, I would go and kind of toggle around between the other different radar stations regionally,” Drucker said. “I was seeing that there are more in Illinois and Michigan than there were in Missouri and Indiana.”

According to Drucker, these remote sensing methods can be used for monitoring biodiversity at a scale that was kind of unprecedented beforehand.

“As we consider this mass biodiversity loss around the globe and how to measure it … and figure out what to do about it,” said Ducker, “weather radar can be a really great tool for that.

In the meantime, sit back and enjoy the show. Sanchez Hererra said don’t worry, too much, if you miss them this year. “You can wait for next year, you might see them then.”

Juanpablo Ramirez-Franco covers climate change and the environment for WBEZ and Grist. Follow him on Twitter at @__juanpab.