Dragonflies are found all over the Chicago lakefront. this is the reason.

Experts say flocks of common green dragonflies are being seen throughout Chicago, and will remain visible through the end of the month.

This is because Chicago is a stopping point on their massive, multi-generational migration across North America, which occurs every spring and fall.

Stand close enough, and you’ll find that the dragonfly’s metallic green chest gives way to a shimmering blue belly. The green runner averages about three inches in length, and is as large as some songbirds.

The dragonfly’s migration will cover more than 900 miles, said Melissa Sanchez Herrera, an entomologist who studies dragonflies at the University of Alabama.

“They will start crossing the lake, because some of these are already coming from Canada,” Sanchez Herrera said.

When you think of migration, most people probably think of birds, bats and monarchs before dragonflies. Of the approximately 500 species of dragonflies, only a small percentage of North American species make long migrations, according to Herrera. The green drummer usually travels to the Gulf of Mexico. Some dragonfly species, such as the wandering glider, have appeared as far south as Colombia.

Sanchez Herrera said dragonflies usually gather the way they do near Lake Michigan for two reasons: to eat or to migrate.

Dragonflies need to find an aquatic habitat to lay their larvae, which will develop over the winter and spring until they emerge when the surrounding water temperature is right.

“But you want to be somewhere where your larvae don’t freeze,” Sanchez-Herrera said. “This is the main reason why they like to look and move around.”

About a week ago, Jacob Drucker, a doctoral student who studies ornithology at the University of Chicago and the Field Museum, noticed massive clusters of dragonflies and began tracking the swarms using weather radar.

“It’s the same weather radars that are designed to look at precipitation, sleet, snow, hail, etc.,” Drucker said. “It is designed to detect objects in the sky, so it also detects birds and insects.”

Drucker, displayed in green and red, pulled out two maps showing huge schools of dragonflies moving across the lake into Illinois. The radar showed how the wind would push the dragonfly toward the lake, then return to land.

“When I was seeing high-density dragonflies on the radar that day, I would go back and forth between other different radar stations regionally,” Drucker said. “I thought there were more people in Illinois and Michigan than in Missouri and Indiana.”

According to Drucker, these remote sensing methods could be a tool for better monitoring biodiversity.

“As we think about this massive loss of biodiversity around the world and how to measure it … and figure out what to do about it, weather radar can be a really great tool for making that happen,” Drucker said.

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