A thousand-mile journey on two-inch wings

Post by Jen Berlinghof

I’ve spent many days this summer living the “lake life,” from the sandy dunes of the Great Lakes to the cattail-studded coves of inland waters. The waterways found within the Lake County Forest Preserves in northern Illinois have been especially rewarding. Each shoreline has provided unique glimpses of wildlife, including an osprey (Pandion haliaetus) diving full throttle to catch a fish at dawn, and a red fox (Vulpes vulpes) pouncing on prey hidden among the beach grass at dusk.

Also present at every waterway this summer has been the jewel-hued, common green darner dragonfly (Anax junius). These ubiquitous insects effortlessly nab multitudes of mosquitoes on the wing. While the monarch butterfly (Danaus plexippus) reigns in our minds when it comes to insect migrations, recent research reveals the green darner takes a multi-generational, miles-long journey of its own each year.

Common green darner dragonflies (Anax junius) are found in the forest preserves. Stock photo.
Common green darner dragonflies (Anax junius) are found in the forest preserves. Stock photo.

Wildlife migrations are typically monitored using small tracking devices called geolocators. Songbirds and monarchs migrate in such large flocks or swarms that their travels can actually be detected by radar. But green darner dragonflies don’t usually migrate in large groups. Coupled with the fact that the technology to fit an insect weighing as little as a paperclip with a teeny transmitter is only in its infancy, scientists have faced a conundrum of how to track the marvelous migrations of these members of the Odonata order.

A video explaining how researchers revealed green darner migration patterns.

A combo of chemistry and community science provided insight into the last century of dragonfly migrations, as well as environmental impacts for future generations of flyers. Research published in the December 2018 issue of the scientific journal Biology Letters was the first of its kind to use the chemical signatures of dragonfly wing samples to describe the nearly thousand-mile journey these insects undertake on two-inch wings.

A green darner dragonfly nymph. Dragonflies and damselflies of Lake County spend the majority of their lives in this underwater stage of development—anywhere from one to four years, depending on the species. Photo © John C. Abbott & Kendra K. Abbott, Abbott Nature Photography.
A green darner dragonfly nymph. Dragonflies and damselflies of Lake County spend the majority of their lives in this underwater stage of development—anywhere from one to four years, depending on the species. Photo © John C. Abbott & Kendra K. Abbott, Abbott Nature Photography.

A record of the birth waters of every dragonfly is stored chemically in the form of a hydrogen isotope. According to the Department of Energy, isotopes are “members of a family of an element that all have the same number of protons but different numbers of neutrons.” The specific isotopes found in water vary geographically.

As swimming nymphs, dragonflies absorb the isotopes into their bodies and eventually incorporate them into the substance that forms their adult wings. Researchers extracted these chemical trademarks from hundreds of wing samples, pinpointing the general area of North America each insect originated from.

With the home bases for different darners known, migration factors had to be determined. Researchers turned to decades of community science data that suggested these dynamo dragonflies were motivated to move based on temperature and the amount of daylight.

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Areas with many dragonflies are represented in red. Areas with no dragonflies at the given point in time are represented in gray. Graphic © Matthew Dodder via Hallworth et al., Biology Letters.

The researchers determined it takes three generations of green darner dragonflies to complete the migration cycle from southern North America to the north, then back again south.

In spring, a first-generation dragonfly nymph emerges from a pond in the American South, sheds its exoskeleton, unfurls its glossy wings and flies 400-500 miles north to lay its eggs before dying. These eggs hatch and complete their metamorphosis by late summer in northern waters.

Some of this second generation will stay put, overwintering as nymphs and waiting out the cold, hunkered down like the rest of us in the Upper Midwest and Northeast. Other members of the second generation have the “travel bug” and hightail it south to lay eggs by late fall along the Gulf and Atlantic Coasts.

This third and final batch of eggs hatches over winter and the adults remain in the warm waters of the South, soaking up the sun. The eggs laid by this third, non-migratory generation will kick off the migration cycle the following spring.

Green darners are named after their resemblance to a darning needle. Photo © Phil Hauck.
Green darners are named after their resemblance to a darning needle. Photo © Phil Hauck.

As summer comes to a close, I’m still striving to live the “lake life.” I walk along the Lake Michigan shoreline at Fort Sheridan in Lake Forest and watch soaring raptors ride thermals overhead as hawk migration gets underway. Closer to the ground, green darner dragonflies zip by on their shimmering wings. Thanks to chemistry and community science, I’m keenly aware of the dual migrations that stratify the skies.

To witness green darners and other wildlife migrating, you might try our Hike Lake County Challenge, now in its 23rd year. Complete seven of the 12 designated trail routes between now and November 30, and you’ll earn a free commemorative shield or zipper pull. Bring your dog along and Fluffy will receive a commemorative dog tag for their collar! See this year’s trails.

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