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The adult green darner dragonfly (Anax junius), also known as the mosquito hawk, is a voracious predator. It snatches and devours small insects as they fly over water. Mosquitoes, gnats, moths, tiny flies, beetles and even smaller damselflies make a delicious meal. Because each of its four wings moves independently, a green darner is highly maneuverable: It can fly forward, backward, sideways, up or down. It can even hover or spin. When still, darners hang from vegetation by their long legs, rather than perching upright.
This dragonfly spends most of its life as an aquatic nymph, though, molting 11 or 12 times before crawling out of the water and molting one last time to become a 3-inch adult with functional wings. Both males and females have bright green thoraxes (the second body segment), but their abdomens are different colors: Females have reddish brown abdomens, while the males have abdomens that are blue.
Like the adults, each nymph is a voracious predator. Its spring-loaded lower jaw snatches prey and pulls it into its mouth. Green darner nymphs are also highly maneuverable, even though they live in water. Each nymph has gills in its rectum, and draws water in and out to breathe. It can expel a burst of water quickly to lunge at prey or to escape an attacker.
Green darners, like monarch butterflies, migrate. From late July to mid-October, peaking in September, swarms fly to the southern United States, the Caribbean or the Gulf of Mexico. (It’s still the Gulf of Mexico to me.) They can fly 25 to 35 mph. As they migrate, they stop to feed, so the journey can take weeks. Like monarchs the complete migration takes several generations.
The green darners that return north in the spring are the great-grandchildren of the dragonflies that migrated south in the fall. Not all green darners migrate, though. Some resident green darners in Massachusetts overwinter as aquatic nymphs, becoming adults later in the season than the green darners that migrate from the south. (In other words, the green darners you see in March have migrated here. The resident green darners, overwintering as aquatic nymphs, become winged adults later in the season, when the water warms.)
To monitor bird migrations, scientists often attach tracking devices to birds. How did scientists figure out green darner migration patterns, when they are too small for this method to work?
Scientists knew green darners migrated because people observe their flights every year. But which generation was traveling was difficult to decipher.
Researchers decided to use chemical clues. Dragonfly wings pick up a mix of hydrogen forms (isotopes) from the water in which the nymphs mature. A form called hydrogen-2 (deuterium) is more common in the south and decreases as you travel northward. Analyzing a pinhead-sized portion of a wing, scientists could tell whether a dragonfly was born a Yankee or a Southerner. They checked 800 green darner wings (including wings from museum specimens) spanning 140 years. This is what they found:
In the spring, the first generation of adult green darner dragonflies emerges in the southern United States, the Caribbean or Mexico. Temperature dictates when they fly north. The air must be at least 49 degrees for them to survive. In Massachusetts, temperatures usually reach 49 degrees in March or April, and that’s when the first green darners arrive.
Upon arrival, these dragonflies mate, lay eggs in ponds (producing a second generation) and die. By late summer, some of the second-generation nymphs have become adults who fly south beginning in late July (but peaking, as I said before, in September). After they arrive at their destination, they mate, lay eggs in ponds (producing a third generation) and die. This third generation of nymphs becomes winged adults in November. This generation cannot return north because the weather has become too cold in the north. So these dragonflies stay in the south all winter where they mate and lay eggs, which become nymphs. Beginning in February, these nymphs become the next generation of winged adults. These green darners begin the cycle all over again. They fly north in spring as the temperatures warm, repeating the journey their great-grandparents made. The first migrants often reach Massachusetts in March.
Unlike monarchs that move north over several generations, green darners move all the way to the northernmost or southernmost parts of their range in one generation.
Green darners are not the only dragonfly species that migrates. In North America there are about 18 migratory dragonfly species, including black saddlebags, wandering gliders and variegated meadowhawks.
Green darners are one of the most abundant dragonflies in North America. You can see them earlier than most other dragonflies in the spring (and now you know why). Their offspring are still around in the late fall. Look for them on your walks near ponds and marshy areas.
Have you taken photos of our urban wild things? Send your images to Cambridge Day, and we may use them as part of a future feature. Include the photographer’s name and the general location where the photo was taken.
Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.
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