Common Green Darner

Common green darners hunt entirely on the wing, catching flies, moths, wasps, mosquitoes, and even smaller dragonflies in mid-air. Their large compound eyes wrap around the head and contain tens of thousands of lenses, giving a near-360-degree view that detects movement from almost any direction, though they see poorly directly above or below. They can also see ultraviolet light. Four independently moving wings let them hover, turn sharply, and even fly backward. Dragonflies in general can eat as many as 100 mosquitoes a day, and green darner nymphs eat mosquito larvae too, an ecological service that benefits wetlands and nearby communities.

In cold northern springs, green darners are often among the first dragonflies on the wing. They are active thermoregulators, contracting and vibrating their flight muscles to generate heat, which lets them fly in cool weather. They can also adjust the flow of hemolymph, the insect equivalent of blood, to spread that heat through the body, and they bask in the sun when it is available. A field study of flying green darners found they regulate thoracic and head temperature at air temperatures down to at least 20 °C (68 °F), producing more heat as the air gets cooler. This lets them patrol and hunt when many other insects remain grounded.

Green darner nymphs, called naiads, live underwater as predators, eating aquatic insects, small crustaceans, tadpoles, small fish, and even smaller darner nymphs. They catch prey with a spring-loaded, extendable lower lip called the labium, and they molt through 9 to 13 growth stages. Development time depends on climate: resident northern nymphs overwinter and take 11 to 12 months, while the offspring of migrants develop in just 3 to 5 months. Growth pauses when the water gets too cold. When ready, the nymph crawls up a plant above the water and emerges as a winged adult, which then lives only about a month.

Green darners make one of the best-documented insect migrations, a relay among three generations each year. The first emerges in the southern US, Mexico, and the Caribbean in late winter and flies north, then lays eggs and dies. The second emerges in the north in summer. Some overwinter as nymphs, but many fly south between July and October. The third develops in the south over winter without migrating, and its offspring become the next spring’s northbound migrants. No single dragonfly makes the full round trip, which averages about 825 miles (1,330 km) across the generations. How the insects know their route and timing is still an open scientific question.

During mating, the male grasps the female behind her head or at the front of her thorax with the appendages at the tip of his abdomen, forming the “mating wheel.” Afterward, the pair often stays linked in tandem while the female lays batches of hundreds of eggs, inserting them into aquatic plants beneath the water’s surface. The green darner is the only North American darner noted for laying eggs in tandem. The male guards her from rival males, which likely helps protect his paternity. Mating is polygynandrous, meaning both sexes mate many times, and the female chooses whether to mate.

Adult green darners are solitary hunters, but in fall they gather in loose coastal swarms as they head south from mid-August to October, sometimes alongside other dragonflies and birds. Cold fronts often trigger departure, and migrants ride thermals and updrafts, stopping often to feed, so the trip can take several weeks. They may also use polarized light to orient south. Birds have learned to follow: at Hawk Ridge, Minnesota, in 1995, 28 percent of passing American Kestrels were seen eating darners, and kestrel counts rose and fell with darner counts. These swarms are not social groups like a bee colony; they form because individuals respond to the same weather and landscape cues.