A male green darner, with its blue abdomen, holds on to the female while she deposits eggsOhio is home to something like 100 species of dragonflies, and to a large extent, they are creatures of summer. Both the abundance and the diversity of dragonflies peak in June and July. But a few species stretch those seasonal boundaries, appearing on the landscape earlier in spring or sustaining activity well into autumn. The earliest of those is the green darner, Anax junius. In 2022, the first sighting of dragonflies in Ohio (as reported to iNaturalist.org, the online database of critter sightings) was a mating pair of green darners observed on 5 March in Montgomery County. For the rest of March and all through April, green darners were pretty much the only dragonflies reported in the state. How do they do it?

Green darners are large dragonflies, up to 8 cm (3 inches) in length and with a 10 cm (4 inch) wingspan. In both males and females, the thorax is green; however, the abdomen is bright blue in adult males, whereas it’s purple/brown in females and young males. Green darners cruise the landscape in search of insect prey. Indeed, more than most species, they seem to fly non-stop most of the time; they can be a real challenge for photographers!

Like other dragonflies, green darners are tied to water for breeding. Dragonfly pairs mate and then, with the male holding on, the female lands on the water surface to deposit eggs in wet vegetation (as pictured above). The juveniles (nymphs) that emerge from those eggs go through several rounds of molting as they grow to become voracious wetland predators; tadpoles beware! Eventually—either later that same season or after an over-winter hibernation—late-stage nymphs climb onto stems of waterside plants, complete metamorphosis, and emerge as adults.

As in all insects, the effectiveness of those bodily functions—flight, hunting, mating, maturation—depends on the dragonfly’s body temperature. Surprisingly, though, despite their early arrival in Ohio, green darners’ range of operating body temperatures is not lower than that of other local dragonflies. So how is it that they—and only they—manage to appear on the scene and stay active during the cool weeks of spring? The answer lies with two features that derive from their strong flight abilities.

The first special feature of green darners, directly responsible for their early appearance in Ohio, is their tendency to migrate. Whereas migration is well known in some insects (monarch butterflies and locusts especially), it is much less well characterized in dragonflies. Among Ohio’s numerous species, only a handful are thought to perform regular, seasonal migrations. Green darners are surely the most well-known of these, as they sometimes move in conspicuous swarms of thousands of individuals, especially during southward migration in autumn.

Modern technology has provided novel insights into green darner migration. Researchers have attached miniature radio transmitter devices to individual dragonflies, then followed the signal from those transmitters using a network of ground-based sensors (www.motus.org, used mostly to track bird migration). Green darners migrate only during the daytime, and they travel up to 120 km (more than 70 miles) in a single day. Not bad for an insect weighing half as much as a penny! Ohio’s migrating darners probably head to the southern US, where they breed and die; their offspring then return north.

In Ohio, the darners that appear and lay eggs in early spring have migrated from the south; at that time of year, Dayton’s wetlands are too cold for nymphs to metamorphose and emerge. Most likely, nymphs that arise from those springtime bouts of egg-laying mature into adults during the summer months that follow, and then may migrate south as the season advances further. However, green darners also continue to breed throughout summer. Those that hatch from later breeders probably over-winter as nymphs (they enter suspended animation, called diapause, as the waters cool) and emerge the following year when waters re-warm.

The second special feature that enables green darners to be active on cool spring days also derives from their strong powers of flight. The considerable muscular work required for these large insects to fly generates a lot of heat. In spring, green darners use that heat to elevate the temperature of their thorax, where the flight muscles are located. They also regulate a warm head temperature, which probably enhances visual acuity. On warmer summer days, darners actually generate too much heat, and they cool off by directing the flow of blood (called hemolymph in insects) to their abdomen, where heat can radiate away. They also change flight behavior, using more gliding and less flapping to reduce heat production. That overall phenomenon, where internally-produced heat is used to regulate body temperature, is known as endothermy. Endothermy is uncommon in insects and has been documented just a few times in dragonflies.

So, green darners use migration to arrive in Ohio early and, once here, they use endothermy to support activity on cool spring days. Still, many questions remain about these phenomena. Why do so few dragonfly species migrate? Why do some individual green darners migrate and others not, and what triggers the migratory flight? March 5, 2022—that early date when green darners appeared in Dayton–was a warm day (22oC, 71oF), good for dragonfly activity; but where did they come from? How common is endothermy in dragonflies?

March is a transitional time of year, and new signs marking the progression of spring seem to appear on a daily basis. Toads and robins sing, mourning cloak butterflies emerge from hiding, and trout lilies color the forest floor. The appearance of green darners seems like a good addition to that list. Keep an eye out for them!

Article and photo contributed by Dr. David L. Goldstein, Emeritus Professor, Department of Biological Sciences, Wright State University.