green and blue slivers
the dragonflies whirl and dart
high and low circles
The above haiku is one of many about dragonflies, an insect contemplated in Asian poetry and prose for hundreds of years. But you don’t need to be haiku master Basho to appreciate these fantastic little predators. We in Ohio are blessed with over a hundred species, each with a cool and unique place in the ecosystem. We’ll illustrate this by focusing on a common large species, the Green Darner (Anax junius).
Green darners are most people’s idea of a dragonfly. They are part of the darner family (Aeschnidae), a widespread family of big dragonflies. Big (up to five inches long), brightly colored, and exceedingly fast, darners are among the master aerialists of the insect world. They have to be to survive in a world of hungry birds who are mostly much larger than themselves. Incredibly, they’ve done so by updating a very primitive flight system that started evolving before there were dinosaurs or birds. Their long wings cannot be folded up or tucked in, so for adult dragonflies, it’s fly or die.
Green darners are the biggest darner in North America and also the most widespread. They have a green thorax and a blue (male) or brownish (female) abdomen. Like most darners, they’re aerial for most of the daylight hours, continually patrolling for food and mates. They can hover and even back up a bit, but they are most comfortable in fast forward flight, usually at least three feet high over land or water, often higher. They can eat any insect smaller than themselves, including smaller dragonflies. They grab their prey in flight with a basket-shaped array of legs and usually eat it while flying. The only times they stop flying are for sleep (when they grasp a stem and hang down) or for parts of mating and egg-laying. They’re like the Swifts of the insect world.
Male green darners are aggressively territorial, patrolling a patch of pond and physically ramming any intruders. If a receptive female should enter the territory, the male performs a looping aerial display. If she doesn’t leave, he’ll seize her behind the head with his abdomen while flying. She will then reach forward under him with her abdomen to receive sperm from a sac just behind his thorax. This leads to a bizarre-looking circle position called the ‘wheel’; the pair often alight on vegetation while in the wheel (so there are quite a few pictures of darners in this position). After 10-20 minutes, the female will uncurl her abdomen, while the male drags her aloft. They will search, as an attached pair, for grass and herb stalks sticking out of a puddle or shallow pool. Alighting on the stalk, the male will back the female down the stem so that she can use her abdomen and ovipositor to insert an egg into the stem below the waterline. Watch this behavior here: www.youtube.com/watch?v=QmXwg0OpNHE. After multiple ovipositions, the pair release and the female flies off.
The egg is protected in the stem until hatching, whereupon the tiny nymph crawls out into the water. They are very different from the aerial adults, being shorter, brown/green-camouflaged, and gilled for survival in the water. They’re ambush hunters, grabbing small worms, insects, tadpoles, and even tiny fish. They have an extensible, hooked lower ‘lip’ that they use to strike at potential prey with lightning speed.
As it grows, the dragonfly nymph starts to take on more adult characteristics, such as long abdomen and wing buds. After it reaches a large size, some combination of size and environment trigger the nymph to crawl up out of the water and eclose, in which it bursts through its old skin and re-forms itself as an adult dragonfly. The process is elegantly summarized in another video.
Green darners have another secret: some of them migrate. Large flocks have been seen along mountain ridges and shorelines in both autumn and spring. Like monarch butterflies, this migration is a multi-generational affair; the dragonflies that journey south will not be the ones that come back north next spring. Unlike monarchs, though, green darners don’t seem to gravitate to a specific area. We know little about the mechanisms or specifics about darner migration, so this is an area where some wow facts will likely come out over the next few years.
What about darners in central Ohio? They return here in April or May, depending on how warm weather is during the spring. They’ve been here since early April this year. They forage over almost any lake or pond or marsh, so large wetlands are usually a good place to search for them. Pickerington Ponds and the Darby Creek wetlands are good places for them, as are some of the park ponds like Thoreau Lake at Blendon or Darby Bend Lakes at Prairie Oaks. Just look for the blue-green blurs circling high over the water. Come September, they’ll make their exit, so enjoy them now.