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NATURE
Jim McCormac

An incredible spectacle unfolded in early September, when untold numbers of dragonflies descended on Ohio and surrounding states. Feeding swarms, ranging from a few dozen to thousands, were reported over fields and meadows in all corners of the state.

In some places, flying dragonflies were so thick that they were visible on weather radar. Many news outlets breathlessly reported the “invasion” of dragonflies. Some Facebook users marked themselves “safe” from the swarms, as if it were some sort of Hitchcockian entomological counterpart to “The Birds.”

Most folks were enchanted by the sight of dozens of insect aerialists, wings glittering in the sun, zigging and zagging as they tore after midges and other small flying insects.

The overwhelming majority of the dragonflies were common green darners (Anax junius). They are big, reaching 3 inches in length, with a slightly longer wingspan. Males have a turquoise-blue abdomen, while female abdomens are purplish-red. The thorax of both sexes is bright green.

Other species were mixed in with the swarms, but in far smaller numbers. Companions included black saddlebag, green-striped darner and wandering glider.

I posted a plea for swarm sightings on my blog, and on a few online forums. Nearly 200 reports came back, from 63 of Ohio’s 88 counties. Hundreds of other postings about swarms were made on Facebook and elsewhere, documenting a massive movement that certainly touched all counties.

This migratory movement was short-lived, with most records falling on Sept. 10 and 11. Some reporters estimated swarms numbering over 1,000 dragonflies, but most observers saw between a few dozen and several hundred.

Dragonfly migration is imperfectly understood. The common green darner has long been known for large autumnal southward movements. Like migratory monarch butterflies, these swarms are generally on a southwest trajectory. They certainly are headed to warmer climes, but exact destinations remain a mystery.

Evidence suggests that most migratory dragonflies are headed to points from the Gulf Coast to Central America. The big movements generally coincide with the passage of a cold front, and in many areas it appears the dragonflies follow prominent landmarks such as lakeshores, rivers, ridges or other land features.

It’s likely that the dragonflies that move south in winter are not the same ones that return in spring. Vernal migrations are far less conspicuous, and spring migrants don’t seem to form swarms. It might be that adults newly arrived on the wintering grounds mate, produce offspring and die. Their spawn are the dragonflies that recolonize the north the following spring.

More conjecture than fact surrounds dragonfly migrations. No one is sure why some species — only a handful of the hundreds of North American species are known to migrate — form enormous aggregations. Some swarms have been estimated to number well over 1 million.

The passage of these “flocks” is typically rapid, and if you’re not looking skyward when they pass over, they will go unseen. Only when the dragons drop down to feed on smaller flying insects do they become conspicuous. Those people fortunate enough to see a feeding swarm bore witness to one of nature’s great fascinations — and enigmas.