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Summer 2012

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

September 11, 2012

Dragonfly migration: A one-way ticket

In mid-September, winds coming from the north create good conditions for observing dragonflies migrating south. Watch for them on light breezy days in the early afternoon, before thermal updrafts lift the insects out of sight. Even when floating just above the treetops, a big dragonfly presents only a subtle profile, that of a darning needle held aloft by four transparent wings.

Of North America's 331 dragonfly species, only nine regularly make long trips down the continent. The most abundant of these is the widespread common green darner.

Young adults, which recently emerged from their stage as underwater nymphs, prepared for migration by packing on fat — a third of their one-gram weight.

Migrating only during daylight hours, darners can fly more than 60 miles a day, often following shorelines. They regularly stop over for several days at a time to prey upon mosquitoes, gnats, flies and bees until falling temperatures urge them on.

But it's not all dining and sailing for the big-eyed insects. They have predators to reckon with; American kestrels, small falcons that follow the dragonfly swarms, picking them off to fuel their own migration.

If falcons don't get them, the clock will. Adult darners live for only a few months. The lucky ones making it to Florida, Texas, Mexico or the Caribbean will enjoy a few weeks of breeding before they die. Their progeny will be the ones heading north in the spring.

Sources: Mike May, Rutgers University; Biology Letters; The Loon; University of Michigan; Migratory Dragonfly Partnership

Common green darner, Anax junius

Anax junius