30 August 2011

Common green darner dragonflies (Anax junius) are fast and powerful fliers, the highly maneuverable “fighter jets” of the bug world.

Like peregrines they capture their prey on the wing but their methods are those of flycatchers.  They chase flying insects and grab them with their mouths.

Though some common green darners overwinter as nymphs, many of them migrate at this time of year.  The adults fly south pretty rapidly but those who fly north in the spring take their time to reproduce so that the individuals then are a later generation than the ones who left.

How fast and far do these bugs travel?

In 2005 a team of scientists attached very tiny radio transmitters to 14 common green darners in New Jersey and followed them on the ground and in the air using radio receivers and a Cessna airplane. 

The results were intriguing.  These dragonflies make many of the same migration choices as songbirds.  They…

• Avoid large bodies of water and will head north to go around them.  Dragonflies that reached the tip of Cape May traveled up the shore of Delaware Bay to find a narrower crossing.
• Rest on windy days (of course!).  They prefer winds less than 15 mph.
• Prefer to fly on the day after two nights of successively lower temperatures.  This is usually a day with a north wind.
• Can fly for 8 hours on stored fat.
• Migrate every three days on average, so their daily progress comes to 7.5 miles/day with idle days included.
• On a good day they can travel 60 miles.
• Their average speed is 5 meters/second = 11.18 mph.
• In two months they’ve traveled 700k (435 miles).

So when you see a green darner, view it with awe.  It’s one of only a dozen dragonflies — out of 400 species in North America — who migrate.  

p.s. The bug in this photo is male; he has a blue abdomen.

(photo by Tim Vechter)