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Dragonflies use their enormous eyes to see almost 360 degrees around their head. Although the eyes can't change focus, they do have depth perception, and are extremely sensitive to motion. If you were a dragonfly, your eyes would be the size of a football helmet!
Dragonflies use their enormous eyes to see almost 360 degrees around their head. Although the eyes can't change focus, they do have depth perception, and are extremely sensitive to motion. If you were a dragonfly, your eyes would be the size of a football helmet! / Photo by Robert Hilbert of Loretta

About the Cable Natural History Museum:

For over 45 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI, at 13470 County Highway M. The current exhibit, “Deer Camp: A Natural and Cultural History of White-tailed Deer,” opened in May 2013 and will remain open until April 2014.

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CABLE — Blue skies and a fresh breeze put a spring in my step as I gathered tools – hoe, rake, shovel – from the shed at the Cable Community Garden. My garden would be fully planted by the end of the afternoon, and I was looking forward to the satisfaction of working the soil, tossing rocks, and nurturing new life.

It was hot, though, in my long pants, long sleeves, and hat. The bugs have been bad at the garden (and everywhere else in northern Wisconsin!), and it’s hard to carefully place seeds in a row and slap mosquitoes at the same time.

Then, as I carried a flat of squash plants to the gate, I looked up. A cloud of dragonflies was darting everywhere with amazing speed and agility. For as far up and as far out as I could see, their tiny silhouettes filled the sky. And then it dawned on me, I hadn’t slapped a single mosquito since I got out of my car!

For weeks now, people have been asking me “When are the dragonflies coming?” “Soon,” I’d answer, or “I’ve seen a few by the lake.” As beautiful as dragonflies are for their own sake, these queries were really about the mosquitoes. Deep snow, a late winter, and a quick spring (once it came) have somehow conspired to bring the little bloodsuckers out in terrible numbers. People yearn for the dragonflies to eat the mosquitoes.

Dragonflies are exquisitely adapted aerial predators. Each of a dragonfly's four wings operates independently, powered by its own set of muscles, resulting in fantastic maneuverability. Dragonflies can fly backward and forward, straight up and straight down, hover, accelerate to full speed in a split second and make hairpin turns. Small antennae seem to have little function except to measure air speed in flight – which can reach 30 mph.

Dragonflies have adapted to see faster, too. Together, a dragonfly's eyes and brain can detect movements separated by only 1/300th of a second! Dragonflies also possess an almost super–human capacity for selective attention. They can focus on a single mosquito in a swarm, track the moving target and adjust their path to intercept the prey with a 95% success rate. Appropriately, the word "dragon" comes from an ancient Greek word that means "sharp-sighted one".

Three pairs of legs attach to a dragonfly’s slanted thorax. This tilts the legs forward, and creates a spiny basket used to catch prey on the wing. Small insects can be scooped into the mouth without pause, but larger prey are taken to a perch. There, the dragonfly can masticate the insects with the serrated teeth on their mandibles. These teeth give dragonflies their scientific family name – Odonata – from the Greek word for “toothed one.”

While dragonflies can eat their own weight in food every 30 minutes, they aren’t actually reliable mosquito control. Dragonflies complete just one generation each summer, while mosquitoes breed multiple times. In addition, eating mosquitoes is like you trying to make popcorn into a satisfying dinner. Dragonflies prefer bigger prey – even other dragonflies – when it is available.

Even so, the dragonflies really seemed to make a dent in the mosquito population in the garden. So why did they take so long to arrive?

Many dragonfly species synchronize their transition from nymph to adult, which results in a “mass emergence.” By all metamorphosing at about the same time, dragonflies can ensure that males and females will mature at the same time and find mate. The cloud of dragonflies will also overwhelm predators, and ensure that at least some survive.

Different species are on different schedules, though. While we see dragonflies from spring to early fall, we are actually seeing a series of different species. Most adult dragonflies live only a few weeks, although some can live up to a year.

The “spring species” of dragonflies that zoomed over my head spent last summer as aquatic larvae growing into their final stage before metamorphosis. Then they stopped growing, and overwintered as almost-adults, ready to emerge when the water warmed enough.

Now they are fierce aerial predators, patrolling the skies with agility and grace. As I happily tend my garden without long sleeves or bug spray, I don’t think it’s too much to call them my angels of mercy. Do you?

Read more posts from Emily Stone.

Emily Stone is a Naturalist/Educator at the Cable Natural History Museum. Visit http://www.cablemuseum.org to learn more about exhibits and programs at the museum. For more from Emily Stone, visit her blog at http://cablemuseumnaturalconnections.blogspot.com/.

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