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August 24, 2011

Dragonflies - A Species Spotlight (or "My Fun With Dragonflies")

Green Darner Dragonfly (anax junius)

I love sitting in the open sun.  I have never been much bothered by our mosquitoes.  Arctic ones are a different matter altogether, as for some reason mosquitoes get fiercer and more numerous the farther north you go.  Caribou, for example, may become so depleted of blood by millions of teeming mosquitoes that they lose weight even while grazing full-time. 

The only animals that consistently prey on mosquitoes are dragonflies.  They have probably been doing so for at least 100 million years.  Mosquitoes seem to have habits that are designed to avoid overlapping with these predators.  They avoid the sunshine, where dragonflies are most active.  But hordes of mosquitoes appear as soon as I step into dense shady woods where there are no dragonflies.  That is, wherever and whenever dragonflies are scarce, mosquitoes are abundant. 

Dragonflies that fly at dusk can cash in on mosquitoes.  I suspect that the dragonflies' extraordinary eyes developed to keep up with prey trying to escape into the dark.  Behavioral adaptations have the same effect:

While walking in the grass recently during the heat of the day, I saw mosquitoes spring up.  I was then followed by several dragonflies that were hawking them.  The dragonflies seemed to have been following me directly, because when I shifted to a slow jog they continued to follow me.  They were acting like some species of birds - cowbirds in North America and cattle egrets in Africa - which also follow large animals because of the prey these animals flush. 

They were so intent on where I was that when I stopped in my tracks they didn't go any further either.  They just flew all around me.  One landed on my arm.  And, as I stood stock still so as not to scare it away, I noticed how beautifully green it was.  This was the Common Green Darner.  We have so many Common Whitetails that I was surprised to see this one. 

It put me in mind of the endangered Hine's emerald dragonfly. 

Hine's Emerald Dragonfly (somatochlora hineana)

As its name implies, this dragonfly has distinctive emerald eyes and a metallic green body with yellow stripes along the sides.  Relatively large, its wings span about 3.3 inches.  The story of the Hine's emerald dragonfly is much like that of the proverbial ugly duckling, except that it dies shortly after its transformation. 

The dragonfly spends the majority of its life in the larval stage.  Nymphs hatch and live in marshes high in calcium carbonate or sedge meadows over dolomite bedrock, where they prey mostly on other aquatic insects.  Molting many times, it eventually crawls onto land after 2-4 years, sheds its skin a final time, and emerges a glorious, beautifully colored, flying adult. 

Adults live only 2-6 weeks, feeding mostly on insects they catch in the air.  Within 7-10 days of emergence, adult males establish and begin patrolling territories, defending them against other males and mating with females who enter.  Females lay over 500 eggs by dipping the tip of their body into shallow water as many as 200 times. 

Both the United States and the IUCN list the species as Endangered.  Its main treat is habitat loss and destruction.  Many of the wetlands vital to its survival are drained for urban and industrial uses.  Contamination of habitat by pesticides and other pollutants and changes in ground water also negatively impact the species. 

Now believed to be extirpated in Alabama, Indiana, and Ohio, Hine's emerald dragonfly is now found only in Illinois, Michigan, Missouri and Wisconsin.  The largest population is in Door County, Wisconsin. 

A healthy dragonfly population is essential to a balanced ecosystem.  Imagine a world where mosquitoes have no natural predators left.

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