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Bitter winds blow across a frozen landscape, but under the ice hide the jewels of summer.
Even during an Arctic cold snap, many quick-flowing and spring-fed rivers maintain an open channel of inky current. This meandering passage often follows the thalweg (one of my favorite words), which is the deepest channel of the riverbed, usually with the swiftest flow. The input of relatively warm groundwater may help prevent ice formation, as does the constant churning of molecules. Even the still waters of lakes and ponds remain liquid below, insulated by the layer of less-dense ice floating on top.
Except for the ice fishermen’s prey, we often forget that anything lives in the dark depths below the ice and chilly water. But crawling around on the river bottoms are some of the most grotesque, fearsome, and ancient predators you may ever encounter. Come summer they will emerge from the depths, shed their gruesome shells, and take flight as shimmering-winged dragonflies.
Understandably, you don’t see those colorful acrobats this time of year. Most dragonfly species do not overwinter as adults. The ones that buzzed your cheek and caught a ride on your canoe paddle? They’re all dead, their genes (hopefully) passed on to the next generation. Common green darners are an impressive exception, as they fly more than a thousand miles to Mexico or Florida with other snowbirds. A few species overwinter as eggs, frozen neatly into the stems of aquatic plants. But most spend the winter hidden under the ice on our lakes and streams, as alien-like nymphs.
Nymphs are the immature stage of insects that go through gradual or “incomplete” metamorphosis. The alternative is “complete” metamorphosis, which is what butterflies do when they enter the inactive pupal stage and then transform abruptly to a flying adult. Dragonfly nymphs shed their skin several times as they grow through stages called “instars,” until finally they emerge as a flying adult.
Dragonfly nymphs may spend anywhere from four weeks to several years growing through the instars. The cooler and shorter the summers are where they live, the longer it takes. In the meantime, they rule the river bottom as fierce predators.
Their hydraulic-powered hunting system is not for the squeamish. To catch food, a nymph draws in water through its anus, and clenches its abdominal and thoracic muscles against the water-filled rectal chamber. The amazing amount of pressure now trapped inside the nymph’s body cavity pushes out its labium, or toothy lower lip, in a high-speed strike. The lightning attack may earn the tiny predator a meal of tasty mosquito larvae, a tadpole, a small fish, or even another species of dragonfly nymph.
The dragonfly nymph’s hydraulic system isn’t just used for hunting. By jetting water out the way it came in, nymphs can propel themselves forward at a speed of 10 centimeters per second. That power of acceleration can help when they are on the hunt, and also allows for quick exits if they become the hunted. Smallmouth bass, for example, might love a nymph for lunch.
As the water goes in and out, it passes by gills in the dragonfly’s rectum, and helps the little critter absorb oxygen. This constant filtering of the stream through their bodies means that dragonflies can’t survive in streams polluted by heavy metals, agricultural runoff, and sewage. Dragonfly nymphs make excellent water quality indicators.
If they survive the winter, dragonfly nymphs will use the abundance of spring and summer to continue growing through their required eight to seventeen instars (depending on the species) before their final mutation into adulthood. The metamorphosis is astounding. From a split down the back of a scraggly, brown, bottom-feeder emerges a colorful, fairy-like being with delicate, dexterous wings.
Their beauty belies the power they retain as a predator. Separate muscles control each wing, and enable dragonflies to swoop acrobatically, move straight up or down, fly backwards, stop and hover, and make hairpin turns – all at full speed or in slow motion.
Huge eyes take in a 360 degree view and allow them to lock onto a moving target, judge its trajectory, and intercept it – with a 95% success rate. Dragonflies’ spiky legs, a relic from their previous life as a water monster, form a net to snag prey on the wing. Those prey include our old friends the mosquitoes, who are (at the moment) “out of sight, out of mind.”
The dragonflies themselves are a bit hidden right now, creeping along the river bottoms and lake beds beneath the ice. They are waiting just as we are – lying low until they can tear off their heavy shells and bask in the warmth of the sun. If you saw one, would you recognize it as a diamond in the rough? Could you imagine it soaring as a jewel of summer?
“I want to think again of dangerous and noble things. I want to be light and frolicsome. I want to be improbable and beautiful and afraid of nothing as though I had wings.”
from “Starlings in Winter”
by Mary Oliver
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.
Find us on the web at www.cablemuseum.org to learn more about our exhibits and programs. Discover us on Facebook, or at our blogspot, http://cablemuseumnaturalconnections.blogspot.com/.