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Sentinel species Scientist Sarah Nelson is using dragonfly larvae to monitor mercury contamination. by Rich Hewitt
Dragonflie on a plant

Dragonflies spend up to five years as larvae living in water, which makes them ideal for monitoring mercury contamination. Larvae species: Aeshnidae, Corduliidae, Gomphidae and Libellulidae.

In the pastoral settings of freshwater ecosystems, dragonflies buzz and flit through the grasses and floating plants of ponds and lakes. But an up-close look at the larvae of these wetland predators brings images from Alien and other sci-fi horror films to mind.

Large, bulbous eyes. Six spindly legs on a somewhat short, slightly stubby body. Gills in their abdomens to breathe. A lower jaw-like feature that’s more like an extendible weapon, shooting out in an instant to grab something edible as it passes by.

And they eat just about anything that moves — from mosquito larvae and tadpoles to small fish. Even other dragonfly larvae.

Indeed, dragonflies spend most of their lives as larvae — up to five years. The larvae hatch from eggs in the water, grasses or mud, grow up to 2 inches long and molt repeatedly before developing wings and taking flight.

That aquatic existence is what endears dragonflies to biogeochemist Sarah Nelson, who studies the insects as bio-sentinels of mercury in freshwater ecosystems.

Nelson, a scientist at the Senator George J. Mitchell Center and the School of Forest Resources at the University of Maine, has been researching mercury in the environment since she first walked onto the Orono campus as a graduate student. Building on a long history of mercury research at UMaine, she is now the primary investigator for several studies regarding mercury, including research involving dragonfly larvae.

At the same time, she has worked with other agencies and organizations to develop educational programs that put teachers, students and citizen scientists into the field as frontline researchers.

Armed only with hip waders and dip nets, students from schools throughout New England regularly go searching for dragonfly larvae under Nelson’s guidance. They are not only discovering science, she says, but also are participating in meaningful research that has bolstered the work being done by professional scientists. Dragonfly larvae are easy to identify and it doesn’t take the students long, as one of their teachers put it, to “get their dragonfly eyes on.”

“They know what they’re looking for,” Nelson says. “It’s hard to get them out of the stream. They go out and bring back what they find on the stream bottom, and they’re just amazed at the critters they see. That’s a great place-based learning outcome in its own right.”

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Spring 2013

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