Potomac River's Creepy Crawlies Help Make the World Go Round

Searching for monsters in the Potomac River can be a lonely and sometimes uncomfortable job in the middle of February. Most people have better things to do.

I threw my waders into the back of my car and headed to Carderock. A short ways down the Billy Goat Trail I found a likely place and slid down the bank, using exposed tree roots as handholds. My boots hit the mud.

In the shallows, a few feet from shore, I plunged one arm into the stream to grab a large, flat rock. I tipped it up on its end and inspected every nook and cranny. In the summer, I would normally find all manner of tiny crawling things, mostly the larval forms of insects. With multi-spiked tales, armored bodies, and fearsome jaws, they look right out of a low-budget horror movie.

But today, nobody was home. Several stones later, my hands began to grow numb, so I gave up.

It was time for plan B. I would find my monsters at the nearby Audubon Naturalist Society (ANS) in a class with a monster of a name: “Benthic Macroinvertebrate Identification.”

The classroom was filled to capacity. Here were people of all ages, eager to learn how these little creatures lived and how to identify them. Armed with this knowledge, they will go to streams to collect specimens and record their findings. Different kinds have different tolerances to pollution, so their presence or absence says something about the quality of the water.

Probably no group of such “citizen scientists,” except birdwatchers, makes a greater contribution to our knowledge of the environment than these stream monitors. Schools here in Potomac and across the country make macroinvertebrates a part of science education. It’s a great way to experience nature in the raw and do some real-life data collection.

I leaned over my microscope to examine the tiny creatures while inhaling the alcohol fumes in which they took their last swim.

Cathy Wiss, ANS Water Quality Monitoring Program Coordinator, dissected the name of the course. “Macro,” she said, means anything you can see with a naked eye. The “invertebrate” part of the name means anything without a backbone, mostly insect larvae.

The final word in the course name was “benthic.” It simply means that these particular insect larvae live on the bottom of our rivers and streams, under rocks, in the mud, or hidden in the weeds.  

Conducting the class this week was ANS stream monitor Gretchen Schwartz. Her subject was one particular group of macroinvertebrates called Trichoptera, the caddis flies.

Enlarged by the lens, many of them looked a little like tiny, elongated shrimp with curved bodies and six legs poking out of their thorax. But actually they are most closely related to moths and butterflies. As larvae living in the water, they make and use silk (more about this later). When they hatch into flying adults, they live on nectar, at least for the few days until they mate and lay their eggs. Then they die.

Schwartz told us other things about Trichoptera that once again showed nature’s infinite variety and inexhaustible ingenuity.

First, they make their living in all different ways. Some kinds scrape algae off of rocks. Others shred up leaves and eat them. Aggressive predators seize and consume other macroinvertebrates.  

Most remarkably, some use their silk to make delicate nets. They set the nets on the stream bottom and collect whatever edibles come by. Other species use silk strands as kind of lifeline to hold them to the bottom in fast currents.

Many Trichoptera use this same silk to make houses, wrapping it around grains of sand and then tying grains together to form a tube that they wear as a kind of a flour sack. (Some enthusiasts give captive Trichoptera colored grains to make their tubes, which they fashion into earrings. It could be a conversation starter, or stopper, depending on the company.)

Beyond useful and clever, these macrointertebrates are also extremely important. They and countless billions of other tiny creatures are what renowned Harvard biologist E.O. Wilson calls “the little creepy-crawlies that run the earth.”

If Pandas or elephants — or us — suddenly disappeared, life on earth would pretty much go on. But remove bees and ants — and benthic macroinvertebrates — and our environment would collapse in a hurry.  

See Trichopters in action in this video on the World News website.