Water quality indicators, it's a bug’s job

Dragonfly posing on a branch

Courtesy Joan Wilson


By Joan Wilson

Most of us can go days if not weeks without thinking about bugs. But we can’t go more than a few days without fresh water. Don’t see a connection? Well, bugs can tell us a great deal about the quality of our water, so maybe we should start paying more attention to these little critters.

Macroinvertebrates—animals large enough to be seen by the naked eye (macro) but having no backbone (invertebrate)—live in streams, rivers, lakes and ponds. And it’s a good thing they do, because these amazing animals can tell us a lot about the health of local waterways.

If you’ve spent any time outside, you’ve encountered macroinvertebrates. The most common types are dragonflies, mayflies and stoneflies, crayfish and water beetles, snails, leeches and black flies. While leeches and black fly larvae can tolerate just about anything, many macroinvertebrates—especially stoneflies and mayflies—are highly sensitive to pollution. When scientists and environmentalists find a lack of “macros” in a water source, they know a problem exists. And a problem for bugs is also a problem for us as we all depend on the same water sources. 

Dobson fly eyes the camera

Courtesy Joan Wilson

Macroinvertebrates spend the first years of their lives—anywhere from one to four years—as larvae or nymphs, fully submerged in the water. Eventually, they shed their exoskeletons and emerge as winged adults. It is the adult version of macros that we see flitting about pursuing mates and laying eggs. Sadly, their adult lives are only a fraction of their childhood—a day to a few months.

Consider mayflies. While mayflies live as nymphs in the water for two years, as adults they live for only 24 hours. The next time you’re at an evening baseball game watching the mayflies swarm the lights, remember that it is the one and only day of their adult lives. Suddenly it becomes a magical and beautiful sight.

But macroinvertebrates would never become adults if they didn’t have clean water. Many nymphs have gills on the outside of their bodies, making them sensitive to environmental conditions. (Just imagine how sensitive to air quality you’d be with lungs on the outside of your body. Indeed, macroinvertebrates are the water equivalent of canaries of the coal mines. 

So the next time you see a bug that looks like a giant mosquito, don’t squash it. It’s an adult crane fly, and it’s telling you that the water is fine.

Joan Wilson is an environmental educator, a photographer, and past Pennsylvania's Chapter Coordinator.

Published March 2006