Stoneflies, even the common species, are increasingly hard to find these days. This stonefly nymph was found in Ferson Creek at LeRoy Oakes Forest Preserve in St. Charles.
Stoneflies, even the common species, are increasingly hard to find these days. This stonefly nymph was found in Ferson Creek at LeRoy Oakes Forest Preserve in St. Charles.

Good Natured: Stoneflies

Pam Otto, Outreach Ambassador for St. Charles Park District 8/22/2023 6:00AM

It may seem, at first glance, that our Fox River bugs are a pretty homogenous bunch.

I mean, really, how different could they be? Head, thorax, abdomen…two antennae, six legs, wings… Glanced from the windows of passing cars, or even up close and personal on a windshield or radiator grill, these often-plentiful insects all appear to share a certain sameness.

But talk to a naturalist, fisherman or anyone else passionate about streams and aquatic ecosystems, and you'll quickly learn that the various groups of River bugs are as diverse as ecological niches they occupy.

The Trichoptera, or caddisflies, are notable for their fanciful larval cases, their hairy little wings, and their tendency to form dense mating swarms that weirdly coincide with river town street festivals.

The Ephemeroptera, or mayflies, are famous for their short adult lives and, in some circles, their gorgeous colors, lacy wings and long, flowing “tails."

And then we have the Plecoptera, or stoneflies. When outdoor enthusiasts speak of these River Bugs, the conversation adopts a notably different tone. Voices hush, and faces take on a look of awed reverence. For stoneflies are the Holy Grail of all River Bug-dom.

Like caddis and mayflies, stonefly juveniles, or nymphs, are aquatic. But unlike their underwater neighbors, most of which are tolerant of some impurities, stoneflies are quite persnickety about their environment. They need cool, well-oxygenated water with a rocky, unsilted substrate—habitat that's hard to find in developed areas. As a consequence, stoneflies are at the present time a rarity here in the TriCities.

But they do persist, in the well-oxygenated riffles of high quality streams like Ferson Creek in St. Charles. In fact, I remember a summer day not long ago where a stonefly turned up in, of all places the dipnet of our summer nature camp kids.

The importance of the find might have been lost on little Joey, a young lad of 6, but Miss Pam was jubilant indeed! All the parts were there, just as we'd seen them in field guides: two tails, or cerci, two claws on each foot, and two sets of wing pads, along with branched, filamentous gills poking out from beneath the thorax.

We eventually decided that Joey's find was a member of the family Perlidae, a group also known as the common or summer stoneflies. As adults, these insects look as though they developed under a rock—which, when you think about it, they kinda did. When viewed from the side, the body appears compressed. The two cerci remain, but usually can only be seen from below as the wings, which fold over the top of the body, obscure them when looking from above.

Stonefly roots date back to the Permian period, some 250 million years ago. Primitive in appearance (that is, no showy swag like wing scales or hairs) stoneflies are strong climbers but weak fliers. Populations are localized and tend to remain that way for eons.

While you won't see stoneflies forming dense mating clusters the way various other River Bugs do, these species have developed rituals that, I dare say, are even cooler: They drum.

Now, we're not talking Buddy Rich-style percussion here. Stonefly drumming is a subtle thing; it has to be. Even giant stoneflies, the largest North American species, measure only about 2 1/3 in. at maturity. If you're not a stonefly, you'll need special equipment to hear the sound. But it's drumming nonetheless, a behavior that, as one scientist puts it, “improves the efficiency of mate-finding in an otherwise silent, drab, relatively sedentary insect."

Male stoneflies are the ones that begin the performance, tapping their abdomens against a surface, such as a log, in a species-specific pattern. Female stoneflies then indicate their interest by drumming back in a simplified response pattern.

If all goes well, mating occurs, females deposit their eggs and the cycle begins again.

Interestingly, the stretch of Ferson where the nature campers found their stonefly is just downstream from where a dam was removed in—if memory serves--2009. Water now tumbles over boulders that had once impeded the creek's flow, creating a rush of clear, cool, well-oxygenated water.

I remember making a similar stonefly discovery in 2008, in the Fox River at Glenwood Park Forest Preserve; the south Batavia dam had been removed a year earlier. This location, where aquatic species diversity has measurably improved, is also a favorite hangout of another species once considered imperiled—the bald eagle.

Stoneflies may not be as iconic as our nation's symbol, but they're no less important members of our Fox River watershed. With continued attention to habitat improvement, including dam modifications and stream re-meandering, there could come a day when stoneflies lose their Holy Grail status.

Wouldn't it be great if they were just another River Bug?

 (Ed. Note: This column is the last of a three-part series on the Fox River's famous, or perhaps infamous, 'River Bugs.')​​

Pam Otto is the outreach ambassador for the St. Charles Park District. She can be reached at

Tags: Around Town Animals Community Community Involvement Education Environment Families Tourism Why Kane
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