Featured Creature: Stonefly

Golden Stonefly Larva (Nymphal Stage); Photo: Allegheny College Creek Connections

What is one of our oldest creatures – dating back to the Dinosaurs – that tolerates no pollution, lives underwater for up to 4 years and then takes flight for a few weeks to mate, and is beloved by trout and mimicked by anglers?

This would be the stonefly, called Plecoptera, which means “braided wings”!

Golden Stonefly Larva (Nymphal Stage); Photo: Allegheny College Creek Connections

So what is a stonefly? What is their story?

Stoneflies love flowing rivers, and crawl around on the bottom among and under rocks, eating moss and algae in their mostly vegetarian diets. The 2,000 to 3,500 (sources differ) species of stoneflies are found all over the world, except for Antarctica, with more than 670 found in North America alone. They are beloved by trout and anglers as staple forage. The largest stonefly is dark colored, about 3 inches long, and hatches in May, but other, smaller species of stonefly hatch at other times of the year, even on the snow in wintertime (melting the snow with its dark color). But they can be black, brown or yellow (called “golden stones”), and they are well-known to fly fishing anglers!

Hatched adult stoneflies have two pairs of wings that fold flat on their backs, while the nymphal (or larval) stage in which they spend most of their lives underwater has six legs, two antennae and two tails. Stoneflies have a segmented abdomen, and tuftlike gills usually positioned at the bases of their legs, and the foot of each leg has 2 claws for clinging to underwater rocks. Stonefly heads have two compound eyes and three simple eyes, along with chewing mouthparts, though not all hatched adults feed.

Golden Stonefly Nymph Photo by Friedrich Böhringer
Adult Stonefly on Leaf Photo from Fir0002/Flagstaffotos

Despite their riverine habitat, they don’t swim very well, but they do have those claws for hanging onto rocks and debris. They also are not strong fliers, so they don’t venture far from their native waters, and some subspecies are wingless; others never even leave the water. A few of these species can even live in temporary streams. When these streams dry out, they can suspend their body development, slowing down their metabolism in order to survive until moisture is restored. Isn’t that amazing? Talk about resilience!

Watch this cool video of a stonefly in water, crawling out to climb up a tree to hatch as an adult:  

What is unusual about their molting, courtship and egg-laying behaviors?

Unlike the butterfly, stoneflies don’t have a pupal stage (like the butterfly’s chrysalis). Instead, they molt directly from their nymphal body into a flying adult with no intermediate stage, as shown in the above video. 

After they molt, they exhibit some unusual courtship behavior. The males drum their abdomens to signal to potential mates, who, if interested, will drum their response as a receptive female. This ceremonial drumming courtship persists until they gradually come together and mate with each other. Females can lay up to 1,000 to 6,000 eggs by either flying over or resting on the water or hanging on rocks or tree branches to drop their eggs into the flowing river. These eggs are well covered with a sticky layer so they will adhere to underwater rocks and not be swept away in the river’s strong current. 

Typically, the eggs take 2-3 weeks to hatch, and then the larval nymphs live for 1-4 years underwater. They molt between 12 and 36 times before finally hatching out into terrestrial adults, which then live for only 3-4 weeks.

Photos from Wikipedia

What do fly anglers love about stoneflies?

They are a popular forage species for all sorts of river-dwelling trout, and so fly anglers tie up an array of flies to imitate them. As a fly fisherman myself, my first encounter with stoneflies was observing their abandoned husks on dry river rocks in May along a stream I used to fish. Later, I came to recognize their adult forms flitting around a porch light at night as the adult stage of that species. 

Fishing in Western rivers, I used to use two imitation stoneflies at once on my line: a big black chenille imitation with added weight at the end of my line, with a smaller golden stonefly replica on a dropper about 18” above. I got some pretty big trout that way, dredging the heads of deep pools in Yellowstone Park and in Northern California waters! Here are some realistic stonefly imitations that are fine works of art!

Imitation Black Stonefly Nymph Photo by Fred Jennings
Imitation Golden Stonefly Nymph Photo by Fred Jennings

How are human activities impacting stoneflies?

Well, we fly anglers struggle to mimic both their appearance and aquatic behavior, which isn’t as easy as you might think! Aquatic entomologists use stoneflies as an indicator species for good water quality, as they are very pollution-intolerant. That means that when stoneflies disappear from a stream, it is a clear sign that something is wrong. They consequently offer a very good case for clean waters, and to stop polluting our streams and rivers so that stoneflies and fish can abound in these environments. Conservation organizations like Trout Unlimited work diligently to protect our cold water fisheries from pollution and warming effects, so that trout and their forage species – such as stoneflies – survive and thrive in a healthy environment that is good for us all. Let us do what we can to keep our rivers flowing cold and clean, for all these special species, and not just for anglers’ recreational enjoyment (though there is also that)!