The Strange Case of the Mystery Hatch

Arlen Thomason lives on the McKenzie River and is  the author of BugWater, scheduled for release by Stackpole Books in January. Oregon Fly Fishing Blog thanks Arlen very much for this discussion of a very underrated emergence. We look forward to the release of Bugwater. Arlen’s photographs and “angler perspective” of aquatic insects will make Bugwater a must have reference.

Come walk with me now, down to the shores of our local river. I’m going to show you some clues left there for us by a mysterious denizen of its flowing waters. Let’s see if you can identify the creature that left this calling card.

Mystery bug shuck

You get partial credit if you said “stonefly.” But what kind of stonefly? It’s a big one—with a body length up to about 1.5 inches. Golden stonefly, perhaps you say? Close, but no cigar. It does belong to the same family, though—Perlidae—and it bears a strong family resemblance.

The hatch started on south Willamette Valley rivers several weeks ago, about mid-July, and will continue off and on through September and into October in the upper reaches of some streams. Observant fly fishers will have noticed the nymphal shucks, like the ones in the picture above, on half-submerged rocks along the banks of the McKenzie and Willamette rivers lately. But the great majority of anglers have no idea that a hatch of big stoneflies is underway locally, nor have they ever heard its name—neither the official scientific designation, Claassenia sabulosa, nor the “common name” of shortwing stonefly. What’s more, you will rarely see a magazine article about it, nor is it even mentioned in most fly fishing books.

 
Yet shortwing stoneflies are found all across the western states, with the highest numbers in certain rivers of Idaho, Montana, and Wyoming. Most larger rivers in Oregon and the Pacific Northwest contain at least decent populations. The McKenzie has moderate numbers in its lower reaches, increasing as you proceed upstream. Every fall I run into adult shortwings above McKenzie Bridge. Adults are only occasionally observed below Leaburg, not only because they are less abundant, but mostly because of their rather secretive daytime habits. Nymphs, on the other hand, are rather easy to find—if you know where to look—in most sections of the McKenzie and southern Willamette. The places to look to find the greatest numbers of these bugs are riffles with cobble predominantly in the range of 3-6 inches in diameter.

Shortwing stonefly nymph

You may be thinking that this looks an awful lot like a golden stonefly nymph, and indeed it does. Here’s how you can tell the difference. We have two species of golden stoneflies locally—the “true” golden stonefly, Hesperoperla pacifica, and the western stonefly, Calineuria californica. They hatch at the same time—late spring to early summer—in mostly the same places, and are both called goldens by nearly everyone. H. pacifica is a little larger, and the adult is slightly different in color (along the yellow-to-orange spectrum) than C. californica. (Note that the bugs in the photo below are not dead, just chilled to inhibit their tendency to scamper off the photo set.)

Golden stonefly adults

The nymphs of both kinds of golden stoneflies have light-colored, feathery gills at the base of each leg; while lacking gills along the abdomen. H. pacifica nymphs have an additional pair between the tails, and that’s how you can easily tell them apart.

Golden stonefly C. californica nymph: no gills between tails

Golden stonefly H. pacifica nymph: gills between tails, hourglass on head

But if you carefully check the photo above of the shortwing stonefly nymph, you’ll see that it also has a pair of light gills between the tails, just like H. pacifica. So how do you distinguish a shortwing stonefly nymph from a golden stonefly H. pacifica nymph? By the marks on their heads. An H. pacifica nymph has a white mark shaped like an hourglass on top of its head. The mark in the same position on a shortwing stonefly nymph is in the shape of the letter W—or an M, depending on the angle you view it.

The “W” on the head of a shortwing stonefly nymph

Luckily for us, when the shortwing stonefly hatches, it leaves behind that tell-tale W / M insignia on its nymphal shuck. If you look back at the shuck in the first photo of this article, and squint a little, you should be able to see it there. So now whenever you’re out on the river and you spot big stonefly shucks bearing that mark, you’ll know that shortwing adults are out and about, prowling the banks.

 
But where are they? Unless they are really plentiful, you may not see any adults, or just a few. That’s because they don’t fly around like golden stoneflies, for the most part they don’t climb vegetation, and they are mainly nocturnal. During the day, they hide under stones along the banks.

 
So what do adult shortwing stoneflies look like? Females are about the same size as—or slightly larger than—golden stoneflies, and have a similar shape. But the color is different—they are brown to gray above, and light tan on the bottom.

Adult female shortwing stonefly

When you turn one over and look at its ventral side is when you’ll really appreciate the difference from golden stoneflies.

Adult female shortwing stonefly, ventral view

Notice that the wings of the female extend well past the posterior of the abdomen, just like a golden stonefly. Now let’s take a look at a male shortwing adult, which has a body length only about 2/3 that of a female.

Adult male shortwing stonefly, a few hours after hatching

Now you can finally see where this bug gets its common name. The wings of male shortwing stoneflies are strangely stunted, extending only about half way down their abdomens. As a result, males can’t fly. Female shortwings can fly, but they rarely do. But both sexes can run like the dickens. Not only on land, but across the water surface. As you can imagine, a big bug like that skittering across the water is going to drive fish mad.

 
Your best chance of seeing adult shortwings is at dusk and a few hours immediately following. The best places? Those same spots where you found the discarded nymphal shucks. Adults that have been on land a while will be running around looking for mates, especially the males. Females run out on the water to lay eggs, and males will scamper across the water from rock to rock seeking females. Newly arriving nymphs will be crawling out of the water to hatch. They never go far, like some stoneflies do, before starting to emerge. Often, they hatch when the nymph is half in and half out of the water.

 
When you go looking, arrive near dusk, and bring a flashlight. If you’re lucky, you might see something like this.

Hatching female shortwing stonefly, and a visitor

This hatching female has so far pulled herself only half out of the nymphal shuck. But the waiting male is overanxious, and has already climbed on her back. Shortwing stoneflies are known to be exceptionally libidinous, for whatever reason. As soon as her abdomen is exposed, he will mate with her. He may stay and guard her from other males, mating with her repeatedly. Or he may go off looking for other females. Real playboys, these guys.

 
Notice how pale—actually quite yellow—the emerging female is. Both sexes look like this at first, then gradually darken over the next few hours after hatching. Males generally hatch first, then patrol the shoreline waiting for the females. That’s why the male on her back is considerably darker, because he hatched earlier, possibly even on a previous night.

 
When adult shortwing stoneflies are around, trout know it. They may see them mainly just before dark, but they seem to remember them all day long. In places where these bugs are really thick, like the South Fork of the Snake in Wyoming and eastern Idaho, big stonefly dry flies are prime producers during the shortwing season. And the great thing about the shortwing season is that it is so long, lasting several months. In that region, a favorite fly during late summer and fall is the big foam Chernobyl Ant. With its realistic stonefly silhouette and wiggly rubber legs, trout really smash it.

Size 6 Chernobyl Ant

You don’t see many anglers in western Oregon employing that pattern, but I can attest that it works here too when the shortwings are hatching.

Upper McKenzie native redside that fell for a shortwing stone imitation

So give it a shot. You just may find that fishing the hatch of the strange bug with the mysterious habits that no one knows about can pay off.