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Big Slate Drakes: The Hexagenia

In search of the nymphs and emergers of this huge and important mayfly.

Big Slate Drakes: The Hexagenia

Hex spinners fall over riffles at twilight, and Pennsylvania fly fishers often erroneously call them Brown Drakes. (Paul Weamer photo)

The day’s final light fades from the sky, fleeing the earth in waves like a dying strobe. With each progressing degree of darkness, the sounds along the creek become more intense—frogs, crickets, the unknown, and eruptions of bursting bubbles and splashes that must be trout feeding in the near-black. The noises surround me, but it’s not fish that bring me here.

I’m within sight of my home along central Pennsylvania’s Penns Creek, if it were still light enough to see that far. I have my camera, and I’m searching for something, something that I know is here but something that I’ve never seen; something that few fly anglers have ever seen. The nymphs and emergers of Hexagenia atrocaudata mayflies, Big Slate Drakes, are waiting to be found and photographed in the darkness.

Big Slate Drakes: The Hexagenia
Deep-burrowing Hexagenia atrocaudata nymphs are seldom seen by trout or fly fishers. (Paul Weamer photo)

Researching the Hexagenia

I’m not the first to notice these giant mayflies, and neither was my friend Charlie Meck when he wrote about them in Meeting and Fishing the Hatches (1977). In his book, Meck wrote about seeing large spinners hovering over Penns Creek near Cherry Run in 1968. He didn’t know what they were, so he collected a male spinner and sent it to professional entomologist Will Flowers. Flowers identified the specimen as Hexagenia atrocaudata.

It’s not surprising that a younger, soon-to-be-legendary hatch master like Charlie Meck was unaware of atrocaudata. The great fly fishing-based entomological works of the time barely noticed the hatch. Ernest Schwiebert’s 1973 Nymphs allotted one brief paragraph to atrocaudata, stating that they were “widely distributed in American trout streams and lakes . . . preferring marshy streams and beaver ponds and the icy, spring-fed, marl swamps of Michigan.”


Schweibert’s 2007 two-volume set, also called Nymphs, has an expanded discussion, highlighted by Schwiebert’s famously dramatic nymph, dun, and spinner descriptions and a brief recollection of a 17-inch brown trout fooled by an atrocaudata imitation. None of these books include drawings or photos of atrocaudata.


Al Caucci and Bob Nastasi’s 1975 master-work Hatches has one sentence about atrocaudata, stating that it was an important Hexagenia mayfly in both the East and Midwest, but no photographs or drawings of it were included in Hatches or Hatches II.

Selective Trout (1971) by Doug Swisher and Carl Richards fails to mention it, and even Charles Wetzel’s books, Practical Fly Fishing (1943) and Trout Flies (1955) omit atrocaudata. Wetzel spent a great deal of time fishing Penns Creek, so he must have seen them. But these shortcomings elevate Charlie Meck’s book as the first significant work for recognizing and understanding the Big Slate Drake.

Charlie made several important contributions to Hexagenia atrocaudata’s demystification. First, he mentions the time of year they occur. H. atrocaudata is an August hatch, often mid-August in Central Pennsylvania, though he mentions seeing the spinners on August 25 in 1977. I’ve seen the flies late in the month, and I even found a single female dun emerging from Penns Creek in September two years ago. But mayfly hatches often seem to be occurring earlier than they did decades ago, so mid-August is most common.

I believe that Charlie may have also coined this hatch’s common name, the Big Slate Drake. I’ve been unable to find any mention of that name before he wrote it. I asked Charlie about that, and he said that he wasn’t sure if he was the first to name it. I’ll give him credit for the name and a pass on his memory—it was 35 years ago when he wrote this in his first book. The name is fitting, as the male atrocaudata dun looks similar to a Slate Drake (Isonychia bicolor) dun, though the nymph, emerger, female dun, and spinner look quite different.




Perhaps Charlie’s most important contribution to Hexagenia atrocaudata’s story is a single photograph of a male atrocaudata spinner’s underside in Meeting and Fishing the Hatches. I believe this to be the first atrocaudata photo included in a fly-fishing volume, and it remains one of a small handful of Big Slate Drake photos published in any fly-fishing book or magazine. A quick Internet search produces several websites that include photos of Hexagenia atrocaudata duns and spinners. But other than one very small, grainy photo of an immature atrocaudata nymph on the University of Iowa’s website, I’ve been unable to find quality photos of atrocaudata nymphs or emergers.

And that’s what left me standing in the dark with my camera on Penns Creek. The nymph and emerger images that I captured, including those in this article and my upcoming book, The Pocketguide to New York Hatches, may be the first ever published for fly-fishing purposes. I can only assume that atrocaudata nymph and emerger photos or drawings exist in aquatic entomology books or academic journals, but I’m not certain. I’ve been unable to find them, though a fly shop customer recently told me about participating in a Messiah University effort to collect H. atrocaudata nymphs on Pennsylvania’s Yellow Breeches Creek. Photos were taken during that study, but if they were ever published, I haven’t been able to find them.

The Hexagenia Nymph

My search for H. atrocaudata nymphs began two years ago. I had just begun a book project that included nymph, dun, and spinner photos for important New York mayfly species. I included atrocaudata because of the excellent spinner falls I’d witnessed on the upper Delaware River.


Each August, I also found the insects emerging from Penns Creek, and trout feeding on the spinners, and I was sure I could easily find them throughout the year because the nymphs take multiple years to ­mature. But I couldn’t find any existing atrocaudata nymph photos, so I wasn’t really sure what they looked like. I knew they were burrowers like Green Drakes, but I knew little more than that.

Penn State University entomologist Greg Hoover helped focus my search. He described atrocaudata’s frontal process (a lip-like appendage on the head, present in all borrowing mayflies), as a rounded projection, unlike the deeply cupped frontal process of the Green Drake.

Hoover also told me to look in the muck. He said Hexagenia atrocaudata nymphs often live in quiet areas near shore in the kind of mud that is ripe with rotting leaves and decaying material; muck that nearly removes your boot when you step into it. Green Drakes do not live in this type of sediment, preferring less muck and more gravelly material and coarse detritus.

Day after day I kicked and shoveled muck into my seine, and I didn’t find a single atrocaudata nymph. It wasn’t until mid-June that the first Big Slate Drake nymph appeared.

I believe these nymphs remain deep in the muck, too deep to seine, until their emergence draws near. Every nymph I found had visible wingpads, showing that they were near maturation. In comparison, I can easily find immature Green Drake nymphs all year long in their preferred sediment, suggesting that they do not spend their lives nearly as deep. This deep-living trait makes atrocaudata nymphs less available to trout, reducing their importance to fly fishers.

Hexagenia Emerger, Dun, and Spinner

After shooting several atrocaudata nymph photos, I decided to focus my attention on the emergers. The duns emerge in the final moments of daylight. So I spent a week, standing with my camera for an hour each evening, in a mucky Penns Creek pool near my house. My plan was to stand as still as possible and photograph the nymph’s transformation once it rose to the pool’s surface and began turning into a dun. This all sounds much easier than it really is, but the effort taught me a great deal about the atrocaudata emergence.

Big Slate Drakes: The Hexagenia
Big Slate Drake duns (an emerger is shown here) emerge from their nymphal exoskeletons in shallow water near shore in August. The subsequent spinner falls cause the heaviest feeding activity. (Paul Weamer photo)

Perhaps my most surprising observation was where atrocaudata emerge. I witnessed one nymph become a dun in less than an inch of water, no more than an inch from dry land, and almost all the emergers were in shallow water near the shore, in spots that would not commonly hold trout in August.

The speed with which the nymphs rose to the surface, escaped their shucks, and flew away was stunning. Most of the time, I had the opportunity to take one photo from the time I noticed the nymph reaching the surface, to when the dun was airborne. I assume this ability—quicker than the often laborious Green Drake emergence process—is an evolved trait that aids large mayflies in escaping fish predation during times of reduced summer stream flows.

But atrocaudata is pursued by another, even more vicious, predator. Any atrocaudata nymph that had difficulty escaping its nymphal shuck, causing it to remain on the surface, was quickly attacked by Water Striders (Gerridae species), which also live near the stream edges.

The proclivity to emerge quickly, in shallow water near shore, usually makes emerger and dun patterns unimportant for fly fishing. But, like most hatch-matching rules, there can be exceptions. Should you witness one of these exceptions, a Gray Wulff, Adams, or even a Brown Drake dry fly, size 8 or 10 2XL, will do a good job of imitating atrocaudata duns and emergers.

Big Slate Drakes: The Hexagenia
Big Slate Drake duns emerge from their nymphal exoskeletons in shallow water near shore in August. The subsequent spinner falls cause the heaviest feeding activity. (Paul Weamer photo)

Both male and female duns have two tails with a minuscule vestigial tail between them. Though there are significant coloration differences between the brown/olive male bodies and the bright yellow female bodies, it is near dark when they hatch, making fly pattern size and silhouette much more important than color.

Fly fishers shouldn’t expect great fishing opportunities from atrocaudata nymphs, emergers, or duns, but expectations sometimes change with the advent of the spinner fall.

Hexagenia atrocaudata are similar to Slate Drakes and March Browns in the way that there often don’t seem to be enough hatching duns to create a surface-feeding hatch event, but there are certainly enough of the big spinners for the trout to take notice.

The spinners do not usually reach the water until just after dark, but great fishing can be had at this time. Pick a slow, flat pool where you can see or hear riseforms and target rising fish with big rusty or brown-bodied spinner imitations. Use heavy tippets (3X or 4X) in case you find large fish feeding nocturnally during summertime low flows.

Fly fishers who target the spinner fall should be aware of two circumstances that can impact fishing. The first is water temperature. This is a summer hatch, and no one should be catching and releasing wild trout in water  exceeding 70 degrees F.

Look for atrocaudata in tailwater rivers and limestone streams, both of which are usually cooler than freestone streams in the summer. The White Fly (Ephoron leukon) hatch presents another issue. The incredible numbers of White Flies in streams where both hatches coexist, like Pennsylvania’s Yellow Breeches, can overwhelm the fish, causing them to ignore the atrocaudata spinners. A White Wulff may be a better option than a Gray one when complicated multiple hatches occur.

Big Slate Drakes: The Hexagenia
Big Slate Drake duns emerge from their nymphal exoskeletons in shallow water near shore in August. The subsequent spinner falls cause the heaviest feeding activity. (Paul Weamer photo)

Hexagenia atrocaudata is not one of the East’s most “famous” hatches. But don’t tell that to the many trout that were eating the spinners all around me late that evening on Penns Creek. If only I had brought a fly rod instead of my camera, perhaps I could further document how much the trout like them, but then I never would have gotten that photo.


Paul Weamer is Fly Fisherman’s Mid-Atlantic field editor and the author of Fly-Fishing Guide to the Upper Delaware River (Stackpole Books, second edition, 2011) and coauthor with Charlie Meck of Pocketguide to Pennsylvania Hatches (Headwater Books, 2009).

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