In Maine, an aquatic insect starts hatching in fast-flowing riverine habitat this week -- maybe. When it does begin each evening, fishing action can rock, creating those daydreams for winter.

This bug normally emerges from the last week in June through September in rivers and large streams with well-oxygenated pocket water. However, the exact beginning of this annual event depends on latitude, elevation and naturally weather.

Knowledgeable fly fishers call it "Isonychia bicolor," but it also goes by the colloquial names "leadwing coachman, mahogany dun, dun variant" or "slate dun" -- the last four descriptive to the core.

Cool whitewater with lots of oxygen proves ideal for swimming mayflies, and Maine has lots of streams and rivers with this requirement. In fact, down through the last century, a Leadwing Coachman wet fly has been extremely popular with fly rodders, and often these folks were inadvertently matching the summer I. bicolor hatch when casting this ancient pattern.

In recent years, the Zug Bug has pushed the old-fashioned Leadwing Coachman wet fly out during I. bicolor time. That iridescent-green body on the Zug Bug and Leadwing Coachman fool trout in this hatch.

In my youth, I tied an Isonychia bicolor nymph according to Ernest Schwiebert directions (chocolate-brown, reddish dubbing) and another one like the Swisher-Richards design (reddish brown), but these days I prefer a Zug Bug on a size 10 or 12, 2x-long, wet-fly hook.

According to Tom Ames, though, I. bicolor can be a size 10 or much smaller, depending on the water and other considerations.

My Zug Bug has a slight variation in the dressing, though. Rather than using peacock herl for the tail, which easily breaks off, I choose the far more durable red-game-cock hackle barbules. (A tail-less mayfly nymph turns trout off because it looks odd to them.) I don't bother with a wing case, either, and put a red-game-cock collar on the nymph rather than the traditional beard design, the former which creates more action on the fly.

It goes without saying that I weight my Zug Bugs so they drift near bottom where fish congregate. Friction from rubble, gravel or weeds slows the current next to the riverbed where bugs find sanctuary. Getting deep to target bottom-hugging trout makes all the difference for consistent success.

Just as one quick example of an Isonychia bicolor hot spot, let's look at the Madison stretch of the Kennebec River beside The Pines on the Father Rasle Road that parallels Route 201a/8 south of downtown Madison. ("Rasle" is pronounced "Rale" with a long "a.")

In late June and early July, this stretch has produced incredibly fast action for me, thanks to a size 10 or 12 Zug Bug tied on a 2x long wet-fly hook cast quartering across and downstream. This large fly imitates the Isonychia bicolor swimming nymph exceptionally well.

As fly fishing goes, this hatch requires a pig-simple tactic. The caster allows the fly to swing in a tight arc in the current and then retrieves it with fairly fast strips -- but not too fast. Even this dynamic swimming insect cannot rocket upstream against the flow, but it moves quite fast.

Astute fly rodders try to watch this natural larva swim upstream in a current, not easy unless the light is right. Folks who watch this mayfly swim then duplicate the proper movement by stripping the line at the same speed and with a similar darting motion. Few mayflies swim as well as this one.

Here's another tidbit about Isonychia bicolor. It's a carnivorous mayfly, bringing up a point. A few years ago, Maine officials put the Tomah mayfly (Siphlonisca aerodromia) on the Threatened Species List and made it sound as if Tomah were unique because it dined on other mayflies. Granted, carnivorous behavior may not be overly common, but the Tomah mayfly is not the sole meat eater in Maine, either.

I once observed Isonychia bicolor on the Kennebec's Shawmut stretch after dam operators lowered the water, stranding some of these bugs in a "tidal pool" near shore, giving me a golden opportunity to watch Isonychia bicolor swimming. They darted around so fast that it was difficult to catch them with a baitfish net I used for bug collecting.

This quartering-across-and-downstream technique for fishing an Isonychia bicolor imitation strikes me as pig simple. After the cast, the fly rodder lets the fly swing on a tight line in the current before retrieving it, so when trout hit the larva imitation, they hook themselves. No one needs to worry about setting the hook.

This makes Isonychia bicolor hatches an excellent event for teaching a child or spouse how to fly-fish. If trout hook themselves, it requires no quick reflexes to be successful. The fly is swinging or being retrieved one second and the next, a fish is on.

When folks fish a dry fly, it takes a quick, timely strike to drive the hook point home. It's so much more simple to have a fish hook itself, which builds instant confidence for novice fly fishers.

When I was in my teens, 20s and early 30s, this hatch hadn't captured the imagination of Maine fly rodders, and here's why:

Occasionally, this mayfly dun may float on the surface and catch our eye, but as a general rule, it moves into shallows, crawls upon a rock and hatches out of the water before flying off with dry feet. When this happens, their nymph cases litter the tops of dry rocks (and fallen logs) sticking out of water, but casual observers do not notice them at all.

Because the duns hatch on rocks, they may never get their feet wet when they're in the subimago stage. If they're not floating on the surface, trout don't bother with them as floating insects, but rather, when their swimming beneath the surface out of our sight, heading to the shallows.

If you have never fished Isonychia bicolor, then make sure to tie up or pick up some size 10 and 12 Zug Bugs or Leadwing Coachmen and have a go at it in the next four weeks. That last 30 minutes of light can create winter memories and be yet another example of the good old days being here and now in the great State of Maine.

Ken Allen, of Belgrade Lakes, is a writer, editor and photographer.