EASTON, Kittitas County — At this time of year, perhaps even today, a tiny underwater nymph will wriggle inside its crusty exoskeleton and begin the most perilous two minutes of its yearlong life.

Latched to the cobblestones on the bed of the Yakima River, this monster movielike bug — horns on its head, spikes running down its thorax and abdomen, grotesquely swollen front legs, gills — has just morphed into something almost beautiful, a delicate flying form. But it's a transformation not without danger.

Now it has wings. But it's still under water.

Nearly defenseless in a shell of its own making, it will let go of the rocks and rise through a river gorged with fish that have an appetite so stimulated they may disregard all other food — that is, except this bug. The bug they'll see. And to these fish, it might as well be pepperoni pizza.

For this large spindly aquatic insect commonly known as a green drake, the danger has only just begun.

The common name for a handful of mayfly species, the green drake is among the largest and most dramatic of North America's river flies. A bug with up to a two-inch wing span, and one that can emerge in such quantities over a short period that it may draw a river's largest fish to the surface, they form an intricate part of the fresh-water ecosystem.

They are so sensitive to pollutants — particularly sedimentation, heavy metals and pesticides — their presence is an indication of a healthy river. They are, in a sense, the composters of the river, spending most of their yearlong life feeding on the slippery algae that coats river rocks and shredding the occasional leaf.

Then, over a two-week to 18-day period in June and early July in Washington, the green drake begins to "hatch," a metamorphosis in which these underwater nymphs become winged mayflies.

Which brings us to the rest of those two minutes.

Like most aquatic insects, mayflies molt as they grow, leaving behind an outer crust of their former selves called an exoskeleton. But unlike most insects, which shed that last underwater layer just below the water's surface, the winged green drake frees itself a good six inches below the surface, relying on turbulence to propel it through that last fish-infested stretch to fresh air.

There, the bug pops to the top of the water, its wings sopping wet, and floats, waiting 30 to 60 seconds to dry off while the enemy swarms below.

"It's like landing on Omaha Beach," said Rick Hafele, an Oregon entomologist and one of the foremost authorities on Western mayflies. "They are sitting ducks."

This risky journey has but one mission: sex, of course, which in this case ultimately leads to death anyway. But that comes later.

Hundreds of species of aquatic insects hatch all over the planet on any given day. On a cloudy day, river watchers can see bugs rising off the water, sometimes in clouds referred to as "blizzards." These hatches can be something to behold. And nature's response is equally remarkable.

Massive hatches can release so many of the same insect at one time they'll coat a person's hair, crawl into the nose and ears and swarm the body.

Hafele has seen mayfly hatches that fill the sky and turn a sunny day dark. He's seen hatches so thick that bugs looked like rafts coating a river. He once witnessed a hatch of black drakes — a relative of the green drake — so thick he opened his car door to get out, and thousands swarmed in. Their husks "were still shooting out of my heater five years later."

Green-drake hatches, which occur across the West, are a sight all their own.

"They're awesome. They just pop up, and their wings stick an inch up in the air — like a big bumble bee on a diet," Hafele said. "The wings are gray-colored, and the body is a rich olive green. You look across the water from the bank and you might see 20 or 30 of them on the water at any given time, popping up and flying off." In response, a seemingly quiet stretch of water will come alive with fish.

"You wouldn't believe it," said Jack Mitchell, a river guide with The Evening Hatch fishing shop in Ellensburg who has fished the river for a quarter century. "On the deepest, darkest, ugliest day, these big bugs will come and suddenly the biggest fish will come up and they'll just be popping out of the water all around you eating up those bugs. It's the coolest."

Scientists don't completely understand how all the elements work together to set off an insect hatch, but water temperature is the biggest driving factor. Typically, when water temperature hits 50 degrees during spring runoff, it will trigger growth hormones in the nymphs that stimulate the metamorphosis.

"But everything's important: weather, air temperature, barometric pressure, light intensity, wind," Hafele said. "How the underwater stage knows about wind, I haven't the foggiest."

And just like soldiers in battle, these thin-bodied creatures' primary source of protection from predators during a hatch is sheer volume. On a good day, on a given stretch of river, green drakes transition from nymph to sexually immature winged "dun" by the tens or hundreds of thousands, hoping to overwhelm the frenzied trout or whitefish, swallows, flycatchers and tanagers that slurp them up as fast as they can.

On breezy days, they'll hatch in small numbers — - if at all — - so sporadically that fish never key in on them, and birds aren't around for the feeding frenzy.

It's like diversifying a financial portfolio.

"On a perfect day, you may get 100,000 of them hatching on one stretch, but maybe 20 percent survive," Hafele said. "On a windy day you may get only 5,000, but perhaps 60 percent will survive. They're spreading their luck out."

Of course, that luck is short-lived. Any surviving green drakes fly off into the trees or foliage on the bank, and rest for their final act — procreation. In this final stage, their mouths and intestinal organs — no longer needed — have disappeared. Within a half hour of leaving the water, they've changed colors, turning from olive to reddish brown.

For 12 to 24 hours they rest. Mayflies, the only insect that ever sheds at the winged stage, molt one more time into something called a spinner. Then the dance begins.

Males rise off the vegetation and form large mating congregations in the air, dancing up and down, 30 to 40 feet above the water, perhaps thousands at a time, for maybe half an hour. Females, their abdomens plump with eggs like little bomb-filled aircraft carriers, dive into the swarm, and males race to be the first to reach her.

The first to arrive will grasp the female in the air, and use his long front legs to rise over the thorax. They mate in midair, and, about the time it takes them to fall 15 feet, the sperm has been exchanged and they separate. In seconds, it's done.

The females almost immediately start dropping to the water to extrude packets of a few hundred eggs at a time that sink to the river floor. Then she'll fly up and drop down again until she's dropped from 3,000 to 4,000 eggs in an act called the "spinner fall."

Exhausted, she'll fall to the river and die.

The male flies to the bank and does the same.

But underwater, in the soft, rich muck, the eggs begin the cycle again.

Natural Wonders appears every other Monday. Craig Welch can be reached at 206-464-2093 or cwelch@seattletimes.com.