Fly5,
Here's a similar link on our other main type of sulpher, the invaria.
http://www.troutnut.com/hatch/11/Mayfly-Ephemerella-invaria-Sulphur-Dun
Invaria - larger (14-16). More orangish as duns and dark brown as spinners. Hatch begins a week or more before the dorothea's most years. But they definitely overlap.
Dorothea - smaller (16-18). Pale yellow or whitish as duns and spinners tend to be grayish. Hatch begins later.
The typical MO is that the duns hatch in the evening. How long depends on weather. Hot sunny days, it may just be a half hour before dark. On cool, cloudy days, it may begin mid afternoon and go all afternoon/evening. They are both swimming nymphs. Aside from hatching time, they look like schools of skinny minnows in slack waters. Prior to hatching, they get in a soft current, get just under the surface film, and go motionless and drift for a while. The back then breaks open and a dun emerges onto the surface, then drifts a while to dry it's wings before taking flight. Fish may key on different parts of this. On drier days, the duns don't float as long, so they're more likely to key on the floating nymphs or emergers. On more humid, or rainy days, the duns struggle to dry their wings so they drift longer, making it more likely the fish key on higher floating dun patterns (catskill ties, thorax ties, etc.).
Then, usually, right at dusk the spinners come back. Usually amassing in clouds over riffles mating and laying eggs and falling spent. More fish suddenly begin rising and the rise form changes. You want spent wing spinner patterns here.
There are some that use a fly that floats flush on the water, like a parachute or comparadun. Under the theory that it can reasonably approximate an emerger, dun, or spinner. Jack of all trades. But master of none. How well it works depends on how picky the fish are.
I usually start off trying to figure it out. I'll tie on a high floating fly. My favorite being cut wing thorax patterns. Then, as a dropper, an unweighted pheasant tail nymph tied on a dry fly hook and greased a little. You're trying to almost float it. Then you have a proper dun pattern and a proper floating nymph. Let the fish tell you what they want. If they take the dropper, fine, keep at it. If they take the dry, cut off the dropper and fish the dry.
Then I'll try to be at the head of a pool near a riffle as dusk approaches. When I see the cloud of spinners flying about, I switch to the spinner. Usually a little too early but that's better than too late, cause once it starts it doesn't last all that long and tying on a fly in the half dark isn't super quick....
