All the signs are there for a solid Sulphur hatch this year. Plenty of nymphs are showing up in stream samples and a few have even begun to hatch on area rivers. Still, when this hatch comes off and it will, it is a major hatch, likely the most prolific hatch of the year. So it is widely anticipated. Two species of mayfly comprise the Sulphur Hatch, the first is the Ephemerella invaria, known as the Dark Hendrickson and the second is the Ephemerella dorothea or Pale Evening Dun. Since both are yellow in color, it is hard to understand why some of the older angling literature listed E. invaria as the "Dark Hendrickson". The Invaria is the larger of the two, usually a size 12-14 and will usually appear first both earlier in the day around 3-6 pm and on the calendar preceeding the Dorothea by at least a week. The Dorotheas are smaller usually size 16-18 and hatch closer to dark. One or the other of these insects hatch prolifically but together they can form a "blanket hatch" with so many bugs on the water the trout may not see the artificial. The daily hatches are ideal for many of us who have real jobs, this is "the workingman's hatch".
Here is a good rule of thumb regarding fly selection: For the Invaria:
Nymph: #12-14 Pheasant Tail Nymph
Emerger: #12-14 Sulphur Emerger, Les Shannon's Lemon Cahill Emerger
Dry: #12-14 Sulphur Parachute, Sulphur Comparadun, Les Shannon's Lemon Cahill
Spinner: #12-14 JC's Invaria Spinner (Invaria spinners are yellow)
For the Dorothea:
Nymph: #16-18 Pheasant Tail Nymph