The Feb-Mar stoneflies in PA are actually 3 different species, but the same dressings work for all 3. They're all skinny and black or very dark brown and tend to be most active in the afternoons.
The first ones to pop are usually the Little Black Stoneflies (Capnia species) size 18 in February
Then the Early Black and Early Brown Stones (Taenioptryx species) size 14/16.
As Webrx mentioned, a black or very dark nymph tied like a Hare's Ear would work well.
Here's another style, with some suggestions to make it easier to tie, here's a pic
Little Black Stonefly Nymph
And here's some info for tying:
Hook: nymph 18-14
Thread: black or dark brown 6/0
Bead: I usually use a black 3.0mm (or 7/64") for size 14-16 hooks and 2.5mm (or 3/32") for size 16-18
Weight: 8-10 turns of .015 or .010 diameter non toxic wire (lead substitute)
Tail: Black or dark brown Goose or Turkey Biots-- these can be a bit tricky to tie in. Much easier would be pheasant tail fibers, or a single strand of rubber leg material tied in the middle to make 2 tails, and extending about a hook gap width from the back of the hook.
Abdomen: Black Brassie wire (or black or dark brown dubbing)
Wing case: Black Swiss straw, folded over black dark gray or very dark brown turkey quill slip, or peacock herl, or just omit it entirely
Thorax: Black or very dark brown dubbing
Legs: Just pick out the thorax dubbing with your bodkin
Antennae: I leave them off-- they always seem to end up tied into the tippet knot anyway
If you're not familiar with biots, they're found on the leading edge of primary flight feathers-- typically goose is used, but Turkey are actually longer and a bit easier to work with. I use Turkey Biot Quill feathers--- they have the biots on one side of the stem, and the quill on the other (used for wing cases etc). Goose biots are typically sold as just a strip of the stem, without the quill section.
Turkey Biot Quills:
Anglers Workshop - Turkey Biot Quills
Goose Biots The stem has been split here with just leading edge of the flight feather (the "biot side") showing. To get at the biots you bend the stem (as in the picture) to fan the biots out.
Spirit River: Fly Tying Goose Biots, Dun, 6 in.