In the winter you spot trout rising to feed on the surface–olive or midge?? The insects are very small but you finally catch one so you can match it. You might not be real good at entomology and you don’t know which one it is, but you know the angling tactics are different. Simple, if it has a tail it is a mayfly, if it doesn’t have a tail it is a midge.
The mayfly is the Blue Wing Olive. Some of the most exciting dry fly fishing I get in the winter is on the Blue Wing Olive hatch. On overcast days, even snowy or drizzly days this hatch can be unbelievably heavy. My favorite flies for this hatch are the Mr. Rapidan Parachute Dry size 18 and the CDC Blue Wing Olive size 18.
If the fly is a midge (no tail) then use a Mr. Rapidan Midge Dry size 22 fished on 7X with a slack line cast. The midge hatches are usually heaviest at dusk.
Tying Material for Mr. Rapidan Midge
Hook: Mustad Signature 94840 or Mustad Heritage R50AP, sizes 18, 20, and 22
Thread: Black 6/0 Prewaxed Nylon
Body: Peacock Herl
Wing: Yellow calf tail or calf body hair
Hackle: Grizzly Rooster neck, dry grade