From stomach pumps and surface seining, I collected duns just breaking out of their nymph shuck with folded wings (no pictures) and duns out of shuck yet wings not developed yet (below).
Here's the pattern that has passed inspection by me, fellow spring-creek anglers and then, of course, trout.
This imitates a dun just escaping its shuck with folded (undeveloped) wings.
PMD: size 16 & 18, straight or curved hook
Baetis (BWO): size 18, 20, 22, & 24, straight or curved hook
Green Drake: size 12 & 14, straight or curved hook
Green Drake don't live in spring creeks however they are one of important mayflies in nearby streams; Yellowstone River = both Yellowstone National Park and Montana sections, then famous Northeast corner of Yellowstone National Park = Lamar, Soda Butte, and Slough.
Instead of emerger hooks above, I use 2XL curved hooks that I typically use for large soft-hackles and bead-head nymphs (not so heavy)
Here are proof pics, approved by trout!!
I was somewhat confident of its effectiveness but I couldn't believe it would be this good!!
SPECIFY Species, Size, Hook style (straight or curved)