This pattern style has several variations that the tier can use to move between a Mayfly emerger (Hairwing Dun) to a Caddis (Matthews X-Caddis & the Elk Hair Caddis). The Elk hair wing is central to the three patterns. The Eld Hair Caddis would have the palmered hackle and no tail. The Hairwing Dun has a modest hackle wound beneath the Elk Hair wing and still can have the tail (feather barbs or synthetic fibers).

Elk Hair Caddis (SwittersB)
Black Quill Hairwing Dun (Tim Hiltz) Here the Hairwing is a bit sparser and you see the hackle wound at the thorax beneath the hairwing. This most often seems to delineate the Hairwing pattern between a Mayfly and a Caddis.