The Stimulator is a dry fly popularized by angler, fly tyer and author Randall Kaufmann to imitate large adult stoneflies.

[1] The Stimulator pattern is a derivative of earlier stonefly patterns—the Improved Sofa Pillow (1940s) and Yellow-bellied Mattress Trasher (1970s).

Others contend the pattern was derived from the Trude style dry flies developed in 1903.

However, the pattern was clearly popularized and promoted by Randall Kaufmann, a fly shop owner, angler and author in Seattle, WA and Portland, OR in the 1980s.

[1] Originally tied to imitate large stoneflies, the Stimulator is also useful to imitate adult caddis, grasshoppers and large mayflies.