Leech patterns are one of my "go-to" patterns that I always make sure I tie up and carry with me for all my fishing whether warmwater or coldwater. They can be tied in many different colors to represent just about everything. The leech yarn has a lot of "movement" which I feel is one of the "trigger" qualities of the pattern. I cannot say that my warmwater/coldwater fish are "hitting" them because they think they are a leech or not because I feel the pattern does a great job of representing a hellgrammite too.