DCHP-1 Online
fly-line 
n. a point or line above which flies do not commonly fly.
- 1947 If you are a horseman, the southern reaches will claim you . . . above timberline, above the fly-line.
- 1963 Another method [of keeping flies off meat] was to tie game or meat to a tall pole and elevate it beyond the fly line, where the sun and air were cooler than nearer the ground.
- 1947 If you are a horseman, the southern reaches will claim you . . . above timberline, above the fly-line.