The yellow cow dung (dry) is very effective on chalk streams and limestone rivers for Trout and Grayling.
All river olives, which this pattern imitates, are a hugely important species and an all season fly taken by Trout in all Trout waters. They are said to be very useful in bright conditions.
Olives imitate a wide variery of flies and insects which inhabit the surface of the water. There is nothing like using dry flies and watching trout and grayling rise to the surface. To purists dry fly fishing is the true art-form of fly fishing, indeed some rivers only permit dry fly fishing.