Sulphur Dun. Also known as the Sulfur Dun or Sulpher Dun is a staple part of a Trouts diet. These Mayflies are generally abundant and when they are on the surface so are the Trout. Hatches of Sulphur Dun flies take place throughout the summer so this is an important dry fly not to be without. Make sure that you have a variety of sized as Trout can be very picky.