The Comparadun is a no-hackle dry fly: a deer-hair wing fanned across the top, a slim dubbed body, and split Microfibett tails. With no hackle to prop it up, the fly sits flush in the surface film like a real mayfly dun, which is why it fools picky trout on flat water. The Sulphur version matches Ephemerella sulphur duns - pale yellow bodies, sizes 14-18.
Sulphurs hatch on slower, clearer water - spring creeks, tailwaters, and the soft glides of freestone rivers - through late spring and into summer, often heaviest in the last hour of light. Best when the water sits 54-64F. Fish it on low to normal flows in clear water, where trout get a long, hard look at the fly.
Fish it on a dead drift, drag-free, in the feeding lane of a riser. The flush profile is its whole advantage, so a clean drift matters more than the cast. If trout refuse the dun and you see them nosing just under the film, switch to an emerger - they may be taking the bug before it climbs out of its shuck.