bjkeledas,
The term "dun" when used to describe the mayfly refers to its stage after emergence. It is technically a "subimago" (I think I spelled that right) meaning immature (sexually). the mayfly then molts and becomes a "spinner" or "imago", i.e. sexually mature. The spinners have clear wings typically. Within hours of molting or maybe a day or so, they will mate in swarms, usually above a riffle. Soon after mating, the males will drop into the water dead or dying, usually with wings spread, allowing them to be well-imitated by a "spent wing spinner" pattern such as the "rusty spinner." The females get to live a little longer, usually long enough to touch down on the water and deposit eggs. When that is complete, they also will drop to the surface dead (spent wing style).
The "coffin fly" is the spinner stage of the Green Drake. When the drake molts, its new body color is starkly white or maybe a bit champagne colored, with clear, but mottled wings.
The "comparadun" is a tying style. It involves a no-hackle pattern with wings made of deer or elk hair. It sits low in the film and may, thus, imitate an emerging "dun," but it can also imitate a spinner in that when they hit the water dying or egg laying, they will be in the film and their wings may be erect as they "take their final breaths" or lay their eggs, as the case may be.
Hope this wasn't more than you were asking and that it is of some help.
