Spentwing patterns, or more commonly known as spinner imitations are those that should be in every serious dry fly anglers arsenal. After mating, mayflies will expire and often over the water in which they mated and become a very reliable and easy food source for trout. Despite being maddening at times due to their prolific nature, spinner falls are an integral part of the life cycle for mayflies, and the food chain for trout. Many times they are the most consistent producers for success when everything else seems to get refused. When the season is in full swing and overlapping hatches are the norm, trout will often key in on the spent naturals because they recognize that they are not only readily available but require the least amount of energy expenditure to consumed calories.
Spentwing patterns are rather easy to construct, but there are a number of ways to tie them. Often you will find that the flies that ride flush in the film, or just under the film are those that get eaten with the most frequency, but are unfortunately difficult to track with the naked eye especially the smaller you tie them.
A gathered parachute hackle spent wing with any number of materials pulled over the top will work rather nicely. The key is using a posting material that will help gather the hackle fibers in the manner that you prefer, or in a way that helps you see the fly as it rides in the film. The use of antron, EP fiber or Hi Vis works rather well in most situations, but when you start tying these style of flies in sizes smaller than a 16 you will find that you not only need something bright to see the fly, but something that will aid in keeping the pattern in the film.
The use of a small diameter foam post works great in these situations, and is my preferred material for these types of spinner pattens. I usually like something that is bright in color, like a chartreuse, orange or yellow as it shows up against the surface of the water, and will stick out better if you are fishing any foam lines. If you are intending to sink your spent wings, then I would use the other synthetics listed prior as they will keep your fly under the film. A quick tip when fishing those, trail them 6-12 inches behind a high floating dry fly and either watch in the general area or wait for that dry to get pulled under.