Midge eggs develop into larva. Larvae are wormlike aquatic versions of the insect.
Prior to emergence, larvae seal themselves inside of an immobile pupal chamber and transform to a pupa.(Dr. Ernest Schwiebert told me the most important thing the presence of midge adults tells you is that there are pupa in the film.)
Prior to emergence the pupa congregate in the slow water, or “cushion” at the bottom of the stream. (Wild Trout also concentrate there because fish can't spend their llives fighting the current). As the concentration of pupae increase, the trout notice them and begin to feed on them. When it's time for the pupa to "hatch" they ascend from the cushion, to the surface. The trout follow them to the surface and suspend.
Upon reaching the surface the pupa encounter the surface film (meniscus.) This is a big deal because it's difficult for the pupa to penetrate the film..Midge pupa suspend below the meniscus and bore a hole through which they crawl to ride the surface, Surface tension holds the pupal shuck under the film. Trout pick off the vulnerable emerging pupa as they struggle to penetrate the film.
The most effective midge pupa imitation I know of is Al's Rat.
The Rat can be sight fished to a visible fish. Dress your 8x tippet to about ten inches from the fly. Fish it dead drift just below the film. Al Miller (father of the Rat) sometimes twitched it. When the fish were lying on the bottom he added a shot and fished the cushion. Some folks sight fish with the Rat some fish it blind, some across and downstream. I believe the “primary trigger” is the silhouette provided by the dark brown monochord.
The "rat" works all day, all year. For many it's the first fly they try every day all year. The "Rat" can be particularly useful during periods of "Behavioral Drift." Gary Borger’s book "Presentation" provides an excellent description of this phenomenon. Try the "Rat" at dawn and dusk when large numbers of midge pupa are in the drift. Al's Rat is always in my fly box. Of all the flies I sold at my shop, the Rat outsold all of them. Try it!