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.
Trout sip them in, leaving only a tiny dimple at the surface.(dimpling rise)
The most effective midge pupa imitation I know of is Al's Rat.