Anglers call Diptera of the Chironomidae family Midges to distinguish them from other small aquatic insects like Mayflies. Midges are aquatic insects like Mayflies and Caddis. Midge eggs develop into larva. Larvae are wormlike aquatic versions of the insect.
Prior to emergence, larvae seal themselves inside an immobile pupal chamber and transform into a pupa. Prior to emergence the pupae congregate in the slow water, or cushion at the bottom of the stream. Wild trout also concentrate there because fish can't spend their lives 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,,,
Photo by Jim Schollmeyer
from the cushion, to the surface. The trout follow them to the surface and suspend.
Upon reaching the surface, the pupa encounters the surface film (meniscus.) which is difficult to penetrate.
Midge pupae suspend themselves below the meniscus and bore a hole through it, 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.
Photo by Don Douple
Point to remember
Adults on the surface mean there is pupa in the film.