Buzzer is the non-scientific, common name for a group of insect of the order Diptera. The Chironomidae family represent large part of the buzzers. The size averages from 1 to 10mm long and sometimes even larger.
They hatch from the egg as worm-like larvae. They turn into pupa to swim to the surface and hatch as winged insect.
They are extremely prolific and they are an important part of the fish diet.
During massive hatches trout and grayling can key on the particular species and feed only on them.
In the picture a pupa has reached the surface and is getting ready to go airborne. Well visible the wing-case, folded on the thorax.