Unlike most aquatic insects, which adopt a horizontal attitude when going about their normal business, chironomid pupae are suspended vertically in the water column. This is an important key to fish. A fly that rides horizontally can't look or act like a chironomid pupa. Midges hatch year round and are most important to the trout fisherman during periods when hatches of other aquatic insects are light, such as late fall, winter and early spring. Midges can be active during all parts of the day, but by and large they like the warmer periods of the day during the cold months and the cooler parts of the day during the warm months.
Trout also feed on midges as they are hatching and after they have hatched into winged adults.
If you look along the edges of the river you will see a stringy yellow green substance ringing many rocks in the splash zone. To the inexperienced observer this substance could look like a form of aquatic pants or algae. Actually it is the egg strings from millions of midges. Watch the rivers edge during mid morning and you will see a multitude of midges laying their eggs. The foam at the head keeps the fly suspended in a vertical attitude near the surface of the water.