Midges are a huge part of the trout diet simply because they are present in nearly all rivers and lakes throughout the year. Moreover they are the only aquatic fly that will hatch pretty much every day of the year.
Midges are frequently used in in still water fly fishing, in the UK we often refer to the midge pupa fly as a buzzer. In rivers however, they are used less frequently but they can be deadly when fished in the right way.
Midge larave, pupa and adult are small. They tend to range in size just a few millimeters up to 2cm in length. Therefore trout will only get a benefit from eating them where they can easily sip them without expending lots of energy. This generally means that midges are best fished in stiller pools where the fish can hold station and sip midges without expending lots of energy as they would in a strong current.
After you have found a stiller pool or glide we need to observe fish rising to midges. First be sure that there are midges hatching by observing the surface of the water and checking you can see them, then observe the behaviour of the fish. Often the rise will take the form of a head and tail rise where you see the fishes back and then tail breaking the surface. You may also see just a small sip and the fish takes and emerging midge pupa. If both these conditions are met then you are in luck!
A fine tippet is required to present a small midge. I would recommend something around 6x. Also a long tapered leader as you will be fishing in still pools where you are more likely to spook the fish. Cast upstream of the fish making sure your fly line does not go over the rising fish. If you loose eye contact with the fly strike at any rise in the vicinity of your fly. When fishing buzzers, grease up the end of your fly line and use greased hi viz braided loop, if the line stops, slows or moves then strike!
Dries, emergers and pupa will work brilliantly. Here are my favourites: