When midges hatch the pupae will hang in the surface vertically with the head up. During this stage they are an easy target for the fish and often appear in large quantites. Due to this, even large fish will start to feed in the surface.
When you fish flies that imitaties the midges pupae, you need to do so with great patient, especially in still water. Instead of picking up the line and trying to another spot, let the fly or flies be still in the same spot for a while. Fish will find them and they will pick up the fly as long as your leader do not spook them and your imitation has the right proportion, size and looks.
They are normally greenish, brownish, red or black in colour.