There are many aquatic living insects and small animals that fish will feed on. Some of the most important once are scuds. These small crustaceans make out a large quantity of the biomass in many waters and many fish species will continue to feed on them all their life.
Scuds are found in both still water and slow moving parts of streams and rivers all around the world. When they swim, they will form there body into a something that almost looks like a circle by bending it so that the tail reaches there head, and then quickly pushing it back into a straight position. When they do so, it almost looks like they are jumping. As you fish your imitation, it is important to imitate this way of swimming by making a short, quick pull in the line and then letting the fly fall slowly down again before the next short pull.