Dry flies that imitate insects are the most diverse group of flies you'll see offered in our shop. There is a good reason for this: trout are selective feeders, which means that they often pick out one type of insect, find out that it's good to eat, and ignore insects that don't have its shape, size, and color. It is usually the insect that is most abundant on the water, regardless of size. Thus, a trout may feed on size 18 mayflies, ignoring larger juicy mayflies that are on the water at the same time. This selectivity may last for an hour or so, or it may last for weeks, depending on the relative abundance of the insect and how long it hatches. The dry-fly fisherman must be prepared to switch flies to accommodate the whims of surface-feeding trout.