These comprise the largest portion of the trout menu. Everywhere. Mayflies range in size from large to extra-tiny. Year around, all the time, any water condition, you can drop a mayfly nymph off any other nymph and, most often, the fish will have the mayfly nymph in it's mouth when landed. Proof enough. This is a great cross-section of the most popular and productive mayfly nymphs imaginable.