The Copper John overall is a relatively good imitation for many types of mayfly nymphs, but in general the Copper John can imitate just about any of the aquatic insects in their nymphal stage. The copper wire help this fly sink fast to get down to where the fish are feeding. I don’t think John Barr realized the versatility of the fly he created. So load up on some. It is effective in sizes #14-#20.