This is the classic blue winged olive fly which has caught millions of fish. Fish love to sip these mayflies. More importantly they work all over the world. Be prepared for the hatch! Some days the fish will want a size 20 and other days they will want a 16. The mayfly is a staple food for trout. It is definitely fun watching the trout sip the fly but even more fun when they are on the line.Our Assortment comes with 4 flies from each of our 3 sizes.