These Barbell Eyes have a recessed end so you can insert for example Adhesive Holographic Eyes or Mirage Dome Eyes with some Zap-a-Gap. Small amount of glue is enough.

Barbell eyes will add weight to your flies and adhesive eyes will make them look even more realistic. Extra weight on your streamer will take the fly where the fish is. You can add the eyes on the top of the hook or under the hook, which will make sure that the hook will stay in correct position.