The Pandemic May – Callibaetis is a great little Callibaetis mayfly nymph pattern that will fool trout with its small amount of flash on the back. The dark olive coloring is right on for many Callibaetis varieties. This is an unweighted fly. Use thePandemic May – Callibaetis under a weighted nymph and see what happens. Size 14 and 16.