The Perfect Fly Little Sister Caddis Pupa trout fly imitates the Little Sister Caddis larva stage of life. Their larvae live in a case type shelter until the pupa is formed just prior to the hatch. Just prior to the hatch, the pupa comes out of its case and accents from the bottom of the stream to the surface where it emerges into an adult. This makes them easy prey for trout. They should be fished from the bottom to the surface of the water by first allowing the fly to sink to the bottom and then bringing the fly up to the surface using the current and the tip of the rod.