Spencer Higa created the SOS fly pattern with Baetis in mind. Once the red wingcase was added this fly took a turn toward an attractor style nymph while retaining its imitative traits. Generally speaking, Baetis nymphs are dark in color with a slim profile and noticeable segmentation in the abdomen. This is a quick tie, and will work great throughout the year with peak times during Baetis (BWO) hatches spring and fall as well as through the winter months when much of a river Trout's diet consists of baetis nymphs and midge larva/pupa. Spencer also uses the SOS on lakes suspended under an indicator or retrieved, usually behind a larger fly like a leech or wooly bugger.
This video link (courtesy of youtube.com and Curtis Fry) has the recipe and tying instructions:
For those who don't tie flies, the SOS is available from Orvis, or Spencer can tie a few for you. He can be reached via email at firstname.lastname@example.org Spencer Higa guides fly fishing and wingshooting at Falcons Ledge in Altamont, Utah.