The California Mosquito pattern goes back to sometime within the 1950's. During the '60's and 70's, this pattern was one of the most popular dry flies within the Sierra Nevada. They even became a popular graying fly within Alaska. The downwing makes this pattern unique. Mosquitos are known for raising their hind legs in the air when they alight onto the surface film of the water. This brings their body into close proximity to their wings. The pattern is still a producer, particularly within the high elevation lakes and meadow streams.
Often, you will find mosquito clusters beneath overhanging branches and next to brushy shorelines. Cast the California Mosquito upstream of the overhangs and let the current bring the fly into the fish zone. It is important that you choose the size of fly that mimics the actual mosquito. If the pattern is larger than the naturals, the trout will ignore the presentation. At times, fish will be keying on subsurface pupa emergers, especially during the evening. If you find that they are not taking the dry, you can modify this fly into a pupa emerger by trimming the hackle so that the fly lies lower within the surface film.