Baetis are very common and generally some of the first mayflies to come off in the spring. Usually smaller, they can be found in most of the river systems of the US and do very well in cooler weather, hatches often being seen during a snowfall. This emerger pattern is very visible and imitates a mayfly struggling to release itself from it's casing. This is when they are most vulnerable and trout key in on them. Color, Blue Wing Olive.Size: 18.