The split case or split back PMD (Pale Morning Dun) mayfly nymph is absolutely deadly when fish are targeting mayflies that are just breaking out of their nymphal exoskeleton, before completely hatching. The yellow spot looks like a nymph's thorax that is in the process of splitting as the nymph hatches. Though it is typically described as representing a PMD, it works well for mimicking many other types of mayflies. Available in sizes 16, 18 and 20.
TIP: Sizes 18 and 20 are an absolutely killer pattern on most tailwaters, like the San Juan river below Navajo Dam.