Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
Maccaffertium is a Latinized version of the name of the world-renowned mayfly systematist, W. Patrick McCafferty. And, mediopunctatum is constructed from roots of medi- meaning middle, and punct- meaning point, so I assume this species has some distinctive feature midpoint in its anatomy. Assuming my logic is valid, perhaps someone else can tell you what the feature is.
The distribution of this species is confined to the states and provinces lying east of the N. American continental divide.