Hi Cussfly,
You're welcome. As to common names, there are four general categories:
1. Accepted scientific common names (not to be confused with the latin ones)
Examples - Springfly, Carolina Stone
2. Angler common names referencing individual species
Examples - Quill Gordon, Hendrickson
3. Angler common names referencing multiple species, sometimes even multiple families
Examples - March Brown, Blue-winged Olive
4. Common names that aren't so common. These are used by anglers on a highly individualized or regional basis for critters that nobody outside their circle has the slightest idea what they are referencing. These are the source for 2 and 3 above from ones that stick and grow in popularity.
As to distribution, there are a multitude of papers that have recorded the distribution of mayflys. Many have been compiled into lists that can be found at places like Purdue University's Mayfly Central and our good friend Taxon's website,
www.flyfishingentomology.com. It must be said that these records are incomplete in many instances, as some locations have not been thoroughly explored and documented, but they are certainly reliable in a general sense. Coupled with incomplete taxonomic characters available in most photos (to a greater or lesser extent), many if not most determinations are somewhere on the scale between "highly probable" and "best educated guess". Some are no better than speculation.
BTW - I inadvertantly left out a few from your post. This was not on purpose, they just got lost in the shuffle:
Quill Gordon & a Grannom - no and yes. The former is probably
Epeorus vitreus (Little Yellow Quill). The latter looks to be
Brachycentrus. Species of this genus go by many common names. American Grannom, Apple Caddis & Shadfly are three common ones, but there are more, depending on which species this is.
Hendrickson - No. This is most likely
Ephemerella invaria (Sulfur Dun). Besides being recorded from SC while
subvaria has not,
invaria does quite well in warmer water. Taxonomically, some
invaria have double banded tibia (as with your specimen) while
subvaria has single bands.
Black or brown Stone - No. Giant Black Stone is the common name traditionally used for
Pteronarcys dorsata, though the western name "Salmonfly" is increasingly being used in more circles. I'm not aware of the common name Brown Stone being used for any particular family, let alone genus or species. My best guess is that this is
Paragnetina fumosa (Smokey Stone).