simply note the similarities of the two nymphs (tails, gills, slender, a bit of difference in antennaes’ lengths).
http://www.pbase.com/michellemahood/image/24839291 (pics by Michelle Manhood)
Both patterns are great for lake fishing. The callibaetis is the more frequent and better known nymph from my experience (not sure, to be honest, I would know a gray drake hatch if I saw it). I am going to incorporate ostrich herl for gills and make tails, legs, eyes, and antennae more pronounced on my nymph pattern along with a darker back and lighter belly ala the Skip Nymph.
In the excellent, upcoming ff mag, FLY FUSION, Spring 2008, there is an article in the ‘Bugs’ section on pages 44-47 by Phil Rowley entitled ‘CONSIDER CALLIBAETIS’. There is an excellent photo of a nymph in its’ nymph pose with splayed tails, legs and gills. Rowley provides precise info on the seasonal evolution of the genus Callibaetis.
Also, with regards to discerning the differences in duns, Hatches Magazine, Spring 2006
http://www.hatchesmagazine.com/page/january2006/86 has photos and wing identifiers to separate out different mayfly duns….’speckled wing’ being the primary identifier as you gaze upon the dun that has landed on your forearm or is still perilously sailing upon the lake’s surface.
Siphlonurus wing (plain) 
Callibaetis wing (speckled)
So, for in the field or on the lake identifying, or for fly tying this helps me calculate which is which, and the materials to use for the wing to get the light dun look (gray drake) or speckled look (callibaetis).
https://swittersb.wordpress.com/wp-admin/post.php?action=edit&post=587 (see here also)
