Snowberry Clearwing Moth (Hemaris diffinis) - black legs are characteristic of this species.

- finally a distinction to write on the guide page! Got any more? Our pages are full of strong opinions on which Hemaris is which, but few that tell us on what grounds.

Well, perhaps that is more of an "observation" rather than scientific fact. So let me just say that the best comparison is by wings as documented at CBIF.
That was not possible for this image so I went for what was visible - the legs. The lateral views at MPG of H. thysbe (by Steve Nanz) and H. diffinis (by John Himmelman) show very light legs for H. thysbe and black legs for H. diffinis. I also checked here at BugGuide for some of the oldest/original images and compared Lynnette's H. thysbe and Patrick's H. diffinis photos. I just hope that an expert confirms this. :) I probably should stop here but it is my observation also that underside H. thysbe has white extending down to rear legs (including white legs) and from there its mostly reddish as shown by Moth Guy here while H. diffinis has white extending down to front legs (has black legs) and from there its mostly black as shown by Bruce's photo.

and as we rarely have wings to work from, I'm sure I'll find these distinctions handy in future. I'd also like to condense some of this for the guide pages. If any of our moth experts is reading this exchange and cares to add anything, that would be helpful, too.

Update: I see another editor has already improved the info on the guide pages (today), probably as a result of this exchange. : )

Most of these observations have already been out there under several images. Looks like the guide pages have been updated before my side by side comparisons. If you can make poetry out of my ramblings that would be fantastic. :)