Thank you so much, Dr. Ascher, Dr. Neff, and Hadel Go, for all of the effort and knowledge that you put in to identifying this bee. And thank you also, Dr. Ascher, for being the person who brought the special nature of this bee to our attention.

amnh has one male bee curated under Megachile chrysophila in addition to the holotype, although the det label is only to subgenus. the specimen does not resemble the bee in the photo - mandibles and t7 are different, among other things. compare: https://www.dropbox.com/sh/hj066tpirlajopp/AAAbZrfnpjJKqobMlZnOidvAa?dl=0

You are right, it is something different. Can you check M. simplicipes and M. toluca (they occur fairly far N in Mexico)? These are in subgenus Tylomegachile. See Michener (2007) re lack of emargination of carina of T6 and large submedian teeth of apical margin of T6.

Compare with description of M. toluca male here:
https://www.biodiversitylibrary.org/item/32790#page/139/mode/1up

Clearly not a Leptorachis. Don't know what I was thinking other than that this subgenus is known from the USA whereas I now realize that this bee most likely belongs to a new SUBGENUS for the USA (was not expecting that level of novelty but it now seems to be the case). Was just remembering the red legs of M. chrysophila I suppose, because T6 doesn't match nor do mandibles (as noted by Hadel).

oops yes i meant t6

we do not have M. simplicipes males, only females.

i've imaged one of the two M. toluca males we have. mandibles are not visible because they're closed and covered by hair in both specimens. this species looks similar to the bee in above photos but i don't know what parts are diagnostic. here are some images (didnt get a chance to edit yet):
https://www.dropbox.com/sh/ab73srxwanehqc4/AAATPj19xLFjaGYh2CdD-zjEa?dl=0

Wow! Thanks for that information. And thanks for taking the time and effort to ID this for us and to convey this information to us.

Can anyone point me to an internet source that explains how to voucher a specimen?

If you collected a specimen, you have at least three options:
1) - Send it to BOLDSYSTEMS for a DNA analysis. (not recommended for a rare species like this, since they don't have a bin for it yet)
2) - Send it to one of our BUGGUIDE scientists for a microscopic examination. A list of specialists is here. You should try to contact one of them, by email for assistance before sending anything. In this case, Dr. Ascher is the prime candidate.
3) - Contact your local university, since they would prefer to properly ID and then house your specimen, in their own entomology collection.

of CTMI affiliated with UT in Austin. He agrees that it could be a Tylomegachile such as M. toluca (but not yet fully confirmed)

Thanks Bob! That's really helpful.

Thanks for your comment, Hadel Go. Couldn't an invasive species like Lithurgus Chrysurus survive in the southern US? It would be nice, though, if it was something else, something native.