Identification request: Pseudomyrmex

I took some pictures recently of this Pseudomyrmex species at the Museum of life and science in Durham, NC. The colony lives in association with an Acacia tree.
Apparently originally, the tree (and the ant colony)comes from Florida.

I would appreciate any help for the identification or what specific criteria I could use to identify them.

Here are the link to the pictures:
http://www4.ncsu.edu/~bsguenar/Pics%20Pseudomyrmex%20spA.html

If the ants are truly acacia-ants, then they did not originate in Florida. An acacia ant exhibit at the Missouri Botanical Garden in St. Louis used to have the obligate acacia ant Pseudomyrmex flavicornis in it, which looks rather like your photos, but the pictured ants could also be the common Ps. cubaensis, an exotic species abundant in southern Florida. The MBG exhibit now has the opportunistic, and much less viciously stinging species Ps. gracilis.

I might be able to figure out which of these two (or another) the pictured ants are from the pictures, but here are the references for you and others to ID them yourself.

Phil Ward's review of the Pseudomyrmex of acacias is the source for identifying the obligate acacia species, none of which should be found in Florida: http://antbase.org/ants/publications/2958/2958.pdf

Phil's review of the Nearctic species will cover those resident in Florida that may have opportunistically occupied the acacia: http://antbase.org/ants/publications/2952/2952.pdf

Thank you for your help.

The plant (and the colony) comes from Florida, but probably from an horticulturist, so it is highly probable that the plant does not originate from Florida, but from an other location.

For Ps. cubaensis, the size of eyes relative to the head seems much smaller on my pictures than on the pictures than I saw on Antweb

I think that I can exclude this species for this reason.

For Ps. flavicornis, the standing pilosity (which is one of the basic trait used in Phil Ward's key) is too important. The standing hair are very sparse on my specimens (you can see it on the pictures).

So now if I follow the key's you sent me. It seems to me that Ps. subtillisimus could be a possibility. I think that on two characters:
- the first one is the sparse pilosity.
- the second one is the petiole that seems flattened dorsaly (at least if I compare with the pictures of Ps. boopis and Ps. ita).
However, for the size, I don't really know, they are 3-4 mm long which is not excessively small, but could it be for Pseudomyrmex?
I was not able to find a picture or a draw of Ps. subtillisimus to make any comparison, so...

I'll clarify:
If the tree is from Florida, and the ants inhabited it spontaneously from a population resident in Florida, it seems unlikely (virtually impossible) that the ants are obligate acacia ants. I'm going with either cubaensis or the somewhat smaller elongatus as the best guess, but this is only a guess. Of course, specimens would clinch the identification. ID from photos, sometimes even those on AntWeb, can be difficult.