The minuscule size of this spider is difficult to convey — I thought it was a lint ball until it moved and tried to pounce on a springtail (which was bigger than it was, to give you an idea of scale).
Attidops cinctipes, no common name, is a well-camouflaged and extremely tiny jumping spider which lives on and under the bark of trees, eating the equally tiny insects that also live there. I can’t imagine finding one of these on an actual tree — this one was practically invisible even on a smooth wooden boardwalk railing.
A. cinctipes is native to the gulf states of the U.S., possibly down into Mexico. Other members of the genus live all over North America, from Canada into Mexico. The Journal of Arachnology (27:7-15, 1999) has a decent description of the Attidops genus, including measurements (2.1-3.1mm long) and a key to the (four known) species.
Forgive me if I squee just a little here — my find was the first Florida ID on bugguide, and the first ID of this species at all on iNaturalist. Squee!
Many thanks to salticidude on iNaturalist and the kind people of the Jumping Spiders (Salticidae) group on Facebook for helping me identify this little lady!