Very cool colony location that is atypical for this species here, but it is still surrounded by some pine trees and within the F. rufa population in the area. We found no F. polyctena.

All workers were hairy on the thorax dorsally, except for one - last slides show an incredibly red worker whose gaster appeared a little swollen and her thorax was almost bald. Her hind tibiae appear to be longer than usual. This could possibly be some sort of parasitic infection that alters the worker's phenotype. Such endoparasites are not yet known in the Formica genus. Specimen was taken.

Found in decaying wood along underside and interior of fallen Populus tremuloides log surrounded by large thatched nest. Several Formica obscuripes workers were present inside the log, and a dealate Formica dakotensis female was also found under the log.
The presence of clavate setae on the legs and eyes confirms Formicoxenus, and the shiny smooth first gaster tergite suggests F. hirticornis.
Images 2, 4-5 show ergatoid females.

Last three images are higher resolution lateral, dorsal, and head views of worker.

https://bugguide.net/node/view/2078519

Observed tandem carrying, multiple workers foraging above leave litter. Found in the botanical garden of Villa Carlotta, however not in a green house but next to an artificial waterfall

For more context see this earlier observation https://www.inaturalist.org/observations/183352558
and this publication
https://doi.org/10.3897/BDJ.12.e123502

Since there are no proper specimen pictures of this specied online(not so surprising), I've decided to throw some in myself. Legs not as red when alive though.
Currently only known from South Korea, this species is rather widespread but more in the western part of the nation. The japanese C. amamianus seem to be the closest known species of this enigmatic endemic.

specimen uploaded to AntWeb: https://www.antweb.org/specimenImages.do?name=antweb1041831

specimen sent to me by @impuls4444

Date is a rough estimation, presumably collected in spring or summer of 2023

Location given as Prasong sub-district, Tha Chana District, Surat Thani Province, Thailand

2.5 mm, in a branch about 3 m above ground inside a tropical greenhouse in ZOO Lešná.
The Yucatan greenhouse: https://www.youtube.com/watch?v=tx41VIEzSX0
in some places it is open and connected to the outside