What's the irrigation hole?
Well, okay.
The Termitat is acrylic sandwiching the round slice of wood, right? Well there are gray plastic spokes holding the round wood in place within the acrylic. Some of the spokes physically reach the wood (bark) surface, but since the wood isn't perfectly round, some spokes do not.
At the very top of the Termitat is a place to inject water. The water goes into the topmost gray plastic spoke, which is hollow. The spoke goes right up to the wood surface. There is a hole drilled into the wood to allow the water to further flow into the nest area at the center of the wood. You can sort of see it in this picture:

(This is a photo from very early on, so you can see there's been changes since then.)
Please note that termites COULD, if they wanted to, walk around on the bark. I saw them exploring out there early on. But mine plugged up the hole leading to the exterior of the wood slice.
Edited by OhNoNotAgain, January 13 2021 - 12:01 PM.
Formiculture Journals::
Veromessor pergandei, andrei; Novomessor cockerelli
Camponotus fragilis; also separate journal: Camponotus sansabeanus, vicinus, quercicola
Liometopum occidentale; Prenolepis imparis; Myrmecocystus mexicanus
Pogonomyrmex subnitidus and previously californicus
Tetramorium sp.
Termites: Zootermopsis angusticollis
Isopods: A. gestroi, granulatum, kluugi, maculatum, vulgare; C. murina; P. hoffmannseggi, P. haasi, P. ornatus; V. parvus
Spoods: Phidippus sp.