This is one of the "slug holes" I have been monitoring since 2006. It is a narrow cavity in a beech tree in the park near my house. There is always water on the bottom. During the day slugs huddle at the far end of the cavity taking advantage of the high humidity. At night they come out of the hole and climb up the tree to feed on the microscopic algae that grow on the trunk.
These are a Megapallifera species that I have not yet identified below the genus level (family Philomycidae). They are native to the area. I have written about them before.
This picture was taken last Thursday afternoon. Once when I checked this hole in December, the water inside was frozen and, not surprisingly, there were no slugs. I intend to check this hole and a few others in the vicinity at weekly intervals this fall to determine when the slugs disappear. I believe they spend the winter deep in the ground.
Watch this blog for updates.