Here is the beetles' "home base" - a sprouting fungus. The fungi grew significantly over the two days that I observed them, and as their sizes increased, so did the number of Fungus Beetles and Tr*iplax sp. that came to feed on them. This image was taken on the first day (May 22).
The beetles engaged in a peculiar behavior. They would start off all within the tunnel they had excavated in the mushroom, then suddenly, march out single-file onto the cap where they would walk the perimeter before disappearing back into the tunnel. They did it at regular intervals of a few minutes, and reminded me of soldiers on a border patrol mission. In this image, the individual on the bottom left is perched at the entrance to the tunnel.