This is one of three stag beetle larvae I found inside the tunnels of Bess Beetles, Odontotaenius disjunctus, in the same small log. It is possible this was actually a grub of Platycerus virescens, which later emerged from this mass of soil:

This grub was a smaller individual than the one of which I took a series of photos, presumed to be a Lucanus:



Based on what came out of this mass, I'm going to move this grub photo to its place alongside its presumed adult. This seems plausible, and better than leaving under Lucanus, where the larger larvae belong.