I found four of these immature tenebrionids -- one under bark of red oak log and the rest in the pithy sapwood of the log. I had never encountered this larval type before. I suspect they are either Alobates pennsylvanica or Tarpela micans. In Alobates' favor, I found one adult of the species at the opposite end of the log. In Tarpela's favor, it is red oak, which Tarpela favors. I'll try to rear them and get a definite ID.