I have found these beetle larvae twice in my area, both times inside rotting logs, closely associated with Bess Beetle, Odontotaenius disjunctus, adults. This individual was found inside a small rotting log in my back yard. The photo below shows one of two others found inside the same log, and that or another grub in a pupation chamber accidentally broken open:

Those two were smaller, later presumed to be Platycerus virescens, based on the emergence of this adult later:


Here are two similar larvae I had originally misidentified as Bess Beetles, but I feel are clearly Lucanus:



Another BugGuide member moved them to the scarabs. (Bess Beetle larvae, I learned, only have four functional legs, the hind ones being modified for stridulation.) Finding identical larvae in my back yard gave me pause. Over nine years, I've never seen any large scarab adults come to my lighted windows, but I have seen adults of the stag beetle, Lucanus capreolus.

Aha! Poking around the Internet, I found Stag Beetle Larvae and was very intrigued by Stridulation in stag beetle larvae, (Lucanus cervus L.). My grubs must not be scarab larvae, but stag beetle larvae! The large size is consistent with Lucanus I think, and not any of the smaller genera. I'm betting they are L. capreolus, given that is more common around here than L. elaphus.

See also: Scarabaeoid Indentification Guide: Lucanidae--By Brett C. Ratcliffe and M.J. Paulsen. This describes the larvae of Lucanidae: The larvae resemble those of Scarabaeidae, but in lucanids the anal opening is longitudinal or Y-shaped, whereas in scarabs it is usually transverse or occasionally Y-shaped.

Wild-eyed Hypothesis: I think it very odd that I've found the Lucanus larvae closely associated with Bess Beetle adults twice. I wonder if the Lucanus is a commensal of the Odontotaenius. Very curious that the Lucanus larvae stridulate. Odontotaenius larvae and adults are known to stridulate. I wonder if there could be some interspecies communication, perhaps even social parasitism. Again, a wild-eyed hypothesis!