I don't think I have heard of any scientific research has been published about larval morphology of Lucanus spp. in the U.S. yet. You see... the insect taxonomy is mainly based on morphology of adult stage. Now, many researchers work on larval morphology as well, however, stag or rhinoceros beetles are not a major concern for such study (unless you are coleopterist/scarabaetologist) as there are a lot more and many different pests out there currently and will be a nuisance for forest, human lives, etc.