The real spelling is "LaRva" because the creators have taken the name from Roman mythology. In ancient Rome, they believed in ghosts of people called MANES or LEMURES (We know that guy :-). Good ghosts of these were worshipped and called LARES. The bad ghosts were called LARVAE, the single word to this is LARVA. Since you wouldn't give a single person a plural name it must be Larva. "Lemures" can be plural and singular.