Spider Baby (1968)

Spider Baby aka The Maddest Story Ever Told aka The Liver Eaters aka Cannibal Orgy, tells the story of how the ironically-named Merrye family manages to cure itself of a horrible family disease. All the Merrye clan suffer from an extremely rare condition called the Merrye Syndrome. As we learn in the movie's introduction, victims of the Merrye Syndrome suffer from progressive age regression that may progress beyond the pre-natal level, reverting to a pre-human condition of savagery and cannibalism. Merrye Christmas! Get it? I don't know why I even bother with you sometimes.