You could say the entire story is an allegory. After all, the setting seems so ordinary that it's tempting to see Gregor's transformation as a symbolic one, rather than an actual one. Maybe he's only a vermin in some moral or psychological sense. Perhaps he did something particularly cretinous – how about that chambermaid he had such "happy, fleeting" memories of? And what about all that cash his supervisor mentioned? Or perhaps his verminous-ness is an indirect expression of his selfish desire to be free of his family obligations. For more discussion on the vermin as it relates to the main themes of the story, check out "Man and the Natural World."