What symbolic objects or other details appear in the story? Do they have connections with earlier mythologies, legends or literature?

I can elaborate on one. The picture clearly symbolizes Gregor's link with his humanity, but different interpretations of this picture are possible. The picture of a pretty woman may represent women in general, along with the possibility of human relationships. Some critics have pointed out that the picture, showing a woman largely concealed in fur as if she is turning into an animal, symbolizes Gregor's own metamorphosis.