Reminds me of a friend I had when I was a teen. I'd go over to his house and he had two Styrofoam cups on his window sill - when I looked in I found fly heads in one, and their bodies in another. I asked him the story, and he said that he liked to cut off fly heads with a razor blade and then collect the heads and bodies separately.
You know what is truly frightening? Today he is a psychiatrist practicing in Los Angeles.