'The Fly' is a poem written by the English poet William Blake (in 1794).
He uses a trimester rhyme scheme in order to symbolize how short is life. What the poet marks by starting a new stanza at line 5 is a comparison of his life and the fly's one (Am not I [...])
He says 'Am not I A fly like thee? Or art not thou A man like me?' meaning that both 'the human being' and 'the fly being' are the same, both are 'being' of God.