Not a proverb I knew; but that means nothing. What is slightly odd is that so far as I can see it isn't in _The Oxford Dictionary of English Proverbs_ (Third edition) in any form. However, now I have the clue to search under 'flies' I have been able to find it in Apperson's English Proverbs and Proverbial Phrases, which cites a version of it from Thomas Fuller's _Gnomologia_ (1732).
no subject