For Beijing's public toilets, three flies are one too many.
The city government is setting new standards in an attempt to clean up its dirty, smelly public toilets.
And in a series of guidelines, it says the number of flies inside should not exceed two.
The curiously precise hygiene ruling set the internet buzzing yesterday, with critics arguing it was too hard to enforce - and just too trivial in the light of all the other problems facing the capital.
But entomologists defended the guidelines, saying that two was a thoughtfully chosen number.
They said it meant eliminating the human waste that attracts flies and that two flies or less would indicate no fly colony was present.
'It is not only feasible, but laudable to reduce the number of flies down to two,' said Dr Qin Qilian , an expert in insect diseases with the Chinese Academy of Sciences' Institute of Zoology.