I have not bothered to look this up in any reference books, so please forgive me if the answer is blindingly obvious.
What, I would like to know, is the quality inherent in the excrement expelled by bats that is related to human madness? I am thinking of the phrase batshit crazy. We do not commonly say, for example, catshit crazy or giraffeshit crazy or puff-addershit crazy, or at least I have not heard these phrases used at the sort of swish sophisticated cocktail parties I try to barge my way into. But reference to the droppings of bats is made frequently.
Answers in the Comments please, unless you are batshit crazy yourself, in which case you would be more appropriately occupied reading something by Will Self.