Batman's Genie

I'm a genie and Batman wangled a one-wish, real deal from me; he said graveyard shift duties have been driving him real nuts lately; he was bored flapping nocturnal, chasing this and that criminal. He wanted to be Spiderman, that celebrity on teevee; I said yes, but identical identities I couldn't guarantee; for crimes of impersonation he shouldn't look at me, I was merely his genie. On his wish he would insist; I granted it with a minor twist: the spider silk wouldn't zip out from his fingers or from his wrist, but through his shoes or the crotch of his pants, whichever opened first. So, if the guy swooshes by upright, that's the Spiderman alright; if he dangles upside down by his butt that's the man who's a bat; if that's unsightly, don't blame me, that's arachnid anatomy!