Shelob, the cat of Sauron

[Balin999]

Another question is: Why didn't Shelob capture Gollum when he was crossing her lair?

I guess Gollum was no good to eat. He was 500 years old and not what you would call "well-nutriated" since he was spending most of his life in a cave.
Somewhere in the books it says that he had worshipped her and promised her to bring her some "sacrifices", if you can call it like that. I mean, Gollum even managed it to keep the two Hobbits from slaying him.
But then, I don't know if Shelob knew what "pity" was.
Hm another question: How could Shelob understand what Gollum was saying to her? And could she talk?