How did Gollum talk with Shelob?

[Kalimac]

One thought while re-reading "Shelob's Lair" which I'd really like to get some opinions on: Could Shelob understand language at all? The way she's described is basically as an extremely malevolent animal, and certainly she never gives any indication of understanding what Frodo and Sam are saying in the scene where they're trapped in the Spider's Pass. Essentially the only thought process she seems to be experiencing there is "Food. More food. Blood. Good. Food."

But in the long passage describing her, Tolkien refers to Gollum having "bowed and worshipped her" and promised to bring her food. And the Orcs refer to Gollum's "having some sort of understanding with Her Ladyship." So how did Gollum *communicate* with her? Hand gestures? Could she understand Westron, maybe just not speak it? For that matter, considering that she's probably only heard Black Speech from the Orcs, and some Elvish long ago, how would she understand Westron? (and certainly Gollum doesn't seem to speak anything else). It's understandable that she wouldn't have bothered to eat the fleshless Gollum, whether or not he could communicate with her, but how did he reach an understanding like that with a spider?