Sigourney Weaver had no problem battling acid-dripping Aliens, but the army of ants in her new movie Copycat sent shivers up her spine. For a scene in the serial-killer thriller, Weaver had to lie in bed while a horde of carpenter ants scampered across her body. ''They were all over! And we couldn't kill any of them,'' she says, explaining that ant wrangler Steve Kutcher ''believes in humanity to ants. He wanted us to give them back to him especially the ones he had defanged.'' But Weaver, who'll next play the Wicked Witch in a live-action version of Snow White, found a way to keep the critters at bay. ''I cheated,'' she admits. ''I put two sheets on the bed, one between me and the ants. I didn't want to go home crying.''