John Carpenter's Tales of Science Fiction: Twitch #2

Astrophysicist Marisa Stringfellow broke out of the observatory thinking she'd be able stop an alien entity on the rampage in the Antelope Valley desert. Lieutenant Pinkerton, meanwhile, thinks Stringfellow may be suffering from heatstroke. "How can you tell they're aliens?" Stringfellow replies, "Because right before they kill you, they'll start to twitch ..."