Story related once you get to
Chapter 12: The Ride.
*** Spoiler - click to reveal *** You will come to a point where you will get on a bus and Kidman starts driving. As you get started an enormous spider like creature attacks the bus and rips it up. Just shoot at it until it falls off. This will happen a couple of times until you come to a tunnel that will end up knocking it off and killing it as you go through.
There are also great guides to get
Along with
Arachnophobia