Minecraft wiki entry for Cave_spider_farming, associated text: Once a spawner is found, there are a few things that need to be done before starting the farm. First, get rid of all the cobwebs with a sword or shears (sword yields string and shears yield the cobweb item itself), and quickly surround the spawner with torches, so that cave spiders won't be able to spawn. Be careful not to accidentally mine the cave spider spawner, because you will not be able to get it back.