Minecraft wiki entry for Cave_spider_farming, associated text: Once your grinder, whether it's automatic or manual, is fully functional, go back to the spawner room and get rid of all of the lighting that you put up. This should allow the cave spiders to spawn. If desired, you can decorate any part of the farm that may be in view of a player. Also, if you want to enhance the mechanics of a farm, make a sorter (this will only work if you have hoppers underneath your grinder). This way, the string and spider eyes dropped from the cave spiders will go into separate chests.