Factual error: The spiders shown in the movie have no adhesive pads like real spiders, but only sharp points on the tips of their legs. Given the mutation, this is possible, but without their gripping apparatus, they should not be able to climb up anything. (01:00:10)
Factual error: In the mine, David Arquette and the others start to smell the methane gas and one of the people even quips "It wasn't me." However, methane is an odorless gas and therefore they probably wouldn't have had any way to tell if it was there. (01:21:55)
Factual error: Due to the way in which a spiders respiratory system works it would actually be impossible for the spiders to breath as their box lungs would collapse and their primitive circulatory system wouldn't get enough oxygen around the body.
Answer: Both! The owner of the spider farm was feeding the spiders bugs from the water that had the chemical spill in it. The bugs were contaminated with the chemical, thus contaminating the spiders. Note: The man who owned the spider farm had no idea about the contaminated water.
SAZOO1975