Spiders are one of the most common enemies found when exploring caves and underground areas. In the inhabited areas of The City they are somewhat scarce, as they are usually exterminated on sight. But in the sewers and deep underground, they are far more common, and come in three varieties, all of them venomous.
Since spiders don't count towards any of the "Don't kill anyone" objectives, the best option is to kill them from a distance with broadhead arrows in a single shot, or with gas arrows and mines for large groups. They can be a most terrifying threat to any player who is arachnophobic.