When I did my first game that default survival, I found a few spiders as I went deeper. Maybe 2 or 3 max. They are annoying to kill, sometimes miss completely or pop up out of no where.
Now I did survival Coastal map. The amount of spiders are crazy. 8-10 or even more found in large underground caverns. Now I find them in small groups which is annoying to kill semi auto gun. I have pull out my flashlight look around, see nothing in the area then get attacked by spider from behind me. Almost to hellstone so many spiders running around, going to suck if I have to keep killing all the spiders to make it my tunnels, guess I have to put walls up and raised platform to avoid them. I will get screen shot to show what I talking about.
Spawn rates I seen 5 mins to 2 hours. I killed a spider outside a small tomb, 1 or 2 skeletons, 2 chests, cleaned it out and went to leave and spider spawn back. I have not found much large animal threats or bandits for all the exploring by horse or on foot.
Since spiders drop no loot, maybe web to make into silk cloth?