Ever heard of the Bolas Spider? It makes a string of silk, and tops it off with a gooey blob of sticky web and goo, then, when a moth flies by, attracted by the goos smell, the spider spins the bola around and freaking lassos the moth from mid air and pulls it to its web.
Neat, I evidently haven't until now. So how would that behavior evolve? It can't be mutated in... really. It's more of something you learn. How exactly would it stop a Cazador rush? Like I said, spiders just can't be competitive with those bastards around. Also I take it as a compliment to their nastiness when Spiders have been wiped off the face of the Mojave desert in a short time span by the arrival of the Cazadors. Amusing really. I don't see Spiders being able to survive unless they learn group behavior. If they gain some kind of venom immunity or resistance (magically) then they wouldn't need quite as large a group.
Also it's not unlikely that something can learn to breathe fire, or grow in size. Those are all plausible. There are beetles that can shoot corrosive burning chemicals that burn a lot, hell a particular combination of gut bacteria could already produce flammable gas. One mutation to allow the digestive track to allow the gas to seep up and out of the body in flammable bursts of fire. Though lighting the gas is another matter. I'm sure I could find some way to do it, but I'm really tired.