Hmm.. Poor spiders can't procreate anymore (because the eggs will burn), can't eat without lots of help (lest the food burns before they get to eat it), might find it hard to hunt (can't spin webs in case they hunted that way), might be setting their abodes on fire constantly, and might be attracting predators (because they're very bright at night).
But, they may have noticed that it's sometimes useful to be on fire: you are less likely to be eaten when you're on fire, you can't freeze in cold weather if you're on fire (and spiders, being small, will have problems with low temperatures; see https://www.youtube.com/watch?v=pO7PmwZc6qE), being on fire can be used to boil water.
So, maybe the spiders would be willing to negotiate to get the ability to switch the fire on and off at will in return for not harming the hapless wizard(?)?
Also, have they already set something on fire? It might be amusing if the frequency of random fires increased around the party and that caused weird (and incorrect) suspicions from random other people in the vicinity.
