As you probably know, make sure you see no darkening rings around lighter bite or dark center as well.
Almost all spider venom is designed to melt shit, like your flesh under the skin.
Got bit by something, which I never did see while cleaning my sauna and it was up to a silver dollar size discoloration when I went to the doc's 5 days later.
Antibiotics, which may not be that effective were given as well as a plug taken out but unfortunately they couldn't identify the critter. It will be two years this Christmas and I still have a nickel sized bruise on the side of my lower leg yet. It's dead stuff under a few layers of skin that will never go away.