If you don't catch and treat it quickly, it may begin to form a necrotizing ulcer. The bite becomes hard in the center and turns an angry red-purple; the wound is inflamed, hot and is extremely painful. Imagine a pit bull clamped on and not turning loose. Most folks will get antibiotics at this stage; I did myself, but then at the time I didn't know what herbs would be helpful. The antibiotics nearly did me in as I went into anaphylactic shock; good thing I was only a block from the ER.
Once you realize you've been bitten look for plantain; also called ribwort or pigs ear. It's a very common "weed" and grows in yards everywhere. (Must be free from herbicides and pesticides). If you cannot recognize it may I invite you to a Green Blessings "weed walk". Or, second best, look it up on the web or in a good herbal. Plantain has tremendous drawing power and has been used for generations to draw out infection. Obviously, the quicker plantain is applied, the better. Chew the leaves then apply to the wound. Yes, I said chew. Your own saliva gives it a little boost. If this doesn't appeal to you, then crush the leaves to release the healing "juice", then apply to wound. Using whole leaves as bandages (hence the nickname "band aid plant") instead of gauze will prevent the bandage from adhering to the wound like a gauze pad does. Ever had to remove a bandage stuck to the wound?! OW! Plus, when a bandage opens up and pulls off skin, it traumatizes the wound and slows the healing process. Wrap and cover the wound with plantain leaves and secure with gauze. Burdock leaves are also WONDERFUL for this because they are so large. The plantain dressing will need to be changed every 2-3 hours.
Can't get wild plantain or burdock? No worries, both will grow nicely in a garden. I planted them from seeds and harvested a marvelous crop.

Skinny leaf plantain

Burdock
If you cannot get fresh plantain, use a good plantain salve. Be sure to get one from an herbalist; it should contain no petroleum products, preservatives, colorings or any other chemical. There are several other drawing agents in the herbal world, but plantain is my first choice, burdock and yarrow being two other good choices.
Now, that takes care of external; let's go inside. I am partial to prairie plants; after all I live among them! The Native Americans called this plant the "Prairie Doctor" and later, folks like my grandparents situated in the heart of America referred to it as the "Farmer Remedy"; gardeners and landscapers call it the Coneflower, but it's well known by its true name: Echinacea. In fact it's so well known that we tend to overlook it as a powerful remedy. Yes, there are naysayers out there who cite scientific studies that negate the efficacy of Echinacea, but for the sake of space, let's save that debate for another time. Plain and simple, it works. I've seen it work its plant magic many times. And, yes, we have "studies" to prove our side as well.
It has been used historically as a remedy for spider bites very successfully. There are thousands of Echinacea preparations out there (this is one of the problems); some are effective, some are not. Look for Echinacea augustifola root tincture. Echinacea has several properties, but its primary use for spider bites is its diffusive action. Diffusive herbs impart a distinctive tingling sensation on the tongue, which indicates its powerful action on the nervous system. Echinacea acts on the blood and lymph which makes it perfect for venomous bites which have caused a septic situation.
Place a couple drops of tincture directly on the wound and take 2 or 3 DROPS under the tongue every three to four hours. (Yes, apply the Echinacea and the plantain to the wound.)
Everyone recognizes this beauty - here's one from my garden.

The herbal world is wonderfully complex and diverse so this is not the ONLY way to treat spider bites, but in my opinion, it's one of the best.