Black widow spider bites may cause stabbing pain in the bite area, but they can also be painless. Look for one or two red fang marks, redness, tenderness, and a nodule at the bite site. Severe muscle cramps, nausea, vomiting, seizure, and a rise in blood pressure may follow soon after. Get medical care immediately. Anti-venom medicine is available. If possible, bring the spider with you for positive identification.