Two of the most medically significant spiders in the U.S. are the Black Widow and Brown Recluse. While their bites are rare, both can cause serious symptoms that require prompt medical care.
Brown Recluse: Light to dark brown with a violin-shaped mark on the back. Prefers dark, undisturbed places. Bites can start painless but may develop into severe tissue damage (necrosis), fever, and chills. Black Widow: Glossy black with a red hourglass marking on the abdomen. Females are larger and more likely to bite when threatened. Bites may cause intense pain, muscle cramps, nausea, and breathing difficulty within an hour.