Which spider is known for its red hourglass marking on the underside?

The spider known for its red hourglass marking on the underside is the black widow spider. The marking sits on the female's abdomen and is usually red or orange, though its shape and visibility vary among species.

Black widows belong to the genus Latrodectus, whose name gives us the medical term latrodectism for the effects of their venom. Females produce a potent neurotoxic venom that can cause severe muscle pain, abdominal cramps, sweating, and spasms. Despite their reputation, bites are rarely fatal, especially with modern medical care.

The hourglass is a useful clue, but not an absolute identification rule. Some widow species have incomplete hourglasses, pairs of spots, or faint markings, while males and juveniles often have stripes or spots on the upper abdomen. False widow spiders in the genus Steatoda are also commonly confused with them, but they lack the classic pattern and generally have less medically significant venom.