Black widow and redback spiders are closely related but have key differences you should know. Black widows are mostly found in North America and have a distinctive red hourglass marking on the underside of their abdomen. Redback spiders are native to Australia and feature a red stripe or spot on the top of their black body. Redbacks are generally smaller and more aggressive, while black widows are usually shy and bite only when threatened. Both species are venomous, but serious bites are rare with prompt medical treatment.