The hourglass marking on a black widow spider is typically red. It appears on the underside of the female’s rounded abdomen and is one of the best-known identification marks of the genus Latrodectus.
The marking is not always a perfect hourglass. Depending on the species, individual, and life stage, it may be split into two red spots, incomplete, or edged with orange or yellow. Juvenile black widows can also look quite different, with pale stripes and spots before developing the familiar dark adult coloration.
The marking is often confused with the spider’s entire identification pattern. Black widows may also have red or orange markings on the top or rear of the abdomen, while males and juveniles are usually much less glossy and conspicuous than adult females. The red underside is therefore a useful clue, but not a substitute for examining the spider safely.
Black widows belong to the genus Latrodectus, whose name is associated with medically significant venom. Their bites can cause latrodectism, a syndrome involving pain, muscle cramps, sweating, nausea, and other neurological symptoms. Despite their reputation, they generally bite defensively rather than aggressively.