The black widow spider, a member of the genus Latrodectus, is one of the most recognizable arachnids due to its distinctive coloration. When trying to identify an adult female black widow, the presence and location of a red mark is the primary feature to look for. The definitive, identifying red mark on a mature female is not located on the back or the top of the abdomen (the dorsal surface). Instead, the characteristic marking is found on the underside (the ventral surface) of the spider’s spherical abdomen.
Location of the Signature Red Mark
The most famous mark associated with the black widow is a bright red or reddish-orange pattern shaped like an hourglass. This striking coloration is positioned squarely on the ventral side of the large, globular abdomen of the adult female. Since the female frequently hangs upside down in her tangled, irregular web, this marking is often prominently displayed to potential threats below. The bright coloring serves as a warning signal, known as aposematism, to deter predators.
The adult female black widow is typically a glossy, jet-black color across her entire body, contrasting sharply with the vivid red mark. Her body length, excluding the legs, generally ranges from 8 to 13 millimeters, with the abdomen forming a large, shiny sphere. The iconic hourglass shape is often complete, appearing as two connected triangles, though sometimes the two halves may be slightly separated. This specific ventral marking is the most reliable feature for confirming the identity of a mature female in North America.
Variations in Males and Juveniles
The misconception that the red mark is on the back likely stems from the appearance of male and immature spiders, which look significantly different from the adult female. Black widow species exhibit pronounced sexual dimorphism, meaning the males and females have distinct differences in size and appearance. Adult males are much smaller, often measuring only 4 to 6 millimeters in body length, and they are generally considered harmless to humans.
Male and juvenile black widows frequently possess markings on the dorsal side of their abdomen. These dorsal markings can be a combination of red, yellow, or white spots and stripes. Juveniles often display these colorful patterns, which can include a row of spots or a central stripe along the back. As a female spider matures through successive molts, she loses the dorsal markings, eventually developing the uniform glossy black body with only the single red hourglass on the underside.
Regional Black Widow Species
Species variations across the Latrodectus genus mean the exact appearance of the red marking is not always identical across different regions. In North America, three common species are recognized: the Southern, Northern, and Western black widows, and each displays subtle differences in their markings. The Southern black widow (Latrodectus mactans) typically features a complete, unbroken hourglass on her ventral side. Although the back is usually unmarked, some Southern black widows may have up to four faint red dots on the dorsal surface.
The Northern black widow (Latrodectus variolus) is commonly identified by a ventral hourglass that is broken into two separate red triangles. The Northern species is more likely to have a series of distinct red spots running down the middle of its dorsal abdomen, a pattern which is rare on the mature Southern female. The Western black widow (Latrodectus hesperus) generally has the hourglass on the lower abdomen, but its color can occasionally vary from red to yellow or even white. These slight regional differences in the red markings contribute to the diversity seen within the black widow group.