Spiders in the genus Latrodectus are commonly known as “black widows” and are globally recognized for the distinctive red marking on the female’s abdomen. This iconic feature leads many to assume all spiders in this group display this bright color. The simple answer is no, as their appearance is far more varied than this single, famous mark suggests. The presence, color, and shape of the red marking depend on the spider’s sex, age, and specific species within the Latrodectus genus.
The Classic Red Hourglass Marking
The quintessential red hourglass is the signature trait of the mature female Southern Black Widow (Latrodectus mactans), one of the most recognized species in North America. This marking is consistently found on the ventral, or underside, of the female’s large, globular abdomen. The vibrant color serves as a form of aposematism, a warning signal to potential predators, such as birds. When the spider hangs inverted in its web, this bright red mark is prominently displayed against the shiny black body, signaling its toxicity.
Sexual and Age-Based Differences in Appearance
The iconic appearance of the female is only one part of the species’ life cycle, as male and juvenile black widows look drastically different—a phenomenon known as sexual dimorphism. Adult males are significantly smaller than females, often measuring less than half the female’s body length, and are typically light brown, tan, or gray, not jet black. Males often lack the prominent red hourglass altogether. If a marking is present, it may be less distinct, appearing white, yellow, or a faint reddish color. They may also have light-colored stripes or spots on the dorsal side of their abdomen.
Juvenile black widows of both sexes are generally lighter, with abdomens that can be white, yellow, or gray, covered in a pattern of white stripes or spots. The young female’s body gradually darkens to a glossy black, and the full red hourglass marking appears only in the final stages of development.
North American Species Variations
The term “black widow” encompasses several different species in North America, and the red marking varies notably among them. The Northern Black Widow (Latrodectus variolus), found primarily in the eastern and northern United States, often displays a variation of the classic hourglass. In this species, the ventral red marking is typically segmented, appearing as two separate spots or triangles that are not completely connected. Female Northern Black Widows also commonly have a row of distinct red spots along the dorsal midline of their black abdomen.
The Western Black Widow (Latrodectus hesperus), which inhabits the western regions, usually possesses the full, connected hourglass shape, though its size and shade can vary. The broader genus Latrodectus also includes species that are not black, such as the Brown Widow (Latrodectus geometricus), which is light brown or tan, and the Red Widow (Latrodectus bishopi), which has a reddish-orange cephalothorax and red spots on its black abdomen.
Identification Beyond the Red Mark
Since the presence and form of the red marking are inconsistent across the genus, other morphological features offer a more reliable way to identify a black widow spider. Mature females are characterized by a smooth, glossy, jet-black abdomen that is noticeably large and spherical, often resembling a shoe button. They also possess relatively long, slender legs compared to their body size, a common trait in the cobweb spider family (Theridiidae).
A distinguishing feature is the structure of the web, which is irregular, three-dimensional, and often described as a messy tangle of silk strands. Black widows generally build these strong, sticky webs in dark, undisturbed areas close to the ground, such as under stones, in woodpiles, or in the corners of sheds.