The size of black widow spiders (Latrodectus) is highly variable between the sexes, leading to confusion about their typical dimensions. The female black widow is the one most commonly encountered and recognized. While she is not a large spider in the general sense, she is substantially more prominent than her male counterpart. Compared to many spiders, the female is of a moderate size, but the male is indeed a very small creature.
The Critical Size Difference: Female vs. Male
The striking difference in size between the sexes is a classic example of sexual dimorphism in the black widow. An adult female’s body, not including her legs, typically measures between 8 and 13 millimeters in length, or about half an inch, with a distinctly rounded abdomen. This size is dramatically larger than the adult male, whose body length is often only around 3 to 6 millimeters. The female can be two to three times longer and up to ten times heavier than the male, a disparity attributed to her need to produce large egg sacs.
Because of this diminutive size, the adult male is often overlooked or mistaken for a juvenile or a different species of spider. Males do not possess the large, globular abdomen of the female, making them difficult to recognize as black widows. Consequently, the female sets the standard for the species’ perceived size, as she is the individual most often seen and positively identified.
Visual Identification Beyond Size
Since the size of a black widow can vary so much, other visual characteristics are necessary for positive identification. The mature female is known for her jet-black, glossy appearance, which covers her entire body and legs. Her overall body shape is characterized by a large, bulbous abdomen that is disproportionately larger than her cephalothorax.
The most definitive mark of the female black widow is the bright red hourglass shape located on the underside of her abdomen. This marking can sometimes appear as two separate reddish triangles rather than a fully connected hourglass, but its presence is a consistent feature. The male and juveniles, in contrast, often exhibit patterns of white, yellow, or reddish spots and stripes on the top of their abdomens, and may have a duller, brown or gray coloring.
Contextualizing Their Size: Comparison to Other Spiders
To put the black widow’s size into perspective, the female’s body length of around 13 millimeters is roughly comparable to a large pea or a common dried bean. The leg span of a large female can extend to about 3.5 centimeters (1.5 inches). This places the female black widow in a medium-sized category when compared to the range of spiders found in North America.
They are noticeably smaller than large species like the tarantula, which can have a leg span exceeding three inches. However, the female black widow is similar in size or slightly larger than other common household spiders, such as the brown recluse, whose body is about half an inch long. Considering the black widow’s relatively long, slender legs, its overall presence is much less imposing than that of a larger, bulkier species.