The black widow spider, a member of the genus Latrodectus, is widely recognized for its glossy black body and the distinctive red hourglass shape on the underside of the abdomen. This striking coloration is often perceived as the sole identifier for all dangerous widow spiders. However, this common perception is inaccurate, as a variety of factors mean that a widow spider may not display the classic red marking. The absence of the red hourglass does not exclude a spider from the Latrodectus genus, which has important implications for identification and safety. The appearance of these spiders changes dramatically depending on their age, gender, and specific species.
Variation in Classic Black Widow Markings
Not all classic North American black widows, such as the Southern black widow (Latrodectus mactans) or the Northern black widow (L. variolus), are solid black with a complete red hourglass. The most significant variation is seen in males and juveniles, who look very different from the mature female. Adult male black widows are considerably smaller, often having a body length of less than half that of the female, and their color is typically lighter, ranging from brown to gray.
Instead of a solid hourglass, males often retain complex patterns from their youth, featuring red, white, and yellow spots or stripes on the sides and top of the abdomen. Juvenile spiders of both sexes start with a pale appearance, often being gray or brown with white, yellow, and red markings arranged in stripes or spots. As female juveniles mature, these markings consolidate and darken, eventually forming the single, shiny black body with the red hourglass characteristic of the adult female. Even among adult females, the hourglass marking is not always perfectly formed; it can sometimes appear as two separate red spots or be reduced in size, especially in the Northern black widow (L. variolus).
Global Diversity of Widow Spiders
The genus Latrodectus includes many species worldwide, and many of these spiders do not fit the “black and red hourglass” description. The Brown Widow (Latrodectus geometricus) is a cosmopolitan species that is tan to dark brown or gray, often with a complex geometric pattern on the back of its abdomen. The hourglass marking on the underside of the Brown Widow is typically a vivid orange or yellowish color, not red.
Another distinct species is the Red Widow (Latrodectus bishopi), endemic to Florida scrub habitats. This species has a reddish-orange cephalothorax and legs contrasting with a black abdomen that is marked dorsally with rows of red or yellow-bordered spots. The Red Widow also frequently lacks a complete hourglass, having only one or two small red marks on the underside instead. The existence of these and other regional species, such as the various “button spiders” in Africa, confirms that the widow spider genus encompasses a broad spectrum of colors and markings.
Reliable Identification Features Beyond Color
Since color and markings are variable, relying on other physical and behavioral traits is a more reliable way to identify a widow spider. The body shape of the mature female is characteristic, featuring a large, smooth, and globular abdomen that is glossy. This bulbous shape is distinct from many other common house spiders.
Widow spiders construct a specific type of web, typically a messy, irregular, three-dimensional tangle, often referred to as a cobweb. These webs are generally found in dark, sheltered, and undisturbed areas, such as under furniture, in woodpiles, or in the corners of garages and sheds. Other identification features include:
- The strength of the silk.
- The female’s use of a “comb-foot”—a row of specialized, curved bristles on her hind legs—to throw silk over captured prey.
- The drastic size difference between the sexes, with the female being significantly larger than the male.
Understanding Widow Spider Venom
Despite the wide variation in physical appearance, all true widow spiders within the Latrodectus genus possess a potent neurotoxic venom. This venom contains latrotoxin, the protein responsible for the systemic effects known as latrodectism, which can include severe muscle pain, abdominal cramps, and muscle spasms.
The potency of the venom can vary between species. The venom of the Brown Widow is often considered less medically significant than that of the Black Widow, although the Brown Widow’s venom is actually slightly more toxic drop-for-drop in laboratory settings. The difference in danger often comes down to the smaller amount of venom the Brown Widow typically injects during a bite. Bites from any widow spider are primarily defensive, usually occurring when the spider is accidentally pressed against the skin or when a female is guarding an egg sac. Medical attention is always advisable following any suspected widow spider bite.