The brown widow spider, Latrodectus geometricus, is a globally distributed species of medical concern that is frequently mistaken for its more infamous relative, the black widow. Recognizing the unique physical features and markings of the brown widow is the most reliable way to distinguish it from other spiders.
Overall Body Characteristics
The body length of a mature female typically ranges from 8 to 12 millimeters, not including the legs. The abdomen is large and globular, giving the spider a distinct, rounded appearance characteristic of the Latrodectus genus. The overall coloration of the spider is highly variable, ranging across a wide spectrum of light tan, dark brown, gray, and sometimes even a nearly black shade. The abdomen often has a slightly dull or velvety texture due to a covering of fine hairs. The legs are usually banded with alternating light and dark rings, a feature that contributes to its mottled appearance.
Key Identification Markings
The most telling feature of the brown widow, common to all widow spiders, is the hourglass marking located on the underside of its abdomen. Unlike the bright red hourglass of its cousin, the brown widow’s hourglass is typically a vivid orange or a yellowish color. This marking can also appear somewhat fragmented or irregular in shape, often looking puffy with an indistinct outline. The top side of the abdomen often displays a distinct pattern of mottling or geometric markings, which is how the species gets its name, geometricus. This pattern can include white stripes or pale-bordered ovals running along the midline of the back.
The Distinctive Egg Sac
The egg sac is frequently the easiest and most definitive way to confirm the presence of a brown widow spider. The sac is spherical and usually a pale tan or off-white color. Its surface is not smooth but is covered in numerous tiny, stiff silk projections or tufts. These silk spikes give the egg sac a fuzzy or spiky appearance. This unique construction serves as an immediate visual differentiator from the smooth, often pear-shaped egg sacs produced by other species in the widow genus. A female brown widow can produce multiple egg sacs, each containing approximately 120 to 150 eggs.
How It Differs From the Black Widow
The brown widow is often confused with the black widow, but several distinctions make them easy to tell apart. The brown widow is typically smaller and less bulky than the black widow, though both species have a similar body shape. The most noticeable difference is color, as the brown widow is typically mottled brown, tan, or gray, while the mature female black widow is a glossy, uniform black. Brown widows tend to build disorganized, tangled webs that are less robust and more exposed, often found in open areas like under patio furniture. Conversely, the black widow prefers to build its stronger, more secluded web in dark, hidden locations. While both spiders possess neurotoxic venom, the brown widow is generally considered less of a medical threat because it is less aggressive and injects a smaller amount of venom when it bites.