Southern Black Widow Spider

Theridiidae—Cobweb Weavers

(Latrodectus mactans)

Lactrodectus mactans female and egg case. Photo by Steven Jacobs, Penn State Extension

The widow spiders, genus Latrodectus, are found worldwide in the warmer regions of most continents. The taxonomy of these spiders is challenging and as few as six to as many as twenty-eight species are recognized. In the United States, there are probably five species: the southern black widow, L. mactans; northern black widow, L. variolus; western black widow, L. hesperus; brown widow, L. geometricus; and the red widow, L. bishopi.

The southern black widow, L. mactans, is found in Pennsylvania. It is probable that the northern black widow, L. variolus, is also present. Occasionally, the brown and the red widow spiders are introduced on potted plants from southern Florida.

Description

The female southern black widow is shiny and jet black. The underside of the abdomen has the well-known orange to red hourglass marking, while the dorsum is unmarked or can have up to four red dots. They are 8 to 13 millimeters in body length and measure 25 to 35 millimeters with legs extended. The male, which is black and has white underbody markings with red spots, is only 4 to 6 millimeters long (12 to 18 millimeters including its legs).

Latrodectus mactans male. Photo by Lynette Elliott, BugGuide.net, photo# 34225

Latrodectus variolus female dorsum showing red spots. Meghan Cassidy, BugGuide.net, photo# 1488945

Life History

Black widows can be found under stones, in stumps or woodpiles, in vacant rodent holes, in the dark corners of barns and garages, and in outdoor privies and other undisturbed cavities. Their webs are skimpy and disorganized.

Males are often killed and eaten by the females shortly after mating, thus the origin of the name "widow. " A female may live for a year or more and produce up to nine 0.5-inch-diameter egg sacs, each containing 200 to 800 eggs. Eggs hatch in about eight days, but the young spiders remain in the egg case for about nine more days, molting once during that time. They then disperse, traveling on thin silken threads through a process known as "ballooning." The female stands guard over the eggs during the summer months—when the majority of widow bites occur.

Medical Importance

Immature and male black widows do not possess fangs large enough to pierce human skin, so bites are almost entirely due to adult female spiders, especially females protecting an egg sac. Reactions to bites can range from no reaction or localized symptoms to a severe systemic reaction, which is described below.

Black widow venom is principally neurotoxic, so at first a bite is almost painless. However, pain will be felt one to two hours later, and occasionally the patient may experience a tingling along the nerves or down the spine. There is almost no swelling at the site of the bite, although the site may exhibit two red fang marks and be surrounded by a rash or erythema.

Generalized body symptoms, which develop within one to three hours, may include any of the following: nausea, chills, slight fever, rise in blood pressure, retention of urine, burning sensation of the skin, fatigue, motor disturbances, breathing difficulty, constipation, and muscle aches, particularly in the abdomen. These symptoms generally dissipate within four days, but they may last as long a week.

Treatment for a bite targets the symptoms and can include the use of muscle relaxants, antihistamines, and analgesics. Latrodectus antivenin is available, but it is generally reserved for extreme cases as there is a risk of anaphylaxis (allergic reaction) to the antivenin.

While a black widow bite is extremely unpleasant, deaths from black widow bites are extremely rare. According to the American Association of Poison Control Centers, approximately 2,200 people are bitten in the United States by black widows every year, but no deaths have been recorded due to black widow bites since 1983.

Reference

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