Latrodectus is a broadly distributed genus of spiders informally called the widow spiders, with several species that are commonly known as the true widows. This group is composed of those often called black widow spiders, brown widow spiders, and similar spiders.
Diversity A member of the family Theridiidae, this genus contains 34 species, which include several North American "black widows" (southern black widow Latrodectus mactans, western black widow Latrodectus hesperus, and northern black widow Latrodectus variolus). Besides these, North America also has the red widow Latrodectus bishopi and the brown widow Latrodectus geometricus, which, in addition to North America, has a much wider geographic distribution.
Elsewhere, others include the European black widow (Latrodectus tredecimguttatus), the Australian redback spider (Latrodectus hasseltii) and the closely related New Zealand katipō (Latrodectus katipo), several different species in Southern Africa that can be called button spiders, and the South American black widow spiders (Latrodectus corallinus and Latrodectus curacaviensis). Species vary widely in size. In most cases, the females are dark-coloured and can be readily identified by reddish markings on the central underside (ventral) abdomen, which are often hourglass-shaped.
Venom
These small spiders have an unusually potent venom containing the neurotoxin latrotoxin, which causes the condition latrodectism, both named after the genus. Female widow spiders have unusually large venom glands, and their bite can be particularly harmful to large vertebrates, including humans. However, despite their notoriety, Latrodectus bites rarely cause death or produce serious complications. Only the bites of the females are dangerous to humans. Due to the presence of latrotoxin in their venom, black widow bites are potentially dangerous and may result in systemic effects (latrodectism) including severe muscle pain, abdominal cramps, diaphoresis, tachycardia, and muscle spasms. Symptoms usually last for 3–7 days, but may persist for several weeks. In 1933, a member of the University of Alabama medical faculty, Allan Blair, conducted an experiment on himself to document the symptoms of a black widow bite, and to test whether someone can build immunity after being bitten. The effects of the bite were so painful and harsh that Blair failed to complete the experiment and did not follow through with being bitten a second time. In the United States each year, about 2,500 people report being bitten by a black widow, but most do not need medical treatment. Some bites have no venom injected—a "dry" bite. In the United States, a 2012 review by the American Association of Poison Control Centers found no deaths from black widows since 1983. Black widows are not especially aggressive spiders, and they rarely bite humans unless startled or otherwise threatened. Contrary to popular assumptions, most people who are bitten suffer no serious damage, let alone death. Fatal bites were reported in the early 20th century mostly with Latrodectus tredecimguttatus, the Mediterranean black widow. Since the venom is not usually life-threatening, antivenom has been used as pain relief and not to save lives. However, a study demonstrated that standardized pain medication, when combined with either antivenom or a placebo, had similar improvements in pain and resolution of symptoms.
Description
Female widow spiders are typically dark brown or a shiny black in colour when they are full grown, usually exhibiting a red or orange hourglass on the ventral surface (underside) of the abdomen; some may have a pair of red spots or have no marking at all. The male widow spiders often exhibit various red or red and white markings on the dorsal surface (upper side) of the abdomen, ranging from a single stripe to bars or spots, and juveniles are often similar to the male pattern. Females of a few species are paler brown and some have no bright markings. The bodies of black widow spiders range from 3–10 mm (0.12–0.39 in) in size; some females can measure 13 mm (0.51 in) in their body length (not including legs). Including legs, female adult black widows generally measure 25–38 mm (1–1.5 in).
Behaviour
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