Latrodectus is a genus of spiders in the family Theridiidae, most of which are commonly known as widow spiders. The genus contains 31 recognized species distributed worldwide, including the North American black widows (L. mactans, L. hesperus, and L. variolus), the button spiders of Africa, and the Australian redback spider. Species vary widely in size. In most cases, the females are dark-coloured and readily identifiable by reddish markings on the abdomen, which are often (but not always) hourglass-shaped.
While rarely fatal, or even serious, the venomous bite of these spiders is seen as particularly dangerous because of the neurotoxin latrotoxin, which causes the condition latrodectism, both named after the genus. The female black widow has unusually large venom glands and its bite can be particularly harmful to humans. However, despite the genus’ notoriety, Latrodectus bites are rarely fatal. Only female bites are dangerous to humans.
Widow spiders can be found on every continent of the world except Antarctica. In North America, the black widows commonly known as southern (Latrodectus mactans), western (Latrodectus hesperus), and northern (Latrodectus variolus) can be found in the United States, as well as parts of southern Canada – particularly in the Okanagan Valley of British Columbia, as can the “gray” or “brown widow spiders” (Latrodectus geometricus) and the “red widow spiders” (Latrodectus bishopi). The blue mud dauber (Chalybion californicum) is a wasp that, in the Western United States, is the primary predator of black widow spiders.
The most prevalent species occurring in Australia is commonly called the redback (Latrodectus hasselti).
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, hyperhidrosis, tachycardia, and muscle spasms. Symptoms usually last for 3–7 days, but may persist for several weeks.
Each year, about 2,200 people report being bitten by a black widow, but most recover within 24 hours with medical treatment (male spiders produce the toxins to help with their own hunting, but they make such a diluted version that they are not harmful to most people). Also, many people who are bitten develop few symptoms since the spider may not inject its venom. Black widows are not especially aggressive spiders, and they rarely bite humans unless startled or otherwise threatened.
Contrary to popular belief, 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 likely to be 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.