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Black Widow Spider, common name for any of several related long-legged, smooth-bodied spiders, chiefly inhabiting the Tropics, but also common in the southern United States and found as far north as Canada. They spin irregular webs in crevices and other dark, protected spots. The fully grown female of the familiar North American species is about 1.2 cm (about 0.5 in) long and is jet black, with an hourglass-shaped red mark on the underside of the abdomen. Males are only about half as long and usually have four pairs of red dots along the sides of the abdomen. Males are rarely seen and are harmless. The female may devour the male after mating, thus giving rise to the common name, but this practice is not uncommon among spiders. The female's bite, poisonous to humans, is followed by local pain and swelling, nausea, and difficulty in breathing and is sometimes fatal. The venom, a neurotoxin, generally affects children more severely than adults. The spider, however, is not aggressive and bites humans only defensively.
At least three other species found in the United States are also poisonous to humans: the brown, or gray, widow spider; the red widow spider, with irregular red and yellow markings; and the northern widow spider, with a row of red spots above and two red bars on the underside of the abdomen.
Scientific classification: Black widow spiders make up the genus Latrodectus in the family Theridiidae. The name is applied especially to the familiar North American species Latrodectus mactans. The brown, or gray, widow spider is classified as Latrodectus geometricus, the red widow spider as Latrodectus bishopi, and the northern widow spider as Latrodectus variolus.