black widow, poisonous spider of the genus Latrodectus, found throughout North and South America and common in the SW United States. The name derives from the fact that the female, like those of many other spider species, may eat the male after mating. The adult is black with a red or reddish-orange hourglass-shaped marking on the lower abdominal surface. The female is somewhat less than 1/2 in. (1.3 cm) long, and the male is much smaller. The bite venom is a neurotoxin and may cause a severe reaction with intense local pain that spreads to other parts of the body. Occasional fatal cases, which result from respiratory paralysis, are usually limited to children. The most effective treatment is an antivenin. Black widow spiders are classified in the phylum Arthropoda, class Arachnida, order Araneae, family Theridiidae.

Any of several black spiders in the genus Latrodectus with a venomous bite that is rarely fatal to humans. Black widow species are found worldwide, with three living in North America. In Australia it is called the redback. The females are shiny black, usually with a reddish hourglass-shaped design on the underside of the spherical abdomen and with a body about 1 in. (2.5 cm) long. The black widow preys on insects. The male, about one-fourth the female's size, is often killed and eaten by the female after mating (the source of its name).

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