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.
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