Black Widow Spider
Lactrodectus spp.
Class: Arachnida
Order: Araneae
Family: Theridiidae
Size: Black widow spider adult female is about 1/2 in (12-13 mm) long, including the abdomen which is 1/4-3/8 in (7.2-9.6 mm) in diameter; length including legs is 1 1/2 to 1 3/8 in (38-43 mm); male is half that size.
Characteristics: The abdomen is almost spherical. The common name comes from the belief that the female eats the male after mating, which rarely happens in nature.
Color: Females are usually black, sometimes brownish, with 2 reddish triangular markings on her underside that are often joined so they look like an hourglass shape, but are sometimes only a single triangular mark. There are usually red markings above the spinnerets. Males usually have a color pattern on upper surface, a row of red spots with white lines or bars going out to the sides. Eyes: 8, with 2 side pairs almost touching. Last tarsal segment of 4th pair of legs has toothed bristles; claws on all lower leg segments. Young spiders are orange and white, becoming more and more black with age, with markings like males on upper side; also 1 or 2 reddish markings on underneath side.
Geographic Range: Around the world; 5 species in the United States. Lactrodectus hesperus, the western widow spider, is found in the western U.S. and western Canada.
Comparison with other species: Widow spiders are all quite similar to each other, but are distinct from other spiders. (See Characteristics above.)
Habitat: Black widow spiders live in protected places such as woodpiles, under stones and decks, in hollow trees or stumps, rodent burrows, or even in low branches; dry places in barns, sheds, meter boxes, barrels.
Food: Insects. Black widow spiders hang upside down from irregular webs one foot in diameter. The female avoids light and tends to seek prey at night.
Biology: Most black widow spiders overwinter as immatures, becoming adults in spring, dying in late July. In some species females may live for 2 or more years after maturity; and some males up to about 6 months. Development from egg to adult takes nearly a year. Incubation period: 8-30 days. Females lay eggs in closely woven silk sacs 3/8-1/2 inches in diameter that turn from white to pale brown. Spiderlings have first molt within egg sac. 2nd instar spiderlings spin silk threads and "balloon" out on a breeze. There are 4-9 instars in 54-107 days, each instar with a different color and/or pattern.
Invasion: They may enter seldom-used, cluttered parts of garages, basements, crawl spaces.
Damage: Only the female's bite is poisonous. The male does not develop venom sacs into maturity, and does not attack prey as an adult. Spiderlings are poisonous only if eaten during first 18 days, but lose their poison. Adult venom is different, a neurotoxin. Female is not aggressive except after egg laying and when guarding her eggs.
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