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Black Widow Spiders

Latrodectus hesperus

The female black widow spider is considered the most venomous spider in North America. The venom of the black widow spider is 15 times as toxic as the venom of the prairie rattlesnake.

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Only the female black widow is dangerous to humans; males and juveniles are harmless. The female black widow will, on occasion, kill and eat the male after mating. More than 35,000 species of spiders occur worldwide. About 3,400 species of spiders in 64 families are found in North America.

Photo of Black Widow Spider

Description/Identification

  • Female black widow spiders are about .5 inch long, to about 1.5 inches long when the legs are spread.
  • Males are about half the female's size, with smaller bodies and longer legs.
    Both females and males have shiny, globular abdomens and are mostly black, and sometimes brown.
  • Females usually have a reddish hourglass shape on the underside of their abdomens. In some species the females have a series of red spots and two crosswise bars on the underbelly.
  • Male black widows frequently have yellow and red bands and spots over their backs, as do both sexes of black widows in their immature stages.
  • Newly hatched spiderlings are predominately white or yellowish-white, gradually acquiring more black and varying amounts of red and white with each molt.
  • Juveniles of both sexes resemble the male and are harmless to humans.

Vital Stats and Information

Genus: Latrodectus
Phylum:
Arthropoda
Sub Phylum:
Chelicerata
Class:
Arachnida
Order:
Araneae
Sub Order:
Labidognatha
Family:
Theridiidae
Species:
6
Weight: 1 gram.
Length: 1-1.5"
Span: 1-3"
Sexual Maturity: 70-90 days.
Mating Season: Spring
Incubation: 14-30 days
No. of Eggs: 250-700/sac
Birth Interval: 4 to 9 egg sacs/summer
Lifespan: up to 3 years
Typical Diet: insects

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Geography – Range

Black widow spiders inhabit most of the warmer regions of the world to a latitude of about 45 degrees N and S. They occur throughout all four deserts of the American Southwest.

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Related Species

Widow spiders (genus Latrodectus) are the best known and largest of the cobweb weavers (family Theridiidae). All widows are venomous, though not all can cause injury to humans. Black widows comprise about six species out of about 2,000 in the large Theridiidae family.

The female black widow spider, though it is the most venomous spider in North America, seldom causes death as it injects a very small amount of poison when it bites. Reports indicate human mortality at well less than 1% from black widow spider bites.

Anyone bitten by a black widow spider should seek medical care; an attempt should be made to catch the spider for identification.

  • The species L. hesperus is common in the western US and is the black widow found in the North American deserts. L. mactans is the black widow common in the eastern and central US. The brown widow spider, L. geometricus, is also found in the US.
  • L. mactans is found in most warm areas of the world.
  • L. geometricus is found in Africa and the US.
  • L. curacaviensis is found in the Americas.
  • L. hystrix, L. dahli, and L. pallidus are found in southern Europe, northern Africa, and southwestern Asia.


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Comparisons

The brown widow spider (Latrodectus geometricus) is not as dangerous as some other widow spiders because the brown widow spider is less likely to bite someone, and injects less poison. Still, it is a venomous creature, and must be treated with respect. It is found throughout the world's tropical regions, and has been introduced into southern Texas, central and southern Florida, and has now also been found in southern California.

Hundreds of Australians are bitten every year by the red-back spider (Latrodectus hasselti), a close relative of the black widow spider. The red-back is found in all parts of Australia except in the hottest deserts and on the coldest mountains.

Behavior

Both western and eastern black widows spin webs that lack shape and form. Their webs are erratic in appearance, and the silk is stronger than almost all other arachnids. The webs are rough and sticky. The black widow spider is shy and nocturnal in habit, usually staying hidden in her web, hanging belly upward. Although not aggressive, she may rush out and bite when her web is disturbed.

Habitat

Black widows are found on the underside of ledges, rocks, plants and debris, wherever a web can be strung. Cold weather or drought may drive these spiders into buildings.

Food & Hunting

Like most arachnids, the black widow preys on insects. Prey caught in the web include a variety of insects (cockroaches and beetles) and other arthropods. After ensnaring its prey in the web, the black widow makes small punctures in the victim's body and sucks out the liquid contents. The black widow is preyed upon by mud-dauber wasps.


Black widow spider

Breeding

Adult male black widows wander in search of females but do not bite. Females may occasionally kill and eat a male after mating but this is more the exception than the rule.

Black Widow SpiderThe female lays several batches of eggs, containing up to 750 eggs each, in one summer. The egg case, about .5 inch in diameter, is suspended in the web. It is white to tan in color and has a paper-like texture. There may be four to nine egg sacs produced during a summer. Young black widows are colored orange and white when they emerge 1 to 4 weeks later. Normally, only one to twelve young survive after the egg incubation period of 14 to 30 days, due to cannibalism by their fellow spiderlings.

The female black widow may live for more than a year and a half. Growth requires two to four months, depending on availability of prey. During this time females molt six to eight times and males three to six times. Females mature about 90 days after egg sac emergence and live another 6 months to a year. Males mature about 70 days after emergence and live only another month or two.

Jim Bremner DesertUSA


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DesertUSA Video Productions

The Black Widow SpiderView Video about The Black Widow Spider. The female black widow spider is the most venomous spider in North America, but it seldom causes death to humans, because it only injects a very small amount of poison when it bites. Click here to view video.

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