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Widow Spiders - the Genus Latrodectus

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Female black widows have shiny, spherical abdomens, usually marked with a red hourglass shape.

Female black widows have shiny, spherical abdomens, usually marked with a red hourglass shape on the ventral side.

Photo: Flickr user peasap (CC license)

The famous black widow is just one of the venomous widow spiders living throughout the world. Bites from female widow spiders are medically significant, and may require treatment with an antivenin. Widow spiders do not attack humans unprovoked, but will bite when touched or threatened.

Description:

Most people will recognize widow spiders by the hourglass markings on the underside of their abdomens. The hourglass mark is not present in all Latrodectus species, however. Females take longer to reach maturity and molt more times than males, resulting in darker, shinier coloration. Males, by contrast, remain lighter and duller.

Female widow spiders are larger than their male counterparts; the body of a mature female measures about one half inch in length. Female Latrodectus spiders have a spherical abdomen and long, thin legs.

Widow spiders belong to the cobweb spider family. They spin irregular, sticky webs to catch insects. Like other cobweb spiders, widows possess a row of bristles on their hind legs. This "comb-foot" helps the widow spiders wrap her insect victims in silk.

Classification:

Kingdom - Animalia
Phylum - Arthropoda
Class - Arachnida
Order - Araneae
Family - Theridiidae
Genus - Latrodectus

Diet:

Widow spiders feed on insects, which they capture in their webs. When an insect touches the web, the widow spider senses the vibration and immediately rushes to capture the prey.

Life Cycle:

A female widow spider lays several hundred eggs, wraps them in a silken egg case, and suspends it from her web. She keeps watch over the eggs, and will defend them vigorously during the month of their development. During her lifetime, the female may produce up to 15 egg sacs, with as many as 900 eggs in each one.

The newly hatched spiderlings are cannibals, and will quickly devour one another until only a dozen or so offspring remain. To disperse, the young spiders parachute down from the web on silken threads. They continue to molt and grow for two or three months, depending on their sex.

Most females live about nine months, but the male lifespan is considerably shorter. Widow spiders, especially black widows, have earned a reputation for sexual cannibalism – the female eats the male after mating. While this does occasionally occur, it is more myth than fact. Not all males get eaten by their partners.

Special Adaptations and Defenses:

Widow spiders do not have good eyesight. Instead, they rely on their sensitivity to vibrations to detect prey or potential threats. For this reason, it's never a good idea to touch the web of a widow spider. A careless poke with a finger is likely to attract a speedy bite from the resident widow.

Mature female Latrodectus spiders inject a neurotoxic venom when they bite. In prey, the venom takes affect fairly quickly; the spider holds the insect firmly until it stops moving. Once the prey is immobilized, the widow injects it with digestive enzymes that begin to liquefy the meal.

Though widow spiders are not aggressive, they will bite defensively if touched. In humans, the venom causes latrodectism, a medical syndrome that requires treatment. Within a few minutes, a bite victim will feel localized pain at the site. Symptoms of a widow spider bite include sweating, rigid abdominal muscles, hypertension, and swelling of the lymph nodes.

Habitat:

Widow spiders stay outdoors, for the most part. They live in crevices or recesses within rock piles, logs, embankments, or outbuildings like sheds or barns.

Range

Widow spiders live on all continents except Antarctica. Five species of Latrodectus spiders occur in the U.S.: southern black widow (L. mactans), western black widow (L. Hesperus), northern black widow (L. variolus), red widow (L. bishopi), and brown widow (L. geometricus). Worldwide, about 31 species belong to this genus.

Other Common Names

Button spiders

Sources

  1. About.com
  2. Education
  3. Insects
  4. Spiders
  5. Widow Spiders - Learn About Widow Spiders, Genus Latrodectus

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