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Latrodectus hesperus

By Ashley Minus

Geographic Range

Westerm black widow spiders, Latrodectus hesperus, live in the warmer regions of the United States. This species can be found in all four of the deserts of the American southwest. (Desert USA, 1996; Encyclopedia Britanica, 1999-2000; Vest and Eagle Rock Research, 1999)

Habitat

Black widow spiders live under ledges, rocks, plants, and different types of debris. Webs are usually built near to the ground. Webs are sometimes built inside trash cans, piles of trash, as well as under or around houses. When the weather changes from warm to cold, black widows are most likely to find places inside of homes in which to live or spin their webs. in order to prevent black widows from invaiding a home, home owners should remove materials where these animals can hide. If a person happens to find an egg sac, it should be removed with an object other than one's hand. (Desert USA, 1996; Jones, 1991)

Physical Description

Adult female black widow spiders are usually black and shiny. Black widows are most commonly recognized because of the reddish hourglass shape on the underside of the abdomen. Females measure an average of 3.5 cm in length, and typically have a very round abdomen.

Adult males are half the size of females. Their bodies are smaller, but they have longer legs than the females. Males have yellow and red bands and spots dorsally.

Newly hatched black widows are white, but can sometimes be a yellowish-white. As they get older, they slowly turn black. Mature black widows can vary in the amounts of red and white on their bodies. (Bisacre, et al., 1979; Desert USA, 1996)

  • Sexual Dimorphism
  • female larger
  • sexes colored or patterned differently
  • Average mass
    25 g
    0.88 oz
  • Average length
    female, 3.5 cm; males, 1.25 cm
    in

Reproduction

Adult male black widows wander around in search of a female. During this time, males do not bite or feed. First, a male black widow spins a very small web, and then he places a drop of sperm on the web or the silk. He then takes the sperm into special receptacles on the ends of his pedipalps. Afterwards, he searches for a female so that he can place the sperm into the female's genital opening. After the female and male mate in this way, the female lays several bunches of eggs, which contain about 750 eggs. A single bunch of eggs is suspended in a web so that nothing happens to to the eggs before they hatch. A single egg case is about 1 cm in diameter. The egg sac can either be tan or white, and usually has a paper-like texture. Within a given summer, a female may make between 4 and 9 egg sacs. Incubation lasts about 14 days, and the young spiders are cannabalistic. Only one to twelve spiders from an egg sac actually live to be 30 days old.

After males are born, it takes about 70 days for them to mature, and then they die after about a month to two months. In part, this is because females sometimes eat a male after mating takes place. Females, on the other hand, only take about 90 days to mature. Female black widows usually live up to a year and a half. (Desert USA, 1996; Jones, 1991; Desert USA, 1996; Jones, 1991)

  • Average gestation period
    14 days
  • Average age at sexual or reproductive maturity (female)
    90 days
  • Average age at sexual or reproductive maturity (male)
    70 days

Lifespan/Longevity

Males live only a couple of months after reaching maturity. Females can live up to a year after reaching maturity. (Desert USA, 1996)

  • Average lifespan
    Status: wild
    males, 4 months; females, 1.5 years

Behavior

Western black widow spiders spin webs that do not take on a definite shape or form. The webs vary in location, but more so in appearance. The silk that creates the web is made up of a liquid from the large abdominal glands. The glands open on the surface of the black widow's abdomen at the spinnerets. As the silk comes outside of the abdomen, it solidifies becuase of the stretching. Black widow silk is stronger than any other arachnid's.

These spiders are shy and nocturnal, and usually stay hidden inside of the web unless frightened. The belly is always facing the upward position. If a black widow is hidden inside a shoe, for example, and a person puts his/her foot inside of the shoe, the black widow will most likely bite. Females are not agressive, but will bite if disturbed.

These spiders produce a potent venom to which humans react, although the incidence of death from a black widow bite is actually quite small. (Desert USA, 1996; Jones, 1991; Virtual Science Centre, 1996)

Food Habits

Western black widows are like most arachnids in that they feed primarily on insects. Because of sticky substances in the silk of a web, black widows are able to catch their prey. The web is made of coarse silk.

The diet of L. hesperus includes beetles, cockroaches, and flies. Black widows kill by means of very small, hollow fangs through which they inject a poisionous venom into their prey. Black widows make small punctures all over the body of their prey, and then proceed to suck out the liquid contents. Because the black widows only take food in the form of liquid, the insect is not fully digested. However, digestive juices of the spider are injected into the insect prey, helping to make more of the prey item edible to the spider. Black widows leave behind the external skeleton of prey insects. These empty shells can be seen near the spider's web. A female black widow may occasionally cannibalize males of the species after mating. (Bisacre, et al., 1979; Desert USA, 1996)

  • Animal Foods
  • body fluids
  • insects
  • terrestrial non-insect arthropods

Predation

Black widows are preyed upon by mud-dauber wasps. (Bisacre, et al., 1979; Desert USA, 1996)

  • Known Predators
    • mud dauber wasps

Economic Importance for Humans: Positive

Black widow spiders are not really beneficial to humans except for the fact that they eat insects. Western black widows are often found around human habitations, which makes them a potential predator of pest insects. (Bisacre, et al., 1979)

  • Positive Impacts
  • controls pest population

Economic Importance for Humans: Negative

Black widow spiders can be dangerous animals. They have a poisionous venom that is fifteen times as toxic as a rattlesnake. When a spider bites, only a little bit of the venom is injected into the prey. Rattlesnakes are considered more dangerous becuase when they bite, more of the venom is injected into their victim. However, it is important to note that if a large amount of the venom created by the black widow were to get into a human, that person would most likely die. In spite of this, most bites are not fatal. The bite of the black widow is more likely to be painful, and it can cause internal problems. Such side effects include abdominal pain as well as swelling of the muscles in the feet. The more mild symptoms include dry mouth, increased sweating, and swollen eyelids. If a person younger than 16 or older than 60 is bitten, he/she might have to go to the hospital. Most times death from black widow bites results in people between these age ranges because of lung or heart failure. Healthy people who are bitten usually recover in two to five days with the proper care. When a person is first bitten, soap and water works well to clean the area, and one should also apply a cold compress on the bitten area. (Desert USA, 1996)

  • Negative Impacts
  • injures humans
  • household pest

Conservation Status

Latrodectus hesperus is not an endangered species. These spiders can produce large numbers of offspring, and people do not hunt them. (Desert USA, 1996; Jones, 1991)

Other Comments

The most well known species of the black widow spider is Latrodectus mactans. The latin name means "murderous biting robber." This species occupies most southern areas. Latrodectus variolus is found in extreme parts of southeastern Canada. They live throughout the New England states and parts of Flordia. Latrodectus bishopi is also called a red widow. Brown widows, Latrodectus geometricus, are found in tropical areas. When this spider bites, it is not as severe as the other species of widows. The subspecies Latrodectus mactans tredecimguttatus is also known as a European black widow. Redback spiders, Latrodectus mactans hasselti, are found throughout Australia and some Southeast Asian countries.

When a woman's husband dies, she is called a widow. These spiders receive their common name, "widows" because females sometimes kill their mates. (Jones, 1991; Vest and Eagle Rock Research, 1999)

Contributors

Nancy Shefferly (editor), Animal Diversity Web.

Ashley Minus (author), University of Michigan-Ann Arbor, Phil Myers (editor), Museum of Zoology, University of Michigan-Ann Arbor.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

agricultural

living in landscapes dominated by human agriculture.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.

diurnal
  1. active during the day, 2. lasting for one day.
ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

fertilization

union of egg and spermatozoan

insectivore

An animal that eats mainly insects or spiders.

internal fertilization

fertilization takes place within the female's body

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

poisonous

an animal which has a substance capable of killing, injuring, or impairing other animals through its chemical action (for example, the skin of poison dart frogs).

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

suburban

living in residential areas on the outskirts of large cities or towns.

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

urban

living in cities and large towns, landscapes dominated by human structures and activity.

venomous

an animal which has an organ capable of injecting a poisonous substance into a wound (for example, scorpions, jellyfish, and rattlesnakes).

References

Bisacre, M., R. Carlisle, D. Robertson, J. Ruck. 1979. The Ilustrated Encyclopedia of Plants and Animals. New York: Exter Books.

Desert USA, 1996. "Black Widow Spiders/Latrodectus hesperus" (On-line). Accessed March 21, 2001 at http://www.desertusa.com/july97/du_bwindow.html.

Encyclopedia Britanica, 1999-2000. "Black Widow" (On-line). Accessed March 21, 2001 at http://www.britanica.com/b.../0,5716,15726+1+15522,00.html?query=latrodectus%20hesperu.

Jones, S. 1991. "Ohio State University Extension Factsheet/ Black Widow Spider" (On-line). Accessed February 15, 2005 at http://ohioline.osu.edu/hyg-fact/2000/2061A.html.

Vest, D., Eagle Rock Research. 1999. "The Widow Spiders" (On-line). Accessed March 23, 2001 at http://hobospider.org/widows.html.

Virtual Science Centre, 1996. "Latrodectus Hesperus" (On-line). Accessed March 21, 2001 at http://www.sci-ctr.edu.sg/events/exhibit/megabugs/blackwid.html.

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