Latrodectus is a genus of spider, in the family Theridiidae, which contains 32 recognized species. The common name widow spiders is sometimes applied to members of the genus due to a behaviour seen in some species in which the female eats the male after mating.^[1] The black widow spider is perhaps the best-known member of the genus. Its bite is dangerous because of the neurotoxin latrotoxin, which causes the condition latrodectism, both named for the genus. The female black widow has unusually large venom glands and its bite is particularly harmful to humans; however, Latrodectus bites rarely kill humans if proper medical treatment is provided.

The prevalence of sexual cannibalism in some species of Latrodectus has inspired the common name "black widow spider". The female's venom is at least three times more potent than that of the males, making a male's self-defense bite ineffective. Research at the University of Hamburg in Germany suggests this ultimate sacrifice strategy has evolved to promote the survival odds of the offspring;^[2] however, contrary to popular belief, females of some species rarely eat their partners after mating and much of the documented evidence for mate cannibalism has taken place in laboratory cages where the males could not escape.^[3]

Description [edit]

Not all adult black widows exhibit the red hourglass on the ventrum underside or top of the abdomen — some may have a pair of red spots or have no marking at all. Female black widows often exhibit various red markings on the dorsal or top side of the abdomen, commonly two red spots. However, black widow young are believed to have at least some sort of marking on their abdomens. Adult male black widows are half the size of the females, and are usually gray or brown rather than black and red; while they may sometimes have an hourglass marking on their ventral abdomen, it is usually yellow or white, not red. Variation in specifics by species and by gender is great; any spider exhibiting a red hourglass or a pair of large red round spots on the ventral abdomen with an otherwise black shiny body is an adult female black widow. The bright red hourglass and spots are never located on the dorsum, which is the more visible aspect; the identifying features are on the underside, anatomically known as ventrum; i.e., the spider must be lying on its back to reveal the markings.

Spiders of the genus Steatoda (also of the Theridiidae family) are often mistaken for widow spiders, and are known as "false widow spiders"; they are significantly less harmful to humans.

In common with other members of the Theridiidae family, the widow spiders construct a web of irregular, tangled, sticky silken fibers. The spider very frequently hangs upside down near the center of its web and waits for insects to blunder in and get stuck. Then, before the insect can extricate itself, the spider rushes over to bite it and wrap it in silk. If the spider perceives a threat, it will quickly let itself down to the ground on a safety line of silk. As other web-weavers, these spiders have very poor eyesight and depend on vibrations reaching them through their webs to find trapped prey or warn them of larger threats. While some species are more aggressive, most are not; many injuries to humans are due to defensive bites delivered when a spider gets unintentionally squeezed or pinched. Some bites are thought to result from a spider mistaking a finger thrust into its web for its normal prey,^{[citation needed]} or in cases where a female is protecting an egg sac, but ordinarily intrusion by any large creature will cause these spiders to flee.

Strength of Latrodectus silk [edit]

Silk from L. hesperus spiders is reputed to be particularly strong compared with the silk of other spiders.^[4][5] However, the results of a study show that this is not the case.^[6]

The ultimate tensile strength and other physical properties of Latrodectus hesperus (western black widow) silk were found to be similar to the properties of silk from orb-weaving spiders that had been tested in other studies. The tensile strength for the three kinds of silk measured in the Blackledge study was about 1000 MPa. The ultimate strength reported in a previous study for Nephila edulis was b1290 MPa ± 160 MPa.^[7] The tensile strength of spider silk is comparable to that of steel wire of the same thickness.^[8] However, as the density of steel is about six times that of silk,^[9] silk is correspondingly stronger than steel wire of the same weight.

Species [edit]

L. hesperus hair and markings

L. hesperus Profile

The southern black widow, as well as the closely related western and northern species which were previously considered the same species, has a prominent red hourglass figure on the underside of its abdomen. Many of the other widow spiders have red patterns on a glossy black or dark background, which serve as a warning. Spiders found in multiple regions are listed in their predominant native habitat.

Widow spiders can be found on every continent of the world except Antarctica. In North America, the black widows commonly known as southern (Latrodectus mactans), western (Latrodectus hesperus), and northern (Latrodectus variolus) can be found in the United States, as can the "gray" or "brown widow spiders" (Latrodectus geometricus) and the "red widow spiders" (Latrodectus bishopi).^[10] The most prevalent species occurring in Australia is commonly called the redback (Latrodectus hasselti). African species of this genus are sometimes known as button spiders.

Americas [edit]

L. hesperus with egg sac

Ventral side of a L. geometricus displaying the hourglass marking

The following widow spiders are indigenous to North America:

• Latrodectus bishopi, the red widow, Florida, USA
• Latrodectus hesperus, the western black widow, western Canada, the Pacific Northwest of the United States, and Mexico.
• Latrodectus mactans, the black widow spider (sometimes called the southern black widow), warm regions of the USA
• Latrodectus variolus, the northern black widow, from the extreme southeastern part of Canada and south to northern Florida, with frequency higher in the northern part of this range

The following are indigenous to Central and South America:

• Latrodectus antheratus, Paraguay, Argentina
• Latrodectus apicalis, Galapagos Islands
• Latrodectus corallinus, Argentina
• Latrodectus curacaviensis, Lesser Antilles, South America
• Latrodectus diaguita, Argentina
• Latrodectus mirabilis, Argentina
• Latrodectus quartus, Argentina
• Latrodectus thoracicus, Chile
• Latrodectus variegatus, Chile and Argentina

Europe, North Africa, the Middle East, and western Asia [edit]

L. tredecimguttatus female

The following widows are indigenous to the Mediterranean region, as well as in western Asia:

• Latrodectus dahli, Middle East to central Asia
• Latrodectus hystrix, Yemen, Socotra
• Latrodectus lilianae, Iberian Peninsula
• Latrodectus pallidus, the white widow or white steppe spider, North Africa, the Middle East, Russia, Iran, Cape Verde
• Latrodectus revivensis, Israel, Palestine
• Latrodectus tredecimguttatus, the Mediterranean black widow or European black widow, Mediterranean area, central Asia, Kazakhstan, also reported in China, some specimens are reported as L. lugubris

Sub-Saharan Africa and Madagascar [edit]

• Latrodectus cinctus, a black button spider found in southern Africa, Cape Verde and Kuwait
• Latrodectus indistinctus, a black button spider found in South Africa and Namibia
• Latrodectus karrooensis, a black button spider found in S. Africa
• Latrodectus menavodi, found in Madagascar
• Latrodectus obscurior, found in Cape Verde and Madagascar.
• Latrodectus renivulvatus, a black button spider found in Africa, Saudi Arabia and Yemen
• Latrodectus rhodesiensis, a brown button spider found in Zimbabwe
• Latrodectus geometricus, a brown button spider found in the southern African savanah

Male L. elegans from Japan

South, East, and Southeast Asia [edit]

• Latrodectus elegans, China, Myanmar, Japan
• Latrodectus erythromelas, Sri Lanka
• Latrodectus ex laos, Laos

Australia and Oceania [edit]

• Latrodectus hasseltii, the redback spider, native to Australia, also found in Southeast Asia and New Zealand, imported into both regions
• Latrodectus katipo, the red katipo, found in New Zealand

Worldwide [edit]

• Latrodectus geometricus, the brown widow, grey widow, or brown button spider is found in Africa, USA, South America, and Australia. It is unclear where this spider originated; however, it has been discovered in many warm, cosmopolitan locales.

See also [edit]

• List of spiders associated with cutaneous reactions

References [edit]

• Insects and Spiders. New York: St. Remy Media Inc. / Discovery Books. 2000. p. 35. 
• Freeman, Scott (2003). Biological Science. Prentice-Hall. 
• Hillyard, Paul (1994). The Book of Spiders. New York: Random House, Inc. pp. 47–50. 
• Hillyard, Paul (1994). The Book of the Spiders. New York: Avon Books. pp. 22–35. 
• Martin, Louise (1988). Black Widow Spiders. Rourke Enterprises, Inc. pp. 18–20. 
• Preston-Malfham, Ken (1998). Spiders. Edison, New Jersey: Chartwell Books. p. 40. 
• "Arthropod". Microsoft Encarta Online Encyclopedia. 2004. 
• Abalos, J.W. (1962). "The egg-sac in the Identification of Species of Latrodectus (Black-Widow Spiders)". ^{[dead link]}
• Levi, H.W. & McCrone, J.D (1964). "North American Widow Spiders of the Latrodectus curacaviensis Group". ^{[dead link]}