Latrodectus

Latrodectus
Black Widow spider
Scientific classification
Domain:	Eukarya
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Arachnida
Order:	Araneae
Family:	Theridiidae
Genus:	Latrodectus Walckenaer, 1805
Species
Approx. 31, see article

Latrodectus is a genus, in the family Theridiidae, that contains approximately 31 recognized species of venomous spider. The common name widow spiders is applied to members of the genus even though the males are not usually eaten after mating and can even go on to fertilize other females.^[1] The Black Widow spiders are well-known members of the genus. The female black widow's venom is particularly harmful to humans (males almost never bite humans). The injection of venom from these species is a comparatively dangerous or lethal spider bite, resulting in the condition Latrodectism which is named for the genus.

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

Along with the southern, western and northern black widows (Latrodectus mactans, Latrodectus hesperus and Latrodectus variolus) with the famed red hourglass, the gray or brown widow spiders (Latrodectus geometricus), and the red widow spiders (Latrodectus bishopi) (Preston-Malfham, 1998) are also found in the United States. But there are widow spiders on every continent of the world except for Antarctica. The single species occurring in Australia, Latrodectus hasselti, has Redback as one common name; African species of this genus are sometimes known as button spiders.

In common with other members of the Theridiidae family, the widow spiders construct a cobweb, i.e., an irregular tangle of sticky silken fibers. The black widow spider very frequently hangs upside down near the center of its web and waits there for insects to blunder in and get stuck. Then, before the insect can extricate itself, the spider rushes over to bite it and swathe it in a silken shroud. If the spider feels threatened it will normally let itself down to the ground on a safety line of silk just as fast as it can. As with other web-weavers, these spiders have very poor eyesight and depend mostly on vibrations reaching them through their webs to orient themselves to prey or warn them of larger animals that could injure or kill them. They are not aggressive, and most injuries to humans are due to defensive bites delivered when a spider gets unintentionally squeezed or pinched somehow. It is possible that some bites may result when a spider mistakes a finger thrust into its web for its normal prey, 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

Silk from Latrodectus spiders is reputed to be particularly strong compared with the silk of other spiders.^[2][3] The results of a study by Blackledge, et al. do not confirm this.^[4]

The ultimate 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 ultimate 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 1290 MPa ± 160 MPa^[5]

Based on the Blackledge study the ultimate strength of spider silk is greater than some steels,^[6] however it is substantially less than 1085A steel wire (1551 MPa to 2758 MPa).^[7] If weight is taken into account, spider silk is much stronger than any kind of steel. The density of spider silk (1.3 g/cm³) is much less than steel (7.8 g/cm³),^[8] so the Latrodectus hesperus silk tested in the Blackledge study was about three times stronger than 1085A steel wire for a strand of the same weight.

See the "Spider silk" article for more details about spider silk in general and the table of Typical tensile strengths in the "Tensile strength" article for a comparison of the strength of spider silk with the strengths of other materials.

The silk of the Latrodectus genus is larger in diameter than the silk of other spiders, and, as with all spider silk, it is stronger than an equal-sized filament of steel. Because of these excellent characteristics it was in great demand to use in the construction of reticles for rifle sights at least as late as World War I.^{[citation needed]}

Species

The southern black widows, as well as the closely related western and northern species which were previously considered the same species, have 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 which are found in multiple regions are listed in their predominant native habitat.

North American widow spiders

Latrodectus hesperus with egg sac

Ventral side of a Latrodectus geometricus displaying the hourglass marking

The following widow spiders are indigenous to North America:

• Latrodectus bishopi, the red widow, Florida, USA
• Latrodectus geometricus, the brown widow, from northeastern US to Texas with predominance in the southeast. Now found as far west as California.
• Latrodectus hesperus, the Western black widow, western Canada, 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.

Widows of Central and South America

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 variegatus, Chile and Argentina

Widows of Europe, North Africa, the Middle East, and western Asia

L. tredecimguttatus female

The following widows 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.
• 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.

Widows of sub-Saharan Africa and Madagascar

• 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 sound 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

male L. elegans from Japan

Widows of South, East, and Southeast Asia

• Latrodectus elegans, China, Myanmar, Japan
• Latrodectus erythromelas, Sri Lanka

Australia and Oceania

• Latrodectus hasselti, 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

Widows found worldwide

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

References

• Discovery Books; 2000; Insects and Spiders; St. Remy Media Inc.; New York; 35
• Freeman, Scott; 2003; Biological Science; Prentice-Hall
• Hillyard, Paul; 1994; the Book of Spiders; Random House, Inc.; New York; 47–50
• Hillyard, Paul; 1994b; The Book of the Spider; Avon Books; New York; 22–25
• Martin, Louise; 1988; Black Widow Spiders; Rourke Enterprises, Inc.; 18–20
• Preston-Malfham, Ken; 1998; Spiders; Chartwell Books; Edison; New Jersey; 40.
• "Arthropod," Microsoft Encarta Online Encyclopedia 2004
• Abalos, J.W. (1962). The Egg-sac in the Identification of Species of Latrodectus (Black-Widow Spiders). Psyche 69:268–270. PDF
• Levi, H.W. & McCrone, J.D. (1964). North American Widow Spiders of the Latrodectus curacaviensis Group (Araneae, Theridiidae. Psyche 71:12-27).PDF

External links

• Media related to Latrodectus at Wikimedia Commons
• Information related to Latrodectus from Wikispecies.
• Tree of Life: Latrodectus
• Black Widow Spider: Large format photographs and information
• Description of crossing experiments between various Latrodectus species