Latrodectus is a broadly distributed genus of informally called the widow spiders,
Diversity
However, the diversity of species is much greater. A member of the family
Theridiidae, this genus contains 34
species,
which include several North American "black widows" (southern black widow
Latrodectus mactans, western black widow
Latrodectus hesperus, and northern black widow
Latrodectus variolus). Besides these, North America also has the red widow
Latrodectus bishopi and the brown widow
Latrodectus geometricus, which, in addition to North America, has a much wider geographic distribution.
Elsewhere, others include the European black widow ( Latrodectus tredecimguttatus), the Australian redback spider ( Latrodectus hasseltii) and the closely related New Zealand katipō ( Latrodectus katipo), several different species in Southern Africa that can be called , and the South American black-widow spiders ( Latrodectus corallinus and Latrodectus curacaviensis). Species vary widely in size. In most cases, the females are dark-coloured and can be readily identified by reddish markings on the central underside (ventral) abdomen, which are often hourglass-shaped.
Venom
These small spiders have an unusually potent venom containing the
neurotoxin latrotoxin, which causes the condition
latrodectism, both named after the genus. Female widow spiders have unusually large venom glands, and their
spider bite can be particularly harmful to large vertebrates, including humans. However, despite their notoriety,
Latrodectus bites rarely cause death or produce serious complications. Only the bites of the females are dangerous to humans.
Due to the presence of latrotoxin in their venom, black widow bites are potentially dangerous and may result in systemic effects (latrodectism) including severe muscle pain, abdominal cramps, diaphoresis, tachycardia, and muscle spasms.
In the United States each year, about 2,500
Contrary to popular assumptions, most people who are bitten suffer no serious damage, let alone death. Fatal bites were reported in the early 20th century mostly with Latrodectus tredecimguttatus, the Mediterranean black widow.
Since the venom is not usually life-threatening, antivenom has been used as Analgesic and not to save lives.
Description
Female widow spiders are typically dark brown or a shiny black in colour when they are full grown, usually exhibiting a red or orange hourglass on the ventral surface (underside) of the abdomen; some may have a pair of red spots or have no marking at all. The male widow spiders often exhibit various red or red and white markings on the dorsal surface (upper side) of the abdomen, ranging from a single stripe to bars or spots, and juveniles are often similar to the male pattern. Females of a few species are paler brown and some have no bright markings. The bodies of black widow spiders range from in size; some females can measure in their body length (not including legs).
Including legs, female adult black widows generally measure .
Behaviour
The prevalence of sexual cannibalism, a behaviour in which the female eats the male after mating, has inspired the
common name "widow spiders".
This behaviour may promote the survival odds of the
offspring;
however, females of some species only rarely show this behaviour, and much of the documented evidence for sexual cannibalism has been observed in laboratory cages where the males could not escape. Male black widow spiders tend to select their mates by determining if the female has eaten already to avoid being eaten themselves. They are able to tell if the female has fed by sensing chemicals in the web.
is referred to as an "opportunistic cannibal" because in dire situations it will resort to cannibalism.
In addition to sexual cannibalism,
Latrodectus hesperus are also known to engage in sibling cannibalism.
Like other members of the Theridiidae, widow spiders construct a web of irregular, tangled, sticky silken fibres. Black widow spiders prefer to nest near the ground in dark and undisturbed areas, usually in small holes produced by animals, or around construction openings or woodpiles. Indoor nests are in dark, undisturbed places such as under desks or furniture or in a basement.
As with 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. When a widow spider is trapped, it is unlikely to bite, preferring to play dead or flick silk at the potential threat; bites occur when they cannot escape.
The ultimate tensile strength and other physical properties of Latrodectus hesperus (western black widow) silk are 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 1,000 MPa. The ultimate strength reported in a previous study for Trichonephila edulis was 1,290 ± 160 MPa.
Spiders of the genus Steatoda (also of the Theridiidae) are often mistaken for widow spiders, and are known as "false widow spiders"; while their bite can be painful, they are significantly less harmful to humans.
Brown widow spider Latrodectus geometricus underside.jpg|Ventral side of a L. geometricus displaying the hourglass marking
Brown widow spider Latrodectus geometricus backside.jpg|Dorsal side of a L. geometricus in Colorado, United States
Black Widow spider, Female.jpg| L. hesperus profile
Taxonomy
The genus
Latrodectus was erected by Charles Athanase Walckenaer in 1805, for the species
Latrodectus tredecimguttatus and
Latrodectus mactans.
Arachnologist Herbert Walter Levi revised the genus in 1959, studying the female sexual organs and noting their similarity across described species. He concluded the colour variations were variable across the world and were not sufficient to warrant species status, and reclassified the redback and several other species as subspecies of the black widow spider.
Levi also noted that study of the genus had been contentious; in 1902, both F. O. Pickard-Cambridge and Friedrich Dahl had revised the genus, with each criticising the other. Cambridge questioned Dahl's separating species on what he considered minor anatomical details, and the latter dismissed the former as an "ignoramus".
Species
Latrodectus cinctus 312114475 563483113.jpg|Female L. cinctus from South Africa
Latrodectus.elegans.male.-.tanikawa.jpg|Male L. elegans from Japan
blackwidow macro.jpg| L. hesperus hair and markings
Blackwidow eggsac silk.jpg| L. hesperus with egg sac
Latrodectus tredecimguttatus female.jpg| L. tredecimguttatus (female swollen with eggs)
, this genus includes 35 species:
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Latrodectus antheratus (Badcock, 1932) – Paraguay, Argentina
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Latrodectus apicalis Butler, 1877 – Galapagos
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Latrodectus bishopi Kaston, 1938 – United States
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Latrodectus cinctus John Blackwall, 1865 – Cape Verde, Africa, Oman, Kuwait, Iraq, Iran
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Latrodectus corallinus Abalos, 1980 – Brazil, Argentina
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Latrodectus curacaviensis (Müller, 1776) – Caribbean, South America
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Latrodectus dahli Levi, 1959 – North Africa, Cyprus, Turkey, Azerbaijan, Kazakhstan, Middle East, Iran, Central Asia
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Latrodectus diaguita Carcavallo, 1960 – Argentina
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Latrodectus elegans Tamerlan Thorell, 1898 – India, Nepal, Myanmar, China, Japan
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Latrodectus erythromelas Schmidt & Klaas, 1991 – India, Sri Lanka
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Latrodectus garbae Rueda & Realpe, 2021 – Colombia
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Latrodectus geometricus C. L. Koch, 1841 – Africa. Introduced to both Americas, St. Helena, Middle East, Pakistan, India, China, Japan, Thailand, Malaysia, Papua New Guinea, Australia, Hawaii
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Latrodectus hasselti Thorell, 1870 – Southeast Asia to Australia. Introduced to Pakistan, India, Japan, New Zealand
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Latrodectus hesperus Chamberlin & Ivie, 1935 – Canada, United States, Mexico. Introduced to Israel, Korea
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Latrodectus hurtadoi Rueda & Realpe, 2021 – Colombia
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Latrodectus hystrix Simon, 1890 – Yemen (mainland, Socotra)
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Latrodectus indistinctus O. Pickard-Cambridge, 1904 – Namibia, South Africa
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Latrodectus karrooensis Smithers, 1944 – South Africa
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Latrodectus katipo Powell, 1871 – New Zealand
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Latrodectus lilianae Melic, 2000 – Spain
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Latrodectus mactans (Fabricius, 1775) – Probably native to North America only. Introduced to South America, Asia
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Latrodectus menavodi Auguste Vinson, 1863 – Madagascar, Comoros, Seychelles (Aldabra)
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Latrodectus mirabilis (Holmberg, 1876) – Brazil, Argentina
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Latrodectus obscurior Dahl, 1902 – Cape Verde, Madagascar
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Latrodectus occidentalis Valdez-Mondragón, 2023 – Mexico
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Latrodectus pallidus O. Pickard-Cambridge, 1872 – Cape Verde to Libya, South Africa, Turkey, Georgia, Kazakhstan, Iran, Central Asia
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Latrodectus quartus Abalos, 1980 – Argentina
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Latrodectus renivulvatus Dahl, 1902 – St. Helena, Africa, Yemen, Saudi Arabia, Iraq
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Latrodectus revivensis Shulov, 1948 – Israel, Iran, Canary Islands?
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Latrodectus rhodesiensis Mackay, 1973 – Namibia, Botswana, Zimbabwe, South Africa
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Latrodectus thoracicus Hercule Nicolet, 1849 – Chile
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Latrodectus tredecimguttatus (Rossi, 1790) – Mediterranean, Ukraine, Caucasus, Russia (Europe to South Siberia), Kazakhstan, Middle East, Iran, Central Asia, China (type species)
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Latrodectus umbukwane B. M. O. G. Wright, C. D. Wright, Lyle & Engelbrecht, 2019 – South Africa
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Latrodectus variegatus Nicolet, 1849 – Chile, Argentina
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Latrodectus variolus Walckenaer, 1837 – Canada, United States
Nomen dubium
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L. dotatus C. L. Koch, 1841
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L. limacidus Cantor, 1842
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L. pallidus Caporiacco, 1933
Distribution
Widow spiders are 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) are found in the United States, equally in western
Mexico (
Latrodectus occidentalis), as well as parts of southern Canada – particularly in the
Okanagan Valley of British Columbia, as are the "grey" or "brown widow spiders" (
Latrodectus geometricus) and the "red widow spiders" (
Latrodectus bishopi).
The most prevalent species occurring in eastern Asia and Australia is commonly called the redback ( Latrodectus hasselti).
They are often confused with spiders in the genus Steatoda, known as false widow spiders, due to their similar appearance.
See also
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List of medically significant spider bites
Further reading
External links
