Approx. 32, see article
Latrodectus is a genus of spider, in the family Theridiidae, which contains 32 recognized species. The black widow spider is perhaps the best-known member of the genus. The venomous bite of these spider species is considered 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 her bite is particularly harmful to humans; however, Latrodectus bites rarely kill if proper medical treatment is provided.
The prevalence of sexual cannibalism, a behaviour in which the female eats the male after mating, in some species of Latrodectus has inspired the common name "widow spiders".  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; however females of some species only rarely show this behavior, and much of the documented evidence for mate cannibalism has been observed in laboratory cages where the males could not escape.
Females of most Latrodectus species are dark or black in colour usually exhibiting a red or orange hourglass on the ventrum underside or bottom of the abdomen — some may have a pair of red spots or have no marking at all. They often exhibit various red or red and white markings on the dorsal or top side of the abdomen, ranging from a single stripe to bars or spots. Females of a few species are paler browner shades and some have no bright markings. Juveniles and adult male Latrodectus are half the size of the females, and are often grey or brown and usually lighter in colour than females; while they may sometimes have an hourglass marking on their ventral abdomen, it may be yellow or white, not red. Variation in specifics by species and by gender is great.
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 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. 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.
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. The tensile strength of spider silk is comparable to that of steel wire of the same thickness. However, as the density of steel is about six times that of silk, silk is correspondingly stronger than steel wire of the same weight.
Arachnologist Herbert Walter Levi revised the genus Lactrodectus 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.P 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".
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). 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.
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
- Latrodectus sp., Reportedly new species due to behavior, discovered in Martha's Vineyard in 1980 and Tuckernuck Island in 2007.
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
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
- 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 Southern African savannah
South and Eastern Asia, Australia and New Zealand
- Latrodectus elegans, China, Myanmar, Japan
- Latrodectus erythromelas, Sri Lanka
- Latrodectus ex laos, Laos
- Latrodectus hasseltii, Australia
- Latrodectus katipo, New Zealand
Australia and Oceania
- 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
- 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.
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, hyperhidrosis, tachycardia, and muscle spasms. Symptoms usually last for 3–7 days, but may persist for several weeks.
Outpatient care following discharge often consists of a weak-to-moderate strength opioid (e.g. codeine or tramadol, respectively) depending on pain scores, an anti-inflammatory agent (e.g. naproxen, cortisone), and an antispasmodic (e.g. cyclobenzaprine, diazepam) for a few days to a week. If the pain and/or spasms have not resolved by this time, a second medical evaluation is generally advised, and other diagnoses may be considered.
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