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. 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 Latrodectus females has inspired the common name "black widow spider". The females frequently eat their male partners after mating. 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.
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.
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, 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 L. hesperus spiders is reputed to be particularly strong compared with the silk of other spiders.However, the results of a study show that this is not the case.
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.
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
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
- In Romania, for the first time Latrodectus were discovered at Sulina in 1961 on a small island of the Razim lake in 1971,when after many years, Latrodectus specie were believed to have dissapeared from Romania .Nowadays the black Latrodectus specie can be found in Dobrogea area and lately in Galati or Constanta were few cases of bitten people has been reported.(the south east area of Romania)
Europe, North Africa, the Middle East, and western Asia
- 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
South, East, and Southeast Asia
- Latrodectus elegans, China, Myanmar, Japan
- Latrodectus erythromelas, Sri Lanka
- Latrodectus ex laos, Laos
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
Bites in humans
Black Widow spider bites rarely cause significant morbidity, and deaths are even more rare.] A significant proportion of bites will not result in envenomation or any symptoms developing. Hundreds or even thousands of people are thought to be bitten each year across the world, although only about 20% of bite victims require treatment.Children, the elderly, or those with serious medical conditions are at much higher risk of severe side effects and death resulting from a bite. No deaths have been reported since the introduction of antivenom in 1956.
In an Australian study of 750 emergency hospital admissions for spider bites where the spider was definitively identified, 56 were from redbacks. Of these, 37 had significant pain lasting over 24 hours, but only six were treated with the antivenom.
The larger female spider is responsible for almost all cases of redback spider bites in humans. The smaller male spider was thought to be unable to envenomate a human, although male bites have been reported. The rarity of male bite reports is probably due to the male's smaller size and proportionally smaller fangs, rather than the male being incapable of biting or lacking venom of potency similar to the female's. Cases have shown the male bite usually only produces short-lived, mild pain.
Bites from Black Widow spiders produce a syndrome known as latrodectism, with symptoms similar to bites from other Latrodectus spiders. The syndrome is generally characterised by extreme pain and severe swelling. The bite may be painful from the start, but sometimes only feels like a pinprick or mild burning sensation. Within an hour, victims generally develop more severe local pain with local swelling and sometimes piloerection (goosebumps). Pain, swelling and redness spread proximally from the site. Systemic envenoming is heralded by swollen or tender regional lymph nodes; associated features include malaise, nausea, vomiting, abdominal or chest pain, generalised sweating, headache, fever, hypertension and tremor. Rare complications include seizure, coma, pulmonary edema, respiratory failure or localised skin infection. Severe pain can persist for over 24 hours after being bitten.
Medical advice is recommended after being bitten by a Black Widow spider. Usually, this requires observation in or near a medical facility for six hours from time of the bite. Treatment is based on the severity of the bite; patients with localised pain, swelling and redness usually do not require any specific treatment apart from applying ice and routine analgesics. In more severe bites, the definitive treatment consists of administering redback antivenom, which will usually relieve symptoms of systemic envenoming immediately.
Antivenom is indicated in anyone suffering symptoms consistent with Latrodectus envenoming. Particular indications for using antivenom are:
- Pain and swelling spreading proximally from site
- Chest pain
- Abdominal pain
- Unusual sweating
Currently, this antivenom is recommended to be given intramuscularly(IM) rather than intravenously(IV), although some have suggested IM antivenom is not as effective as IV antivenom, because IM antivenom takes longer to reach the blood serum. Adverse reactions to Black Widow antivenom are rare. Antivenom may be effective for up to 3 months after a bite, but in the vast majority of cases, it is administered within 24 hours. Doses are the same for both children and adults.