Latrodectus

Latrodectus
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Chelicerata
Class: Arachnida
Order: Araneae
Infraorder: Araneomorphae
Family: Theridiidae
Genus: Latrodectus
Walckenaer, 1805[1]
Species

31 described species,[2] estimated 65 species living

Latrodectus is a genus of spiders in the family Theridiidae, most of which are commonly known as widow spiders. The genus contains 31 recognized species[2] distributed worldwide, including the North American black widows (L. mactans, L. hesperus, and L. variolus), the button spiders of Africa, and the Australian redback spider. Species vary widely in size. In most cases, the females are dark-colored and readily identifiable by reddish markings on the abdomen, which are often (but not always) hourglass-shaped.

The venomous bite of these spiders is considered particularly dangerous because of the neurotoxin latrotoxin, which causes the condition latrodectism, both named after the genus. The female black widow has unusually large venom glands and its bite can be particularly harmful to humans. However, despite the genus' notoriety, Latrodectus bites are rarely fatal. Only female bites are dangerous to humans.

Description

Female widow spiders are typically dark brown or a shiny black in color 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. Females of a few species are paler brown and some have no bright markings. The bodies of black widow spiders range from 3 to 10 mm in size; some females can measure 13 mm in their body length.[3]

Behavior

An atypical black widow spider, with multiple red spots on her back, and vaguely yellow stripes on her legs.

The prevalence of sexual cannibalism, a behavior in which the female eats the male after mating, has inspired the common name "widow spiders".[4] This behavior may promote the survival odds of the offspring;[5] however, females of some species only rarely show this behavior, 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.[6][7]

Like other members of the family Theridiidae, widow spiders construct a web of irregular, tangled, sticky silken fibers. 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 wood piles. Indoor nests are in dark, undisturbed places as well such as under desks or furniture or in a basement.[8] 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. To feed, it uses its fangs to inject digestive enzymes, liquefying the prey's internal organs.[9] Their prey consists of small insects such as flies, mosquitoes, grasshoppers, beetles, and caterpillars.[9] If the spider perceives a threat, it quickly lets 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. When a widow spider is trapped, it is unlikely to bite, preferring to play dead or flick silk at the potential threat; bites are the result of continual harassment.[10] 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 1290 ± 160 MPa.[11] The tensile strength of spider silk is comparable to that of steel wire of the same thickness.[12] However, as the density of steel is about six times that of silk,[13] silk is correspondingly stronger than steel wire of the same weight.

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.

Taxonomy

Male L. elegans from Japan
L. hesperus hair and markings
L. hesperus profile
Ventral side of a L. geometricus displaying the hourglass marking
Dorsal side of a L. geometricus in Colorado, US
L. hesperus with egg sac
L. tredecimguttatus female

The genus Latrodectus was erected by Walckenaer in 1805, for the species Latrodectus tredecimguttatus and Latrodectus mactans.[1][14] Arachnologist Herbert Walter Levi revised the genus in 1959, studying the female sexual organs and noting their similarity across described species. He concluded the color 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.[15]

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".[15]

Species

As of July 2017, the World Spider Catalog accepted the following species:[1]

Distribution

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 well as parts of southern Canada – particularly in the Okanagan Valley of British Columbia, as can the "gray" or "brown widow spiders" (Latrodectus geometricus) and the "red widow spiders" (Latrodectus bishopi).[16] The blue mud dauber (Chalybion californicum) is a wasp that is the primary predator of black widow spiders.[17]

The most prevalent species occurring in Australia is commonly called the redback (Latrodectus hasselti).

Bite

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.[18] Symptoms usually last for 3–7 days, but may persist for several weeks.[19]

Each year, about 2,200 people report being bitten by a black widow, but most recover within 24 hours with medical treatment (male spiders produce the toxins to help with their own hunting, but they make such a diluted version that they’re not harmful to most people). Also, many people who are bitten develop few symptoms since the spider may not inject its venom. Black widows are not especially aggressive spiders, and they rarely bite humans unless startled or otherwise threatened.[20]

Contrary to popular belief, 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.[21]

Since the venom is not likely to be life-threatening, antivenom has been used as pain relief and not to save lives.[22] However, a study demonstrated that antivenom and placebo added to standardized pain medication had similar improvement in pain and resolution of symptoms.[22]

See also

References

  1. 1 2 3 "Gen. Latrodectus Walckenaer, 1805", World Spider Catalog, Natural History Museum Bern, retrieved 2016-01-28
  2. 1 2 "Species list for Latrodectus". World Spider Catalog. Natural History Museum Bern. Retrieved 2016-01-28.
  3. ↑ "Black Widow Spiders". Orkin Pest Control. Retrieved 19 June 2017.
  4. ↑ Breene, R. G.; Sweet, M. H. (1985). "Evidence of insemination of multiple females by the male Black Widow Spider, Latrodectus mactans (Araneae, Theridiidae)" (PDF). The Journal of Arachnology. 13 (3): 331–335.
  5. ↑ "Male spiders let females eat them for kids' sake: Study". livescience. 23 December 2011. Retrieved 23 December 2011.
  6. ↑ Johnson, J. Chadwick; Trubl, Patricia; Blackmore, Valerie; Miles, Lindsay (August 2011). "Male black widows court well-fed females more than starved females: silken cues indicate sexual cannibalism risk". Animal Behaviour. 82 (2): 383–390. doi:10.1016/j.anbehav.2011.05.018.
  7. ↑ Baruffaldi, Luciana; Andrade, Maydianne C.B. (April 2015). "Contact pheromones mediate male preference in black widow spiders: avoidance of hungry sexual cannibals?". Animal Behaviour. 102: 25–32. doi:10.1016/j.anbehav.2015.01.007.
  8. ↑ "Black Widow Spider - Facts, Bite & Habitat Information". Animal Corner. Retrieved 4 November 2016.
  9. 1 2 "Black Widow Spiders". National Geographic. 10 September 2010. Retrieved 19 June 2017.
  10. ↑ Nelsen, David; Kelln, Wayne; Hayes, William. "Poke but don't pinch: risk assessment and venom metering in the western black widow spider, Latrodectus hesperus". Animal Behaviour. 89: 107–114. doi:10.1016/j.anbehav.2013.12.019. Retrieved 12 October 2015.
  11. ↑ Blackledge, T. A.; Swindeman, John E.; Hayashi, Cheryl Y. (15 May 2005). "Quasistatic and continuous dynamic characterization of the mechanical properties of silk from the cobweb of the black widow spider Latrodectus hesperus" (PDF). Journal of Experimental Biology. 208 (10): 1937–1949. doi:10.1242/jeb.01597.
  12. ↑ "Astm a36". OnlineMetals.com. Archived from the original on 9 May 2013.
  13. ↑ Elices, Manuel; Guinea, Gustavo V.; Pérez-Rigueiro, José; Plaza, Gustavo R.; et al. (2005). "Finding Inspiration in Argiope Trifasciata Spider Silk Fibers". JOM. 57 (2): 60–66. Bibcode:2005JOM....57b..60E. doi:10.1007/s11837-005-0218-7. Retrieved 2009-01-23.
  14. ↑ Walckenaer, C.A. (1805), Tableau des aranéides ou caractères essentiels des tribus, genres, familles et races que renferme le genre Aranea de Linné, avec la désignation des espèces comprises dans chacune de ces divisions, Paris: Dentu, pp. 81–82
  15. 1 2 Levi, Herbert W. (1959). "The Spider Genus Latrodectus (Araneae, Theridiidae)". Transactions of the American Microscopical Society. 78 (1): 7–43. JSTOR 3223799. doi:10.2307/3223799.
  16. ↑ (Preston-Malfham, 1998).
  17. ↑ Drees, Bastiaan M.; Jackman, John A. (1999). "Mud Daubers". Field Guide to Texas Insects. Gulf Publishing Company. ISBN 978-0-87719-263-3. Archived from the original on 12 July 2007. Retrieved 10 July 2007.
  18. ↑ Ushkaryov, YA; Rohou, A; Sugita, S (2008). "alpha-Latrotoxin and its receptors". Handbook of experimental pharmacology. Handbook of Experimental Pharmacology. 184 (184): 171–206. ISBN 978-3-540-74804-5. PMC 2519134 Freely accessible. PMID 18064415. doi:10.1007/978-3-540-74805-2_7.
  19. ↑ Peterson, ME (November 2006). "Black widow spider envenomation". Clinical techniques in small animal practice. 21 (4): 187–90. PMID 17265903. doi:10.1053/j.ctsap.2006.10.003.
  20. ↑ Why Black Widow Spider Venom Is So Potent. By Jennifer Viegas, Discovery News January 6, 2015
  21. ↑ Bettini, S (1964). "Epidemiology of latrodectism". Toxicon. 2 (2): 93–102. PMID 14301291. doi:10.1016/0041-0101(64)90009-1.
  22. 1 2 Isbister, Geoffrey K.; Page, Colin B.; Buckley, Nicholas A.; Fatovich, Daniel M.; Pascu, Ovidiu; MacDonald, Stephen P.J.; Calver, Leonie A.; Brown, Simon G.A. (2014). "Randomized Controlled Trial of Intravenous Antivenom Versus Placebo for Latrodectism: The Second Redback Antivenom Evaluation (RAVE-II) Study". Annals of Emergency Medicine. 64: 620–8.e2. PMID 24999282. doi:10.1016/j.annemergmed.2014.06.006. (Subscription required (help)).

Further reading

Media related to Latrodectus at Wikimedia Commons
Data related to Latrodectus at Wikispecies

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