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The spider genus Steatoda, in the family Theridiidae, includes over 120 recognized species, distributed around the world (including many cosmopolitan species which are found among human populations worldwide).
These usually dark spiders have in most species a white line around the anterior back, in addition to other lines or spots.
Many spiders of the genus Steatoda are often mistaken for widow spiders (Latrodectus), and are known as false black widows; however Steatoda are significantly less harmful to humans. Steatoda are shaped similarly to widow spiders, with round, bulbous abdomens. However, not all Steatoda species resemble widows – many have distinct coloring, and are significantly smaller than Latrodectus specimens. Some species of Steatoda will actually prey on widows, as well as other spiders which are considered hazardous to humans.
Some members of this genus do have bites which are medically significant in humans (such as S. grossa and S. nobilis), however bites by Steatoda species generally do not have any long-lasting effects. The symptoms associated with the bite of several Steatoda species are known in the medical profession as steatodism; and have been described as a less-severe form of latrodectism (the symptoms associated with a widow spider bite). The redback spider antivenom has been shown to be effective at treating bites from S. grossa, after it was mistakenly administered to a S. grossa bite victim who was erroneously believed to have been bitten by the far more dangerous redback.
In common with other members of the Theridiidae family, the Steatoda spiders construct a tangle web, i.e., an irregular tangle of sticky silken fibers. 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 to 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, but ordinarily intrusion by any large creature will cause these spiders to flee.
There are currently over 120 recognized species in Steatoda.
Those commonly mistaken for widows include:
- S. borealis. A common species in North America, often mistaken for the black widow (despite being smaller and having colored markings on the dorsal side of the abdomen, rather than the ventral side).
- S. capensis, the black cobweb or false katipo spider. Originates in South Africa and is found in Australia and New Zealand; in the latter location it is often confused with the katipo spider.
- S. grossa, often known as the cupboard spider. A dark-colored spider which resembles specimens of Latrodectus, though without the characteristic bright marks found on most widow spiders. This spider is known to occasionally prey on true widows. Bites by S. grossa have been known to produce symptoms similar to (but far less severe than) the bites of true widows. Originally from Europe, but now found worldwide.
- S. nobilis. This spider, a native of the Canary Islands, has since been introduced into the United Kingdom and across Europe. Sensationalist news reports in the UK have focused on its reportedly-painful bite, including a case of man needing treatment for symptoms of heart seizure.
- S. paykulliana, another spider which is often confused with Latrodectus. This one is generally found in the range of Latrodectus tredecimguttatus and is frequently confused with it. Has a medically significant (but not serious) bite.
Other notable and recognizable species
Other notable and recognizable species in the genus include:
- S. bipunctata. A common house spider in Europe.
- S. triangulosa, the triangulate cobweb spider, a common household spider noted for a pattern of triangles on the dorsal side of its abdomen. Not known to bite; found worldwide.
- S. hespera, the western bud spider. This species is commonly found in the western United States and Canada, where it is an effective predator of the hobo spider. It is often confused with the black widow, despite being significantly smaller (7 to 8 mm) and having no bright-colored markings. Not known to bite humans, but has a venom which is similar to S. paykulliana (a medically significant spider of this genus).
- "Steatoda Spiders" (PDF). Washington State University. Retrieved 30 December 2013.
- David Sapsted, "Watch out, the black widow's sister is ready to bite you", Daily Telegraph, 2006-11-17
- Levi, H.W. (1962). The Spider Genera Steatoda and Enoplognatha in America (Araneae, Theridiidae). Psyche 69:11-36. PDF (with key to American species)
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