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Tegenaria agrestis, known in the United States as the Hobo Spider, is native to Europe but is now well established in the Pacific Northwest of the United States and spreading eastward (Baird and Stoltz 2002). Hobo Spiders are medium-sized brown spiders that build funnel webs and resemble many other agelinid spiders. Beginning in the late 1980s, the claim was made that this spider was responsible for necrotic skin lesions seen in the Pacific Northwest that had previously been attributed (without justification) to the Brown Recluse (Loxosceles reclusa) (Vest 1987a,b; Vest et al. 1996), although the Brown Recluse (the bites of which can indeed produce necrotic skin lesions) is absent from or extremely rare in the Pacific Northwest (Vetter 2008). Although Brown Recluses clearly cannot be responsible for the symptoms in question in this region, the claim that Hobo Spider bites are instead responsible has been seriously challenged (Binford 2001; Vetter and Isbister 2004, 2008; Gaver-Wainwright et al. 2011), although this question may not yet be fully resolved. Many serious conditions having nothing to do with spiders can produce necrotic skin lesions. For example, methicillin-resistant Staphylococcus aureus (MRSA) bacterial infections are a potentially very serious cause of skin and soft tissue injury and are often presented by patients as spider bites. Suchard (2011) found that the great majority of patients seeking medical attention for a "spider bite" were actually suffering from skin and soft-tissue infections. Misdiagnosis as "spider bite" can lead to delayed or inappropriate treatment (Vetter and Isbister 2008).
The Hobo Spider is one of two European agelinid spiders that became established in the Pacific Northwest of the United States early in the 20th century. The first of the two aliens to be noted, the Giant House Spider (T. duellica) has been considered harmless in both its native range and in North America. In Europe, the Hobo Spider, like the Giant House Spider, has been considered medically benign and, as noted above, recent investigations have suggested that despite repeated assertions that Hobo Spiders in North America are not so benign, it now appears likely that in fact North American Hobo Spiders are generally as harmless as those from European populations. Both the Hobo Spider and Giant House Spider have expanded their ranges and occur together in some areas, although the Giant House Spider is still mainly restricted to the Pacific Northwest west of the Cascades (Vetter et al. 2003).