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The spider family Sicariidae includes 132 described species placed in two genera, Loxosceles and Sicarius (Platnick 2014).  Loxosceles species, which account for most of the sicariids, are found in the Mediterranean region, northern Africa, and South and Central America; Sicarius includes just around two dozen described species, which are found in Africa and South and Central America. (Platnick 2014) Thirteen sicariids (all Loxosceles) are known from North America north of Mexico (Ubick 2005).

Sicariids are among the minority of spiders with six eyes. This family includes several well known spiders whose bites are medically significant for humans, such as the Brown Recluse (L. reclusa) in North America, L. boneti in Mexico (Ramos-Cerrillo et al. 2004), and L. intermedia, among others, in South America (Fischer and Vasconcellos-Neto 2005 and references therein).

The genus Loxosceles has a worldwide distribution. In North America north of Mexico, most species are restricted to the southwest, from southern California to southern Texas. The Brown recluse has a wide natural distribution along the Mississippi Basin and adjacent parts of the southcentral United States (future climate change may result in a northward expansion of this distribution; Saupe et al. 2011), but has spread synanthropically to isolated localities outside this region. (Ubick 2005)  Loxosceles laeta is a South American species now known from a few localities along both coasts of North America, including southwestern Canada (as well as Finland and Australia) (Gertsch and Ennik 1983)  The Mediterranean Recluse (L. rufescens) is native to the Mediterranean region; it has been inadvertently transported by humans around the world, but is typically restricted to local (sometimes dense) populations in particular buildings. Loxosceles rufescens is now established over much of the southern United States and in New York (as well as much of the rest of the world). Although L. rufescens is frequently found outdoors in all but the northernmost parts of its Old World range, all confirmed North American collections have so far been from inside structures, mostly institutional (e.g. governmental, academic, museums) or commercial rather than smaller residential buildings. Greene et al. (2009) reported on L. rufescens populations in a number of  buildings in Washington, D.C. Unlike L. reclusa, D.C. populations of L. rufescens are essentially troglophilic, concentrated mainly in basements, foundation walls, and other man-made subterranean habitats, typically in close association with American Cockroaches (Periplaneta americana) and/or Reticulitermes termites. (Greene et al. (2009)

Loxosceles are found in a range of habitats from deserts to mesic habitats and, as noted above, some species are often synanthropic. They are typically hidden in crevices they line with silk and extend beyond their retreat. The silk is sticky, readily capturing prey, and somewhat resembles cribellate silk although it is structurally different (Stern and Kullmann 1981 cited in Ubick 2005). Studies on the biology of L. reclusa indicate that it is a generalist feeder with a univoltine life cycle (i.e., with a single generation per year), but may live up to three years in captivity (Hite et al. 1966 cited in Ubick 2005). Observations on the biology of Loxosceles deserta (discussed as Locosceles unicolor) were presented by Ennik (1971 cited in Ubick 2005) and on L. arizonica by Richman (1973).

Although Loxosceles spiders are widely perceived to be a major public health menace, at least in the case of L. reclusa in the United States it is clear that many supposed cases of Loxosceles bites actually have other causes. In fact, Brown Recluses are often blamed for supposed bites in areas where the best available data indicate they are not even present (Vetter et al. 2003; Vetter 2005, 2008; Vetter and Isbister 2008) or at times of the year when bites are unlikely (Vetter 2011). Such misdiagnoses do not merely endanger the reputation of L. reclusa, but more importantly, they can endanger human patients who are often actually suffering from potentially more dangerous dermatologic conditions such as MRSA infections (e.g., Moran et al. 2006). A similar situation has been described in Australia, with necrotic ulcers being routinely attributed to spider bites on very weak evidence (Isbister 2001 but see Young and Pincus 2001; Swanson and Vetter 2005).

In the southcentral United States, however, Brown Recluses can, in fact, be very abundant in houses (Sandidge and Hopwood 2005) and a bite can be a serious medical event. Loxosceles reclusa venom is potent and can produce lesions and persistent sores in humans, although rarely death (Gertsch and Ennik 1983). In South America, Loxosceles species present a far greater public health issue, although here, also, these spiders are likely blamed for significantly more medical events than they are actually resposible for. Although Loxosceles venom can do serious damage to vertebrates, its primary evolved function is believed to be to immobilize insect prey (Zobel-Thropp et al. 2012).

The Nearctic and Neotropical Loxosceles have been revised by Gertsch and colleagues (Gertsch and Mulaik 1940; Gertsch 1958; Gertsch 1967; Gertsch 1973; Gertsch and Ennik 1983). Loxosceles and Sicarius were at one time placed in their own families (Loxoscelidae and Sicariidae) and have also been included in the Scytodidae, another family of six-eyed spiders. (Ubick 2005)  Binford et al. (2008) and Duncan et al. (2010) investigated the phylogenetic and phylogeographic relationships within Loxosceles and Sicarius.

(Ubick 2005; Bradley 2013)

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