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At least by name, the Brown Recluse (Loxosceles reclusa) is one of the best known spiders among non-arachnologists. The Brown Recluse is a small, yellowish brown spider with a characteristic darker mark on the carapace, broader at the front and narrowed behind (to some observers, this mark resembles the shape of a violin). Length is around 9 mm for females and 10 mm for males (Kaston 1978). Although this spider is not hard to recognize for those familiar with it, inexperienced observers can easily find other small brown spiders with markings that could conceivably be described similarly. One additional character that is unambiguous is that Loxosceles (and other spiders in the family Sicariidae) have only six eyes (rather than the more common 8), which are arranged in a recurved row (i.e., median eyes farther forward than the laterals) of three pairs (Ubick 2005). Spiders without this distribution of eyes are not Brown Recluses.
Loxosceles spiders are widely known because their bites can cause significant skin necrosis (sometimes referred to as cutaneous loxoscelism) and other serious symptoms--although in the great majority of cases, no serious symptoms develop following a Brown Recluse bite. Loxosceles spiders were not documented in the literature as medically important until the mid-20th century. In North America, once they were determined to be a public health threat, there was great interest in defining the distribution of the Brown Recluse. This was followed by many reports of bites, both verified and unverified, in both the medical and popular literature. Most symptoms attributed by patients to "spider bites" in fact have some other cause. This misattribution of symptoms can have serious medical consequences if it results in delayed or inappropriate treatment. 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. Medical professionals also overdiagnose spider bites, with potentially serious consequences (such as failing to diagnose Lyme disease or other pathologies). More accurate diagnosis may sometimes be assisted by an ELISA immunoassay (Gomez et al. 2002; Stoecker et al. 2006).
Of the approximately 100 Loxosceles species, 51 are native to North and Central America and 33 to South America (one, L. rufipes, is shared between the two continents). The Brown Recluse is normally found only in the south-central United States, with several related species in the southwestern United States and southward. Although the general public and many physicians across the United States are quick to attribute mysterious skin lesions or other symptoms to Brown Recluse bites, Vetter (2005) could find no evidence that significant Brown Recluse populations exist outside the central and south-central United States (from southeastern Nebraska through the southernmost strip of Ohio and south into Texas to northern Georgia and western South Carolina), although reports of bites outside this region are not rare. Brown Recluses belong to a group of spiders that does not disperse by ballooning, which may partly explain why local infestations outside the primary range tend not to spread. Other studies have also found a pattern of Brown Recluse bite diagnoses inconsistent with the distribution and abundance of Brown Recluses (Vetter 2009a; Vetter et al. 2003, 2009). In many cases, the spider is never seen and diagnosis is based on poorly interpreted circumstantial evidence; in other cases, medical personnel, pest control operators, and other "authorities" have a specimen, but identify it incorrectly (Vetter 2009b).
Although symptoms from Loxosceles bites are usually mild, they can ulcerate or cause more severe, systemic reactions. These injuries mostly are due to sphingomyelinase D in the spider venom. There is no proven effective therapy for Loxosceles bites, although many therapies are reported in the literature. (Swanson and Vetter 2006) Vetter (2008) reviewed biological, medical, and (interestingly) psychological aspects of envenomation by Brown Recluse spiders and this key reference should be consulted by anyone interested in Brown Recluse (and other Loxosceles) spiders and, in particular, their interactions with humans. Medical aspects have also been reviewed by Swanson and Vetter (2006) and Vetter and Isbister (2008).
Although they do have the potential to be quite serious, bites are uncommon even where Brown Recluses are common. As their name implies, Brown Recluses are reclusive and have a predilection for crevices and other tight locations. In nature, they can be found under rocks and the loose bark of dead trees. In synanthropic environments (i.e. where they cohabit with humans), they are found in cardboard boxes (especially under folded flaps), in cupboards, behind bookcases and dressers, in trash, under broken concrete and asphalt, and (of particular medical concern) in shoes and clothes left out on the floor or stored in closets and garages.
These spiders can be found in very high density in synanthropic situations. A Kansas family collected 2,055 Brown Recluses in their home in the course of 6 months and a survey in Kansas showed that 22 of 25 homes sheltered Brown Recluses, with an average of 83.5 to 114.9 spiders per home (range 1 to 526). In an Oklahoma barn, a team of arachnologists collected 1,150 Brown Recluses over three consecutive nights with an essentially undiminished capture rate, although the size of the spiders decreased slightly as collecting progressed.
Brown Recluses are active hunters that do not make typical webs for prey capture. Instead, they extend lines of silk from a retreat to opportunistically alert them to the presence of entangled prey. For more information about their biology, see Vetter (2008).
More information about Brown Recluse (and other) spiders is available from the University of California-Riverside Spiders Site.
(Vetter 2008 and references therein)