Brown Recluse Spider Anatomy

—This article by Jerry Cates, first published on 7 February 2010, was last revised on 23 April 2016. © Bugsinthenews Vol. 11:02(04).

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Related Links:

• Brown recluse encounters in Austin, Texas
• Brown recluse behavior; encounters in Joplin Missouri & Grove, Oklahoma
• Brown recluse encounter in Wichita Falls, Texas
• Brown recluse encounter near Lake Travis, Texas
• Brown recluse encounter in Collin County, Texas

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Sicariidae: Brown Recluse Spider (Loxosceles reclusa); Showing dorsum of male body, Annotated; Collected on 101510 at Home of Jane T., Joplin, MO–Photographed 08 Dec 2010

The brown recluse spider (Loxosceles reclusa) is one of thirteen species of recluse spiders recognized in the genus Loxosceles that are native to North America.

The genus is distributed worldwide, and a total of 100 species of recluse spiders have been described thus far.

The generic name, Loxosceles, is derived from the Greek adjective λοξος (loxo = “slanting, crosswise, oblique”), and the Greek combining form -σκελης (sceles = “leg”).

The following notes about the brown recluse spider were gleaned from multiple sources. References are listed at the end of the article, with links to Internet sources that admit all inquiries without subscription, whenever such sources are available.

Drawings depicted here were made in the EntomoBiotics lab from photographs supplied by others or taken by our personnel, in the field or in the lab, of captive live or preserved specimens.

Anatomical photographs used in this article were taken in the lab, using a digital camera with a macro lens, further magnified–in most cases–through the lens of a dissecting microscope:

Anatomy

• Abdomen has the form of a flattened oval and is hairy (Ubick et al., p 222); see photo 001, below.
• Body sizes for mature Sicariidae specimens range from 5.0-13 mm (Ubick et al., 2005, p.222)
• Carapace is flat, pyriform [pear-shaped], longer than wide, the fovea [dorsal thoracic groove] is longitudinal (Ubick et al., 2005, p. 222); see photo 001, below.
• Chelicerae [jaws]: fused basally such that they are connected by a membrane [which, while limiting lateral articulation of the jaws, would seem to add leverage to the fangs, possibly enabling deeper penetration during envenomation], with median lamina [a sclerotized ridge on the cheliceral margin or mesal–i.e., toward the midline of the body–surface of each jaw] which have, at their distal extremities, an apical tooth [in photo 003, below, the apical tooth of the median lamina of the left chelicera is betrayed by its incised edge, where it nests the distal fang]; ectal [away from the midline of the body] margin with stridulatory file [fine grooves used in conjunction with thorns to produce sound] (Ubick et al.,  p. 222); outer side of chelicera with stridulatory file of coarse grooves; femur of* pedipalp with a single, short, black stridulating pin near the base on the prolateral [i.e., the anterior surface of the appendage] side (Gertsch, 1958, p. 3: *note that, in Gertsch & Ennik, 1983, p. 278, a similar–though slightly restructured–description is offered, except the preposition “of” is replaced by “or”; inasmuch as the original 1958 rendering appears most likely to be what Gertsch intended, and the 1983 rendering the likely result of an editorial corruption, the original wording has been retained here); see photos 003 & 004, below.
• Coloration is yellowish to grayish brown with cephalic area often darker (Ubick et al., 2005, p.222); body coloration partially dependent on diet, such that specimens that have recently fed on darker organisms, such as field crickets and carabid beetles, tend to have darker brown or grayish bodies (Sandidge, 2009, p. 6); see photo 001, below.
• Eyes are six in number with the AME [anterior median eyes] absent, arranged in three diads which form a strongly recurved [i.e., the lateral eyes are posterior to median eyes] row (Ubick et al., p. 222); anterior median eyes have been lost, such that the posterior median eyes have migrated to the forward position (Gertsch & Ennik, 1983, p. 278); see photos 006 & 008, below.

Sicariidae: Brown Recluse Spider (Loxosceles reclusa) 002

Sicariidae: Brown Recluse Spider (Loxosceles reclusa) 003

Sicariidae: Brown Recluse Spider (Loxosceles reclusa) 004

Sicariidae: Brown Recluse Spider (Loxosceles reclusa) 005

Sicariidae: Brown Recluse Spider (Loxosceles reclusa) 006

Sicariidae: Brown Recluse Spider (Loxosceles reclusa) 007

Sicariidae: Brown Recluse Spider (Loxosceles reclusa) 008

Sicariidae: Brown Recluse Spider (Loxosceles reclusa) 009

• Fangs are so small many wonder how they can pierce human skin; fang size is so diminutive that thin clothing, gloves, socks, undergarments, etc., cannot be penetrated, thus clothing of any kind provides a layer of protection against bites (Sandidge, 2009, p. 7); see photos 002, 003, & 005.
• Genitalia: haplogyne, with the female lacking external modification, and having unremarkable palps (see photo 005). The male palp has a swollen tibia, with a short tarsus, a spherical bulb, and an embolus consisting of  a simple prong (Ubick et al., 2005, p.222); immature specimens are not obviously assignable to either sex, though males become evident  in the penultimate stage [the developmental stage produced in the next to last molt] when the tarsi of the palpi become moderately to distinctly enlarged in the basal and median portions [see photo 004]; maturity in the female is usually signaled in the penultimate stage by the reddish coloration of the tibia and tarsus of their pedipalps [see photo 005] (Gertsch & Ennik, 1983, pp. 278-279); female pedipalp is without a claw (Gertsch & Ennik, 1983, p. 277); see photo 005.
• Legs are slender (Ubick et al., 2005, p.222); legs and body clothed thickly with two kinds of covering hairs: 1. long, suberect, denticulate hairs, those on tibiae, metatarsi and tarsi in eight rows, two on each surface, and some of those on tibiae and metatarsi shorter and thickened; 2. fine, procumbent, basally feathered hairs lying between the suberect ones (Gertsch & Ennik, 1983, p.278); see diagram 000, and photo 001.
• Mouthparts consist of an elongate labium that is flexibly attached to the sternum. Endites are strongly convergent (Ubick et al., 2005, p. 222)
• Respiratory system consists of one pair of book lungs and a tracheal system with a single spiracle near the spinnerets (Ubick et al., p.222).
• Sternum is subcircular, with extensions at coxae (Ubick et al., 2005, p.222)
• Spinnerets with large, pointed  colulus (nonfunctional cribellum that may be as large as a cribellum but that is commonly reduced to a small fleshy lobe or even just a pair of setae) about twice as long as wide; ALS (anterior lateral spinnerets) largest, separated by width of colulus; PS contiguous (Ubick et al., 2005, p.222)
• Tarsi with two claws (Ubick et al., 2005, p.222) [by comparison, the closely related spitting spiders in the Scytodidae family have tarsi with three claws (Ubick et al., 2005, p. 217)]; tarsal claws arise from a single cuticular platelet (Foelix, 1996, p. 18); tarsal claws not supplemented with scopulae as in [most] entelegyne Dionycha (See photo 009).

Behavior:

• Cannibalism: Dozens of Loxosceles spiderlings of the same species can be reared in close quarters in a single jar with minimal cannibalism as long as there is adequate prey to eat and crevices in which to hide (Vetter, 2008, p.154)
• Synanthropy (Greek syn “together with” + anthro “man” = “an animal that resides with man”):  Brown recluse spiders are notoriously synanthropic, to the point that populations of the species increase in association with humans, and decrease in the absence of such an association (Vetter, 2008, p.151).

Common Names:

• Sicariidae and Loxosceles reclusa: Brown spiders, violin spiders, brown recluse spiders (Ubick et al., 2005, p.222)

Distinguishing Characteristics:

• From other haplogynes: Basally fused chelicerae, six eyes in three diads forming strongly recurved row (Ubick et al., 2005, p.222)
• From Scytodidae: Relatively flat carapace, tarsi with only two claws (Ubick et al., 2005, p.222)

Distribution:

• Sicariidae: Worldwide; most native Nearctic species are restricted to the southwest U.S., from southern California to southern Texas (Ubick et al., 2005, p.222)
• Loxosceles reclusa: Of 15 synanthropic spiders known to inhabit homes in Kansas, the three most common are L. reclusa, Achaearanea tepidoriorum, and Steatoda triangulosa (Guarisco, 1999, p.218); L. reclusa is distributed naturally along the Mississippi Basin and adjacent parts of central U.S.A.; distributed synanthropically to isolated localities beyond (Ubick et al., 2005, p.222)

Envenomation:

• Four categories of Loxosceles bites are recognized: (1) unremarkable, with very little damage, and self-healing; (2) mild reactions such as redness, itching, slight lesion, also self-healing [most Loxosceles bites produce the signs described in categories 1 and 2]; (3) dermonecrotic, producing the necrotic skin lesion considered by many as the typical reaction; and (4) systemic or viscerocutaneous, affecting the vascular system and potentially fatal, but these are very rare [less than 1% of cases of suspected L. reclusa bites]; in the most severe manifestations [of category 3 bites, as just described], loxoscelism lesions can grow to 40 cm in size, and healing can take several months and leave a disfiguring scar; cutaneous loxoscelism damage is greater in obese victims because the venom enzymes readily destroy poorly vascularized adipose tissue (Vetter, 2008, p.155).
• Misdiagnosis of dermonecrotic lesions [from other causes], incorrectly attributing them to brown recluse spider bites, is so common that, according to Philip Anderson, a dermatologist, “because the well-accepted rules of evidence have been ignored, a large part of the total clinical literature on loxoscelism is invalid”; many medical maladies [unrelated to loxoscelism] manifest in necrotic skin lesions that present signs and symptoms similar to loxoscelism but are caused by a variety of other dermonecrotic etiologies [in 2008 the list of such etiologies included 39 entries] (Vetter, 2008, p.157-158).
• MRSA and CA-MRSA: nosocomial [hospital acquired] cases of methicillin-resistant Staphylococcus aureus [MRSA, or HA-MRSA], and similar cases of community-acquired MRSA [CA-MRSA] have emerged as a major etiology of necrotic skin and soft tissue injury; such lesions are often diagnosed incorrectly as brown recluse spider bites, partly because the general public  tends to use “spider bite” as the common explanation for their skin lesions (Vetter, 2008, p.158).
• Risk of a Loxosceles spider bite is small, even in heavily infested structures; in a Kansas home where 2,055 L. reclusa were collected in 6 months time, no one in the family of four had sustained a perceptible loxoscelism event in the first 6 years of occupancy [at the time the study was first conducted]; in the 11th year of occupancy the mother was bitten on the finger while reaching into laundry, and subsequently shook a brown recluse from a shirt sleeve; the finger turned red and swelled slightly, but healed without incident (Vetter, 2008, pp. 156-157).
• Risk of secondary infections, particularly MRSA infections,  emanating from spider bites were considered common  by Shawn Fagan, M.D., in an article published in 2003; since then, however, other investigators have concluded otherwise, observing that in cases screening for MRSA on randomly-collected house spiders in Chicago showed no evidence of the bacterium on spider body parts [presumably in spite of a prevalence of such bacteria on humans occupying the same structures] (Vetter, 2008, pp. 158-159).
• Viscerocutaneous loxoscelism: is a systemic illness that [infrequently] occurs after a Loxosceles envenomation; it has a higher incidence in the pediatric population, and is accompanied by the low grade fevers and arthralgias occasionally observed in the cutaneous form of the bite; however, the systemic illness soon progresses, with diarrhea, vomiting, coagulopathics, disseminated intravascular coagulation, hemolysis, petechia, thrombocytopenia, and urticaria; these usually occur from 48-72 hours after envenomation, but can occur as early as 24 hours; prevalence of viscerocutaneous loxoscelism ranges from 0.7%  to 27%, and varies geographically; in the U.S. the systemic illness is rare, with a low incidence of mortality; from 1958 to 1995 only 39 cases of hemolysis were found, 6 resulting in death (Hogan et al., 2004, p.613).

Historic Significance:

• Since 1872 physicians have recognized a peculiar skin lesion now referred to as “necrotic arachnidism” (ed: most recently as dermonecrosis) or more specifically “loxoscelism” (Gertsch & Ennik, 1983, p. 276). In 1934 Machiavello presented convincing evidence that the bite of Loxosceles laeta in Chile produced serious dermonecrotic wounds; later, in 1937, Machiavello followed up this finding with a study of the medical and iological aspects of 70 bites; in 1957 Dr. Curtis W. Wingo and his associates at the University of Missouri published the first convincing–though largely circumstantial–evidence incriminating L. reclusa as the likely cause of similar dermonecrotic lesions in the U.S. (Gertsch, 1958). The cytotoxic, and to a lesser extent, haemotoxic and neurotoxic, nature of Loxosceles venom has been established and documented with clinical and experimental evidence; clinical signs ranging from mild cutaneous necrosis to serious systemic reactions and death have been reported (Gertsch & Ennik, 1983, p.276).
• The first documented case of Loxosceles envenomation [in the U.S.] was in 1879, when a Tennessee man developed fever, jaundice, and hematuria after a spider bite (Hogan et al., 2004, p.609).

Physiology:

• Heart rates of brown recluse spiders deviate from the norm in that, though having low body rates, their heart rates are among the lowest of all spiders known. This correlates with observations that brown recluse spiders can survive indoors for months or even years without food or water (Carrell et al., 1976 ).
• Heat and cold tolerance: The activity limits of L. reclusa are 4.5° to 43° C (Hite et al., 1966). No mortalities were recorded in 30-day exposures of L. reclusa at temperatures of 0° C, or in 4-hour exposures to temperatures of 3, 1.5, 0, -2 and -5° C; when held for 4-hours at -7 and -10° C, however, groups of L. reclusa experienced 47% mortality, and at -14° C mortality was 100% (Cramer & Maywright, 2008, p. 137).

Taxonomy:

• Family name: Sicariidae (from a Greek word meaning assassin, murderer), first described in 1880 by Eugen von Keyserling (March 22, 1833, Pockroy, Lithuania – April 4, 1889, Dzierżoniów, Silesia) was a German arachnologist. Keyserling was the author of Die Spinnen Amerikas. In addition he completed Die Arachniden Australiens (1871-1883) on behalf of Ludwig Carl Christian Koch.
• Generic name: Loxosceles (from the combination of two Greek words meaning “slanted + leg”): first described in 1832 by Heineken & Lowe.
• Species name: Loxosceles reclusa, first described by Gertsch & Mulaik, i940.

Similar families:

• Scytodidae

References:

• Carrel, James E., and R.D. Heathcote, 1976. Heart Rate in Spiders: Influence of Body Size and Foraging Energetics. Science, 193: 148-150.
• Cramer, Kenneth L., 2008. Are Brown Recluse Spiders, Loxosceles reclusa (Araneae, Sicariidae) scavengers? The influence of predator satiation, prey size, and prey quality. J. Arachnology 36:140-144.
• Cramer, Kenneth L., Alex V. Maywright, 2008. Cold temperature tolerance and distribution of the brown recluse spider Loxosceles reclusa (Araneae, Sicariidae) in Illinois. J. Arachnology 36:136-139.
• Gertsch, Willis J., 1958. The Spider Genus Loxosceles in North America, Central America, and the West Indies. AMNH Novitates 1907: 1-46.
• Gertsch, Willis J., 1979. American Spiders, 2nd Edition. Von Nostrand Reinhold Company.
• Gertsch, Willis J., Franklin Ennik, 1983. The Spider Genus Loxosceles in North America, Central America, and the West Indies (Araneae, Loxoscelidae). Bull. AMNH 175: 254-360.
• Guarisco, Hank, 1999. House Spiders of Kansas. J. Arachnology 27:217-221.
• Hite, J.M., W.J. Gladney, J.L. Lancaster & W.H. Whitcomb, 1966. The biology of the brown recluse spider. University of Arkansas, Fayetteville. Agricultural Experiment Station Bulletin No. 711.
• Hogan, Christopher J., Katie C. Barbaro, Ken Winkel, 2004. Loxoscelism: Old Obstacles, New Directions. J. Annemergmed 08: 608-624.
• Parks, Jennifer, William V. Stoecker, Charles Kristensen, 2006. Observations on Loxosceles reclusa (Araneae, Sicariidae) feeding on short-horned grasshoppers. J. Arachnology 34:221-226.
• Sandidge, Jamél S., 2009. Brown Recluse Spiders: A knowledge based guide to control and elimination. Pubkished by BRS Pest Control, McLouth, KS.
• Ubick, D., P. Paquin, P.E. Cusing and V. Roth, editors, 2005. Spiders of North America. Published by the American Arachnological Society.
• Vetter, Richard S., 2008. Spiders of the genus Loxosceles (Araneae, Sicariidae): a review of biological, medical and psychological aspects regarding envenomation. J. Arachnology 36:150-163.

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