Identifying and Misidentifying the Brown Recluse Spider
Department of Entomology,
University of California Riverside
The brown recluse spider, Loxosceles reclusa, is often implicated as a cause of necrotic skin lesions.[1-3] Diagnoses are most commonly made by clinical appearance and infrequently is a spider seen, captured or identified at the time of the bite.[1, 2, 4-6] The brown recluse lives in a circumscribed area of the U.S. (the south central Midwest) with a few less common recluse species living in the more sparsely-populated southwest U.S. In these areas, where spider populations may be dense, recluse spiders may be a cause of significant morbidity. However, outside the natural range of these recluse species, the conviction that they are the etiological agents behind necrotic lesions of unknown origin is widespread, and most often erroneous. In some states such as California, unsubstantiated reports concerning recluse spider bites have taken on the status of "urban legend" leading to overdiagnosis and, therefore, inappropriate treatment.
General information regarding recluse spider life history characteristics has been published.[7-9] Recluse spiders, as their names imply, are rather secretive in their habits. They are nocturnal spiders that actively attack prey and subdue it with venom. Although they don't use silk for prey capture, they do use it to line their diurnal refugia. In nature they are found under rocks and in crevices and are considered "synanthropic" meaning their populations benefit when associated with humans. When a habitat is conducive to recluses, dense populations are found. Part of the reason is that recluses are highly tolerant of conspecifics; they are one of the few spiders that can be reared communally in a jar, given that there is sufficient prey availability. As an example of their abundance, in Missouri, the author and a colleague collected 40 brown recluses in a barn within 75 minutes. In Kansas, the brown recluse is an extremely common house spider. Finally, recluses have a prevalence for hiding in boxes which allows them to be transported out of their range by commerce or residential relocation. Despite this opportunity for range expansion, remarkably few verified populations have established outside the shaded area in the map shown. When they do establish, it typically is in the basement of a building and there is little expansion beyond the structure unless connected to other structures by underground pipes or passageways.
Despite their reclusive habits, they do occasionally bite humans. Recluses typically bite when they are trapped between flesh and another surface, as when a sleeping human rolls over on a prowling spider, or when putting on clothing or shoes containing spiders. Ways to reduce bite risk from recluse spiders include: 1) keep beds away from walls; remove bed skirts and items under the bed so that the only pathway to the bed is up the legs. 2) Keep clothing off the floor; if it is on the floor, shake it vigorously before dressing. 3) Store all intermittantly used items such as gardening clothing, baseball mitts or roller skates in spider-proof boxes or bags.
The common name "brown recluse" refers specifically to one species of spider that lives in the south central Midwest U.S. (Map). It may be found in less dense populations around the margins of the shaded area on the map. Many reports, both media and medical, forebodingly state that the brown recluse can be transported outside its range. Although this is true, it is then erroneously projected that one spider is the "tip of the iceberg" for rampant populations. In fact, verified finds of brown recluses outside of its range are rare and almost every collection is that of a single itinerant spider. Subsequent searching of the vicinity typically results in no additional recluses. The undeserved infamy that this spider has achieved outside of its range is nothing short of mind boggling. The few known instances of any recluse spider population establishing in non-native habitats typically are limited to circumscribed areas, with only rare reports of expansion from its locale.
The name "brown recluse" spider correctly refers only to the midwest species; additional species are known by common names such as the desert recluse, the Arizona recluse, etc. Unfortunately, non-arachnologists incorrectly lump them all under the "brown recluse" moniker. This is a potentially incorrect extrapolation because only the brown recluse has been intensively studied. All recluse species are probably capable of inflicting necrotizing bites, however, there may be behavioral and toxicological differences among the various species.
Two other spiders that have the potential to produce necrotizing wounds, though much less well-documented than the brown recluse, are the hobo spider and the yellow sac spider. The hobo spider (Tegenaria agrestis) may be found in the Pacific Northwest as far east as Montana and south into Oregon and Utah. The two yellow sac species (Cheiracanthium spp.) are found all over the United States, but probably only produce minor necrotic wounds.
Identifying the Brown Recluse Spider
One can readily learn how to identify recluse spiders with less than a minute's training. Whereas most U.S. spiders have 8 eyes, typically arranged in 2 rows of 4, the recluse spiders have 6 eyes arranged in pairs (dyads) with one anterior dyad and 2 lateral dyads (Fig. 1). All 13 species of U.S. recluses (11 native, 2 non-native) share the same eye pattern. In many publications, the violin pattern on the cephalothorax (the first body part to which the legs attach) is mentioned as a diagnostic characteristic (Fig 2). Although it is quite consistent in adult brown recluses (although it can fade in preserved specimens), many western U.S. recluse species and some young brown recluses have virtually no contrasting pigmentation in the violin region (Fig. 3, 4). In addition, recluse spiders have abdomens that are devoid of coloration pattern and their legs are covered with fine hairs but lack thickened spines.
Misidentification of spiders as brown recluses is not uncommon both in the lay and medical communities. Many of these mistaken spiders are similar in only one trait with actual recluse spiders, with some only sharing the characteristics of brown color and eight legs.Six-eyed spiders
Spitting spiders (Scytodes spp., Family Scytodidae) are taxonomically related to recluses, are non-poisonous and probably often mistaken as recluses throughout the U.S. They share the same eye pattern (Fig. 5), however, the several known species have black stripes and/or maculae on the dorsal surface of both the cephalothorax and abdomen which should quickly eliminate them as recluse spiders (Fig. 6). In addition, in side view, the cephalothorax is definitively humped (Fig. 7), an anatomical modification necessary for housing the large spitting glands that are only found within this genus.
The woodlouse spider (Dysdera crocata, Family Dysderidae) (Fig. 8) has six eyes which are grouped closely together in triads near the anterior margin of the cephalothorax. Despite this and the lack of bodily pigmentary pattern, the woodlouse spider is commonly misidentified as a brown recluse. It is found throughout the U.S.
Spiders with "violin" markings
There are several common and ubiquitous non-poisonous U.S. spiders that have dark markings on the cephalothorax which are erroneously and creatively misinterpreted as the violin marking of a brown recluse. These include the long-legged cellar spiders (Psilochorus spp., Physocyclus spp.; Family Pholcidae) (Fig. 9) and pirate spiders (Mimetus spp., Family Mimetidae) (Fig. 10). On the Pacific coast, the marbled cellar spider, (Holocnemus pluchei, Family Pholcidae) has often been submitted by the public as a recluse despite the fact that its brown "violin" pattern is on its sternum and ventral abdomen (Fig. 11). All of these spiders have eight eyes although some eyes are quite reduced in size or obliterated from view by black pigment; microscopic examination is needed to see them.
Other brown arachnids
Fearing that they might have recluse spiders, the public has brought in many other brown, eight-eyed spiders in addition to non-spider arachnids such as solpugids and daddy-long legs. The latter is differentiated from spiders in that it has one major body part as opposed to two, lacks venom glands, does not make silk and therefore, is not found in webs except as spider prey. Unfortunately, the urge to misidentify common, virtually harmless spiders as brown recluses is not restricted to the lay community.
Although bites from the brown recluse and other recluse spiders can be a source of significant morbidity, diagnoses implicating these spiders as the culprits should be restricted to those regions of the country that support populations of the spiders. On a broader scale, spider bites in general are overdiagnosed.  A call for more judicious evaluation has been made several times.[1-3, 11,12] Spider bites are the result of an incidental and accidental encounter between arachnid and human. In areas outside the range of recluse spiders, it has been suggested that physicians consider more strongly as differential diagnoses, many of those arthropods (fleas, hard ticks, soft ticks, mites, bedbugs, assassin bugs, etc.) that purposely seek out humans for their blood meals rather than the accidental spider encounter. Wounds from these animals could stem from reactions to the animal's saliva, to toxins or to bacteria introduced while feeding. Stringent guidelines have been put forth in attempt to stem the overdiagnosis of spider bites.[1,2,12] Verified spider bites require the presence or sighting of a spider in the act of biting. In the absense of this, a necrotizing wound should be evaluated thoroughly for infectious, thrombotic, and vasculitic causes. Without verification, the diagnosis of necrotizing spider bite should be one of exclusion.
Richard Vetter, M.S., is a Staff Research Associate in the Department of Entomlogy, University of California Riverside. He studies the systematics, distribution, and public health impact of arachnids in Southern California. To assist the medical community in identifying spiders and differentiate those with medical significance from harmless varieties, Mr. Vetter has kindly volunteered to identify spiders sent to him. Please place the spider in alcohol in a leakproof vial with a note describing the circumstances of finding the spider as well as the location and date of collection.
Department of Entomology
University of California, Riverside
Riverside, CA 92521
fax (909) 787-3086
References1. Russell FE & Gertsch WJ. Letter to the editor. Toxicon 1983; 21:337-339
2. Kunkel DB. The myth of the brown recluse. Emerg Med 1985; 17(5):124-128
3. Vetter RS & Visscher PK. Bites and stings of medically important venomous arthropods. Intl J Dermatol 1998; 37:481-496
4. Foil LD & Norment BR. Envenomation by Loxosceles reclusa. J med Entomol 1979; 16:18-25
5. Delozier JB, Reaves L, King LE Jr. & Rees RS. Brown recluse spider bites of the upper extremity. So Med J 1988; 81:181-184
6. Wright SW, Wrenn KD, Murray L & Seger D. Clinical presentation and outcome of brown recluse spider bite. Ann Emerg Med 1997; 30:28-32
7. Gertsch WJ & Ennik F. The spider genus Loxosceles in North America, Central America, and the West Indies (Araneae, Loxoscelidae). Bull Amer Mus Nat Hist 1983; 175:264-360.
8. Hite JM, Gladney WJ, LancasterJL Jr. & Whitcomb WH. The biology of the brown recluse spider. Univ Arkansas Agric Exp Sta Bull 1966; 711: 1-26
9. Ennik F. Laboratory observations on the biology of Loxosceles unicolor Keyserling (Araneae, Loxoscelidae). Santa Barbara Museum Natl Hist Contrib Sci 1971; 3:1-16
10. Guarisco H. House spiders of Kansas. J Arachnol 1999; 27:217-221
11. Vetter, RS. Envenomation by an agelenid spider, Agelenopsis aperta, previously considered harmless. Ann Emerg Med 1998; 32:739-741.
12. Anderson PC. Loxoscelism threatening pregnancy: five cases. Am J Obstet Gynecol 1991; 165:1454-1456