The Brown Recluse Spider:

Other Common Names: Brown fiddler, violin spider, fiddleback spider

The brown recluse spider, Loxosceles reclusa, has a bad and largely undeserved reputation. Across the US, people fear the bite of this spider, believing it is an aggressive attacker and certain to cause devastating necrotic wounds. Research on brown recluse spiders has proven these assertions to be false.

Description:  The best known feature of the brown recluse spider is the fiddle-shaped marking on the cephalothorax. The neck of the dark brown fiddle points toward the abdomen. Other than this marking, the brown recluse is a uniformly-colored light brown, with no stripes, spots, or bands of contrasting color. The violin marking is not a reliable identifying characteristic. Young recluses may lack the mark, and other Loxosceles species also display the fiddleback detail.

Along with other Loxosceles species, brown recluses have six eyes, arranged in a semi-circle pattern of three pairs. This feature distinguishes Loxosceles spiders from most others, which commonly have eight eyes. The brown recluse lacks any stiff spines on its body, but is covered with fine hairs.

The only definitive way to identify the brown recluse spider, Loxosceles reclusa, is to examine the genitalia. With a body size of just a quarter inch long, this requires a high magnification microscope. Suspected brown recluse spiders should be brought to your county extension agent for expert identification.

Classification:

Kingdom – Animalia;  Phylum – Arthropoda;  Class – Arachnida;  Order - Araneae

Family – Sicariidae;  ;  Genus – Loxosceles;  Species - reclusa

Diet:  The brown recluse spider feeds at night, leaving the security of its web to search for food. Current research reveals the brown recluse is primarily a scavenger, feeding on dead insects it finds. The spider will also kill live prey when needed.

Life Cycle: Brown recluse spiders live about two years. The female lays up to 50 eggs at a time, encasing them in a silken sac. Most egg production occurs between May and July, and a single female may lay five times within a year. When the spiderlings hatch, they remain with the mother in her web until they have molted a few times. Over the first year of life, the spiderlings will molt up to seven times before reaching adulthood.

Special Adaptations and Defenses: Brown recluse spiders use short fangs to inject a cytotoxic venom into prey. When provoked, a brown recluse spider will bite, and this venom may cause necrotic wounds to the person or animal that has been bitten.  Venom is not the brown recluse's primary defense, however. As the name recluse suggests, this spider is quite timid, and spends the daylight hours in retreat, usually in its web. By remaining inactive during the day, the brown recluse limits its exposure to possible threats.

Habitat:  Brown recluses prefer dark, undisturbed areas with low moisture. In homes, the spiders find shelter in basements, storage closets, garages, and sheds. During the day, they may hide in cardboard boxes, folded clothing, or even shoes. Outdoors, brown recluse spiders are found beneath logs, in wood and lumber piles, or under loose rocks.

Range:  The established range of the brown recluse spider is limited to US states in the central Midwest, southward to the Gulf of Mexico. Rare and isolated encounters with brown recluse in areas outside of this range are attributed to interstate commerce. Brown recluse spiders may seek shelter in cardboard boxes, and make their way to places outside their known range in shipments of goods.

Identifying and Misidentifying the Brown Recluse Spider
The brown recluse spider, Loxosceles reclusa, is often implicated as a cause of necrotic skin lesions. Diagnoses are most commonly made by clinical appearance and infrequently is a spider seen, captured or identified at the time of the bite. The brown recluse lives in a circumscribed area of the US (the south central Midwest) with a few less common recluse species living in the more sparsely-populated southwest US 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 over-diagnosis and, therefore, inappropriate treatment.

Natural History

General information regarding recluse spider life history characteristics has been published. 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 intermittently 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 US 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 US 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. All 13 species of US recluses (11 native, 2 non-natives) 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. Although it is quite consistent in adult brown recluses (although it can fade in preserved specimens), many western US recluse species and some young brown recluses have virtually no contrasting pigmentation in the violin region. 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 US They share the same eye pattern, 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. In addition, in side view, the cephalothorax is definitively humped, an anatomical modification necessary for housing the large spitting glands that are only found within this genus. The woodlouse spider (Dysdera crocata, Family Dysderidae)  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 US

Spiders with "violin" markings

There are several common and ubiquitous non-poisonous US 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) and pirate spiders (Mimetus spp., Family Mimetidae). 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. 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.

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 over diagnosed.  A call for more judicious evaluation has been made several times.  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 over diagnosis of spider bites. Verified spider bites require the presence or sighting of a spider in the act of biting. In the absence 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.

Note:  This is the second in a series that I am doing on Spiders, enjoy - Bird