Venomous brown recluses exist within a smaller range than many realize, and their existence may be threatened by climate change.

The brown recluse spider is well-known for its appearance and poisonous bite. It is the most common and widespread of the brown spiders, but it is found only in the south and central United States.

Brown recluse spiders live in a region comprising Kansas, Oklahoma, Texas, Louisiana, Arkansas, Missouri, Mississippi, Alabama and parts of Georgia, Tennessee, Kentucky, Ohio, Indiana, Illinois, Iowa and Nebraska. According to the entomology department at the University of California, Riverside, if you do not live in those areas, "it is highly unlikely that you have a recluse spider. It is possible but incredibly unlikely."

This map shows the range of different species of recluse spiders, including the brown recluse (reclusa, in red); Texas recluse (devia, in yellow); Big Bend recluse (blanda, in green); Apache recluse (apachea, in light blue); Arizona recluse (arizonica, in blue); and desert recluse (deserta, in purple). (Image credit: University of California, Riverside)

Appearance

The brown recluse is part of the Loxosceles genus of spiders. Members of this group have violin-shaped markings on the top of their cephalothorax (fused head and thorax) and may be informally referred to as fiddleback or violin spiders, according to The Ohio State University Extension Entomology Department.

The brown recluse's violin marking can vary in intensity depending on the age of the spider, with mature spiders typically having dark violin shapes, according to The Ohio State University. The violin shape points toward the spider's bulbous abdomen. The violin shape is easy to misinterpret, so it is best to look at the eyes when determining if a spider is a brown recluse.

The recluse's eyes are one of its most distinctive physical characteristics. "They have six eyes, instead of eight like most spiders," said entomologist Christy Bills, invertebrate collections manager at the Natural History Museum of Utah. Other types of spiders have eight eyes arranged in rows of four. Recluses, however, have six equal-size eyes arranged in three pairs, called dyads, in a semicircle around the front of the cephalothorax.

Another distinguishing characteristic of the brown recluse spider is its uniformly colored abdomen (though the shade of brown varies from spider to spider) covered in fine hairs, which give it a velvety appearance. Their long, thin legs are also covered in fine hairs. According to the Integrated Pest Management Program at The University of California, Berkeley, the scientific name Loxosceles means "slanted legs," and refers to the fact that recluse spiders hold their legs in a slanting position when at rest. Bills also noted that the brown recluse's legs do not have spines, only fine hairs.

Ohio State University reports that the brown recluse is typically about three-eighths of an inch long and about three-sixteenths of an inch wide (about 1 centimeter long and half a centimeter wide), with males being slightly smaller than females but possessing longer legs.

Classification/taxonomy

According to the Integrated Taxonomic Information System (ITIS), the taxonomy of brown recluse spiders is:

Kingdom : Animalia

: Animalia Subkingdom : Bilateria

: Bilateria Infrakingdom : Protostomia

: Protostomia Superphylum : Ecdysozoa

: Ecdysozoa Phylum : Arthropoda

: Arthropoda Subphylum : Chelicerata

: Chelicerata Class : Arachnida

: Arachnida Order : Araneae

: Araneae Family : Sicariidae

: Sicariidae Genus & species: Loxosceles reclusa

Habits

The brown recluse gets its name from its color and its "shy nature," Bills said. "Most spiders go out of their way to avoid humans, which makes sense, considering we are thousands of times larger than they are and don't have a great record of behaving politely toward them."

Brown recluses often hide in dark, secluded places, like under porches or deep in closets. The brown recluse thrives in man-made areas, and may be found under trash cans, tires, etc. It is primarily nocturnal and lays its eggs from May to July.

Brown recluse spiders get around by hitchhiking on furniture boxes and other items from infestedstructures, according to the Illinois Department of Public Health. They are well adapted for establishing themselves by hitchhiking. They are long-lived, can go for many months without eating, and are adapted to the hot, dry conditions found in many structures. What's more, a female brown recluse needs to mate only once to produce eggs throughout her life, and can produce 150 or more spiderlings in a year. Thus, a single female hitchhiking into a structure is all it takes to establish an infestation. The need to inspect items before moving them in is clear.

Once established within a structure, brown recluses are often difficult to control. Though hundreds of brown recluses may be present in a house, they may not be easily observed because of their reclusive, nocturnal habits.

Brown recluse bite

The brown recluse has a venomous bite, and anyone bitten should seek immediate emergency medical help, according to the National Institutes of Health.

Like most spiders, the brown recluse typically only bites when disturbed — though it is possible to inadvertently threaten them. The Texas A&M AgriLife Extension Program reports that this may happen if a spider is caught in bedding or clothing.

"People react differently to bites," Bills said. According to The Integrated Pest Management Program at UC Berkeley, 90 percent of bites heal without medical attention or scarring. Reactions to a brown recluse bite vary depending on the amount of venom injected and the individual’s sensitivity levels, reports The Ohio State University. Some people may experience a delayed reaction, others an immediate reaction, and others no reaction at all. Many brown recluse bites leave a small red mark that heals quickly, and the vast majority of bites do not leave scars.

For those with higher sensitivity levels, a small white blister appears at the bite site soon after the bite. The tissue may become hard. Lesions are dry, blue-gray or blue-white patches with ragged edges surrounded by redness. This color pattern has yielded the nickname "red, white and blue," and, in severe reactions, the bite site can develop a "volcano lesion," according to The Ohio State University. The damaged tissue becomes gangrenous and leaves an open wound that can be as large as a human hand. It can take eight weeks or longer for full recovery, and scars may result.

According to the NIH, symptoms of a brown recluse bite may include itching, chills, fever, nausea, sweating and a general feeling of discomfort or sickness.

After being bitten by a brown recluse spider, a 10-year-old girl in Mexico required two medical procedures to remove blackened, dead tissue from her leg. (Image credit: The New England Journal of Medicine ©2013)

Treatment

There is no effective commercial antivenin. If you are bitten, the NIH recommends calling 911 or poison control or getting to an emergency room immediately.

The NIH says you should wash the area of the bite with soap and water, then wrap ice in a washcloth and place it on the bite area for 10 minutes. Remove the washcloth for 10 minutes, and repeat the process.

Then, go immediately to the emergency room and bring the spider, if possible, for identification purposes.

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