The fearsome reputation of the brown recluse spider (Loxosceles reclusa), shown here, and its relatives far outpaces their actual impact, researchers in Florida say. Their new study concludes that recluses are in fact scarce, reluctant to bite, and in most cases unlikely to cause the severe harm so often attributed to them. (Photo by Louis A. Coticchio)

By Ed Ricciuti

Ed Ricciuti

“Everything’s a brown recluse.” That’s what a would-be wag quipped on a Facebook spider identification page, in response to the interminable photos of brown spiders posted by people panicked that they might be sharing their digs with the feared brown recluse spider (Loxosceles reclusa). Most aren’t. But, even if they were, it is no big deal because, according to researchers in Florida, recluse spiders are not nearly as scary as advertised.

“Recluse spiders are reluctant to bite. Recluse spiders are unlikely to account for most necrotic skin wounds attributed to them,” say Louis A. Coticchio of the University of South Florida and Deby L. Cassill, Ph.D., of the University of Florida in a study published in February in the Journal of Medical Entomology.

They qualify their conclusion by noting that their study was confined to Florida, although Coticchio says it busts myths about recluse spiders anywhere in the country. The authors hope their research will reduce the public perception that recluse spiders are like the proverbial monster under one’s bed. “Media coverage highlighting these rare but severe outcomes has fueled the public’s fear of spiders in general and recluse spiders in particular,” the authors write.

“Recluse bites are defensive, not aggressive,” says Coticchio, who conducted the primary research. As proof that these spiders do not prowl around looking for people to bite, Coticchio describes a check he made for recluses at a business, whose owner insisted there were none there. Coticchio shined a flashlight around a cabinet and immediately found three of them crawling where workers were likely to place hands and fingers, yet no one had been bitten.

Louis A. Coticchio of the University of South Florida, conducted a five-year study documenting recluse spiders in several locations in Florida, as well as testing their propensity to bite in lab experiments. Of the 220 homes, businesses, and other properties eh checked, breeding populations of recluse spiders turned up in only 19 sites, and none were the brown recluse spider (Loxosceles reclusa)—instead, they were all an introduced relative, the Mediterranean recluse spider (L. rufescens). (Photo by Mike Ringer, Skill Capture Media)

Even when a brown recluse spider does bite, for example, symptoms are usually mild, with only 10 percent producing the severe, necrotic lesions that popular imagination sees as inevitable. “In the final analysis, the risk to humans has been overstated, warranting a reevaluation of their role in suspected necrotic lesions by the medical community and the public,” Coticchio and Cassill write.

Physicians should heed the study, says Cassill. “Lou’s study matters most because many skin wounds are routinely blamed on ‘spider bites ‘without evidence,” she says. “That can delay proper diagnosis and treatment of infections, allergic reactions, or other medical conditions. My hope is that the AMA [American Medical Association] and the FMA [Florida Medical Association] contact Lou to present his findings at their annual conferences.”

During the five-year-study, ending in 2024, Coticchio looked for recluses in St. Augustine, Orlando, and several other locations in Florida. In the laboratory, he pinched and prodded recluse spiders with laboratory tools tipped with soft material that replicates human skin. It took major provocation to elicit a bite from a recluse spider, akin to when attacked by a predator or the squeeze it can feel when trapped between tight clothing and a human arm or leg. Indeed, the only person who reported a bite during the survey of properties where recluse spiders were found was a woman who experienced exactly that sort of contact. She had shared her home with recluses for years.

Of the 220 homes, businesses, and other properties checked, breeding populations of recluse spiders turned up in only 19 sites (and ranged from of one to 171 individuals). Moreover, none were the brown recluse spider (Loxosceles reclusa)—instead, they were all an introduced relative, the Mediterranean recluse spider (L. rufescens). This, say the researchers, counters the misconception that brown recluse spiders, or recluse spiders in general, are abundant in Florida.

The verified United States distribution of the brown recluse, the authors note, centers in the Midwest, westward to Colorado and New Mexico and eastward to northern Georgia. The brown recluses used in the study came from Kansas.

Eleven species of recluse species live in the United States. They can live in homes but mostly are found outdoors, under rocks, logs and similar concealed spots. Their similar appearance also can make them tricky to differentiate, as illustrated by this pair of spiders, a brown recluse (Loxosceles reclusa) at left and Loxosceles laeta, sometimes known as the Chilean recluse, at right. (Photos by Louis A. Coticchio)

All told, 11 recluse species live in the United States. They can live in homes but mostly are found outdoors, under rocks, logs and similar concealed spots. Next to the brown recluse, the Mediterranean has the most extensive range, found scattershot in almost two dozen states, from New York City to Texas. As the climate warms, the species could expand its range by hitchhiking along with humans, in shipping crates or luggage, the researchers suggest. The remaining species have limited, even tiny, ranges, mostly in southwestern deserts and parts of California.

“Some of the ranges are so small they are only a dot on the map,” says Coticchio.

Perhaps the most common misconception about recluse spiders of all, and one that causes considerable misidentification, Coticchio says, is that a dark, violin-shaped mark on a spider’s top side (in recluses on the cephalothorax) is a sure sign that its bearer is one of them. Not always so, he says. It is not always visible in recluse spiders, and several other types of spiders that have similar markings are often mistaken for recluses, to their misfortune.

In their paper, Coticchio and Cassill stressed that their findings “underscore the importance of expert identification, diagnostic caution, and public education to correct misconceptions. By disentangling myth from evidence, this study provides a clearer understanding of the behavior and medical significance of recluse spiders and highlights the need for careful evaluation of their true impact in human-dominated environment.”

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His most recent book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.