If you live in the United States, you've probably heard of the brown recluse spider. Probably the younger sister of your childhood best friend was bitten the summer she was 10 and she was in bed for a week and it's a lucky thing she didn't die. Or maybe your neighbor found a spider in her bathroom and posted a photo on Facebook and somebody "who would know" confidently identified it as a "for sure" brown recluse, and she's probably going to have to find a new place to live now.
While it's true the venom of a brown recluse spider (Loxosceles reclusa) can cause nasty necrotic skin lesions in some, and could possibly be deadly for a minute portion of the population, it's pretty unlikely that any of the stories you've heard about brown recluses are true — even if they happened to your dad's cousin.
Recluse spiders belong to the genus Loxosceles, meaning "crooked legs," so named because their legs look like a bunch of old, bent-up metal coat hangers lying in a heap. They're a hunting spider, meaning they don't sit around in a web waiting for their supper to come to them — although they get around on strands of silk and make "silken retreats," or little shelters to hang out in, they hunt their prey down on foot. Their ancestors were cave spiders, so in the wild they like the dry underneath surfaces of rocks, bark, shingles, woodpiles, etc. But like rats, racoons, flies and house sparrows, brown recluses are synanthropic, meaning their numbers increase in association with humans — and human habitations are actually very much like caves. A brown recluse loves nothing more than to lay her egg sac on the dry underside of the flap of a cardboard box and while away the hours guarding it in the eternal twilight of your attic.
But that doesn't mean brown recluses are everywhere.
"The distribution of brown recluses in the U.S. is a little like a rain cloud over the midwest," says Richard Vetter, a retired technician in the University of California Riverside Urban Entomology Department and author of The Brown Recluse Spider and articles in two medical journals — one in The New England Journal of Medicine and one in JAMA Dermatology — that have changed the way doctors diagnose spider bites. "If you're standing in the center of the storm — in Kansas or Oklahoma — you're going to get very wet, a lot of water droplets are going to hit you. If you're standing at the edge of the thunder shower in Georgia, the droplets will be much fewer and farther between."
In one 2009 paper, Vetter and his coauthors discovered that at the southeastern edge of their range in the state of Georgia, only 25 brown recluses could be found over the course of a five-year search. Compare that with the one Kansas residence in which 2,055 brown recluses were collected by the homeowner over the course of six months. The homeowner lived in the house for 11 years before knowingly sustaining a brown recluse bite, which ended up being asymptomatic.
"She reached inside a shirt, felt a pinch and the spider fell out," says Vetter. "She says she took a Benadryl and went on with her day. She was fine. I'm not sure the Benadryl did anything."