I love a good natural history question to ponder! And the brown recluse spider, Loxosceles reclusa, furnishes a fine one. Academic spider researchers consider this species, one of very few spiders capable of giving humans a serious bite, a creature of the south-central portions of the United States. Although an avid student of bugs of all kinds, I’ve never seen a recluse here (or indeed anywhere else). Yet I regularly receive reports of brown recluses on the Vineyard, and recurring discussion of this species on a Vineyard Facebook page I frequent shows many Islanders firmly persuaded that brown recluses are an abundant and well-established peril here.
Biologists, more often than they like to admit, overlook data or miss changes in the distribution of wildlife. But also, untrained observers routinely misidentify spiders, often letting fear, rumor, or expectation influence their perceptions. So what’s the reality?
Well, some help comes from the basic biology of the recluse, which, because it represents a significant human health hazard, has been well studied. While its core range appears to be slowly expanding northward, perhaps as the result of our warming climate, it remains essentially a spider of the southern plains states, ranging north about to the level of downstate Illinois.
Newly hatched recluses don’t “balloon” like some other spiders do, drifting on the wind with a strand of silk for buoyancy. And adults don’t generally range far from their home site. So this is not a species that disperses well on its own. Moreover, the brown recluse, the northernmost of a mostly tropical and subtropical complex of species, does not like cold: One experiment found that prolonged exposure to temperatures in the low 20s reliably kills this spider. So the ability of brown recluses to get here on their own, or to establish enduring outdoor populations on the Vineyard, is questionable.
However, brown recluses, like many other species, sometimes disperse with the unintentional aid of humans. Recluses are known to hide in produce, packaging, or other goods being shipped, and in theory they could arrive nearly anywhere by this means. Once arrived, a recluse could certainly bite someone, and if a fertile female were transported, her offspring could start a new population. There are documented cases, mainly around port areas in warmer parts of the East and West Coasts, of such transported recluses setting up shop.
But even this mechanism for establishment turns out to be far less frequent than rumor would have it. A biologist with the Florida Department of Agriculture, investigating hundreds of purported brown recluses bites in the Sunshine State (edis.ifas.ufl.edu/in576), was only able to verify a handful of solid records of this spider occurring in Florida, and was able to identify only a single bite that could unequivocally be attributed to a recluse.
Other studies have found that most skin lesions blamed on brown recluse bites actually have other causes, with secondary bacterial infections following minor cuts or nonvenomous bites perhaps the most common one. A major 2008 study in the Journal of Arachnology, viewable at americanarachnology.org/JoA_free/JoA_v36_n1/arac-36-1-150.pdf, presents the best summary of such research.
Moreover, the spider ID skills of the general public prove to be pretty sketchy: According to the Wikipedia account for this spider, of nearly 600 spider specimens submitted to researchers as suspected recluses in California, only one proved to be the real thing. Even doctors and pest control specialists, unless they’ve had special training dedicated to spiders, are demonstrably not up to the task of correctly IDing recluses, or their bites, consistently. For many people, any orange butterfly is a monarch; likewise, many observers, motivated by fear and not well informed on the diversity of spiders or the proper means of identifying them, summarily execute any brown spider they find because they’re “sure” it’s a recluse.
The brown recluse is a smallish spider, its body roughly half an inch long and its stout legs spanning about the diameter of quarter. The legs have fine hair on them, but no spines or markings. Recluses have a distinctive arrangement of eyes, six of them (eight is the most common number in spiders), arranged in three pairs. If you’ve killed a “huge” spider, or a “hairy” one, or one with more or fewer than six eyes, or one with spindly legs, you didn’t kill a recluse. And as its name suggests, this spider is mainly nocturnal, a skulker in dark crevices. So if you killed a spider walking about in daylight, that one probably wasn’t a recluse, either.
So what does this all add up to? While some uncertainty remains about the status of the brown recluse on the Vineyard, I’m unaware of any evidence rising much above hearsay that they occur here. Most reports of the species come from people with no particular ID skills, and don’t involve a good photo or (an intact) specimen. If the patterns established elsewhere outside this spider’s core range also apply on the Vineyard, most Island reports are incorrect and most lesions blamed on brown recluse bites are the result of other causes.
Absence of proof, to be sure, is not proof of absence. There’s no reason individual brown recluses couldn’t be transported here, and they could establish colonies, though probably small and short-lived ones, within human structures. If your house had such a colony, you’d likely have the impression that recluses were everywhere! Likewise, there could surely have been genuine recluse bites here. Given that the lesions caused by the bites of this spider can be gruesome and slow to heal, we should not complacently dismiss the possibility of brown recluses here: This is one of the few spiders that really does represent a threat, and nobody wants to see it become established.
But the biology of the species, and what is known of it from studies conducted elsewhere, suggest that there’s no need to panic. Brown recluses appear unsuited to establishing themselves here in any meaningful way. The arrival of introduced individuals is probably infrequent. And even in areas where this generally docile spider is common, it doesn’t live up to its fearsome reputation. Stay alert for the brown recluse, by all means. But you can put down the blowtorch.
