The Spider That Came in From the Cold

The recluse spiders (Loxosceles spp.) have been held up as perfect examples of pests that are intrinsically resistant to casual attempts at control, especially when it comes to the glib advice on “sealing” or “pest-proofing” that characterizes much of the IPM advocacy literature.

Particularly as tiny juveniles, these highly thigmotactic arachnids can easily penetrate into most types of structures through a variety of crevices that are far too numerous and obscure to ever locate, much less eliminate. Where food and harborage are abundant, the spiders can build up to astounding numbers, although their habit of sheltering in dark and secluded crannies makes even the densest populations surprisingly cryptic.

BUILDINGS ARE BETTER. From an ecological viewpoint, Loxosceles is one of the best examples of organisms that are almost perfectly preadapted to utilize buildings as highly improved versions of the type of habitats in which they evolved. Many of the most striking features of these spiders — the reduced number of eyes; long, thin legs; starvation resistance; and diminished intraspecific aggression — are characteristic of arthropods that are specialized for life in caves, and the genus contains numerous species that are commonly found in subterranean environments. Quite a few of them live nowhere else.

On a global basis, the über-recluse is Lo. rufescens, commonly known as the Mediterranean recluse because it is believed to have evolved in that general region. It is the most widespread Loxosceles in the world, having surreptitiously hitchhiked throughout history from Europe, North Africa and the Middle East to much of southern Asia, Japan, Australia, numerous Atlantic and Pacific Islands and the United States.

Across most of its Old World and Oceanic range, the species exploits virtually any covered or claustral habitats available, indoors or out — under bark, stones, human debris, leaf litter or within buildings. It also shows markedly troglophilic tendencies, and has become a dominant arthropod in a limestone cave in Thailand and some of the drier volcanic caves on the Hawaiian island of Kauai.

The subterranean connection has been taken to the extreme in the United States and southern Canada. Lo. rufescens has now been collected from 26 states, from Massachusetts to California, and probably occurs in all of them. But it has so far never been found outdoors, even in warmer areas that have no native congeners that might serve as competitors. Nor is it a common inhabitant of residential dwellings — almost all of its identified New World abodes have been inside large governmental, academic or commercial structures, and thus the species is essentially restricted to cities (and, of course, college towns).

Many of the most striking features of these spiders — the reduced number of eyes; long, thin legs; starvation resistance; and diminished intraspecific aggression — are characteristic of arthropods that are specialized for life in caves.

THE BATHTUB SPIDER. Since a typical state or city record involves one or a small number of specimens encountered in a single building, they have been traditionally regarded as representing isolated introductions due to the spider’s pronounced anthropochoric (dispersed by humans) tendencies. However, the advent of widespread sticky trap deployment by the pest management community that began in the 1990s has revealed there is much more to the story, particularly throughout the densely populated corridor from Washington, D.C. to Boston.

In this hyperurbanized region, Lo. rufescens is widespread and common, but hardly ever detected by casual inspection because its densest concentrations are typically far below ground amidst the old masonry foundations that constitute the roots of the metropolitan ecosystem. Living for the most part in complete darkness, preying extensively on vast herds of cockroaches and termites, they have colonized the Western Hemisphere as true urban troglophiles — tiny Morlocks beneath our feet.

Which brings us to Sheila Parram’s November 1978 encounter with an unusual spider in the bathtub of her home in Bellevue, a suburb of Johannesburg, South Africa. No doubt many of its ancestors had been similarly noticed over the past 2,000 years of permanent human settlement in that region, and either ignored or promptly dispatched. However, Parram herself was unusual in that she was fascinated by arachnids and regularly sent specimens to the leading South African arachnologist at the time, Gerald Newlands.

Newlands immediately recognized it as a new species, published its formal description three years later and christened it Loxosceles parrami in honor of its discoverer. A striking photograph of a preserved adult female is included in his paper, its incongruously small body relative to its elongate, skinny legs making it look very much like a pycnogonid (“sea spider”).

Lo. parramae (the spelling was corrected in 2012 since the grammatical rules of zoological nomenclature mandate that a species name Latinized from a modern female name must end with an appropriate female suffix) is not the only arthropod species that has revealed itself to science in a structural environment. It is actually one of two to do so in the giant pitfall trap known as a bathtub, sharing the distinction with a tiny, blind diving beetle that inhabits underground aquifers and was first collected by an Oregon couple in 1984.

What makes this spider so noteworthy — so disconcertingly noteworthy — is that 40 years after its discovery, we still have very little idea where it lives in the wild. The Transvaal (essentially northeastern South Africa, where Johannesburg is located) constitutes one of the planet’s oldest karst areas, and its complex maze of largely inaccessible, subterranean fissures is home to an assortment of distinctive, cavernicolous Loxosceles, several of which have only recently been described.

Newlands noted that Lo. parramae much more closely resembles the other known cave-dwelling forms (reduced eyes, plainer pigmentation, characteristic male palp anatomy) than their relatives found above ground on the savanna. However, encountering this species anywhere outdoors is still exceedingly rare, and it was only in 2017 that the first non- anthropogenic, underground location inhabited by Lo. parramae was reported.

On the other hand, there is no difficulty whatsoever to collecting large numbers of this species in structures within about a 30-mile radius in the Johannesburg area. Much like Lo. rufescens, Lo. parramae thrives in older stone or masonry foundations in dry, dusty microhabitats that at least superficially resemble the Transvaal caves, readily penetrating the interiors of both residential and commercial buildings. Due to its frequently dense indoor populations and the hemorrhagic/necrotizing ability of its venom that is typical of its genus, Lo. parramae is considered to be one of the most medically important spiders in South Africa.

Gerald Newlands, described by one colleague as a “multi-talented genius,” (he is best known as an inventor in the field of microphotography) left South Africa in 1992 during its period of exceptional unrest prior to the end of apartheid, began a new career in the University of Calgary’s (Canada) Department of Archaeology, and never looked back at a considerable body of unfinished work with Loxosceles.

For many years, his native homeland has had a thriving arachnological community, so it is only a matter of time before Lo. parramae yields up its secrets. But for the moment, the delicate little species is an enigma. It is the spider that came in from the cold, poster bug for urban entomology’s challenging future.

The author is an industry consultant and longtime “Bug Czar” of the U.S. General Services Administration who retired from government service in 2018.