By Matt Dhillon
Early on a fall morning, the stubble of a field glitters, interlaced with drooping strands of spiderwebs. Set to snare insects that flick through the grass, they’ve caught mostly dew in the still hours before the world wakes up. Spiders are all around us, as the webs they leave behind reveal, but they prefer to stay hidden from humans, sticking to shadows, cracks, caves and corners, hunting in dark, forgotten places.
Most people prefer to keep them out of sight. For many, even the thought of spiders sends a shiver down the spine. But the webs they weave are immensely beneficial to us and important in holding the ecosystem together.
Spiders are crucial in controlling insect populations. They are so effective in this role, scientists have estimated that the world would quickly be faced with widespread famine without them. Insects they consume, such as flies, ticks and mosquitos, are common disease vectors, whereas over 99% of spiders are harmless to humans.
While some people run away from spiders, biologists Marshal Hedin and Marc Milne have spent years looking for them. In February, they co-authored an extensive, 130-page study culminating over 30 years of collected specimens — approximately 2,100 — which identified 10 new species that had previously managed to stay hidden from scientists in the caves of Appalachia.
The Nesticus genus, to which these spiders belong, now contains 37 known species in Appalachia and almost 300 worldwide. Nesticus are in the family of comb-footed spiders, named for the serrated bristles on their hind feet that they use to pull silk from their spinnerets.
“Nesticus is unique, however, in the family because they are largely cave-adapted, and they live in subterranean habitats,” Milne says.
Suited to their environment, these spiders are troglomorphic, meaning they’re adapted to the perpetual darkness of caves.
“When we talk about troglomorphy, we’re looking at features such as a smaller body size, having reduced eyes, sometimes the eyes are even missing, and reduced pigmentation, so a lot of them will have a lighter color,” Milne says. “Most Nesticus have an orangish color, but some that are more cave-adapted are a lighter shade of tan.”
Life in these dark reaches can be delicate. There are not many resources to go around. Troglobites lose the features that are no longer useful to them, leaving some eyeless, with a milky or translucent color. Many cave-dwellers develop exaggerated sensory organs to compensate. For Nesticus, the spider’s web is particularly potent in the cave’s confines.
“It’s the opposite of that perfect, round, Charlotte’s Web that you think of,” Milne says. “A Nesticus web will be between a couple of blocks and it’s a bunch of threads of silk going here and there. It’s what we call a hackle-band web, and they commonly hang upside down in the middle of that. And they’ll wait for little arthropods to come along, and they’ll consume them.”
In their subterranean habitat, those arthropods will usually be tiny insects like springtails, beetles and millipedes. The spiders will be preyed upon in turn by bats and small mammals. Spiders are often an important bridge between invertebrates and vertebrates in the food web, according to Milne.
One of the incredible things the study found, Milne says, is that the spiders avoid living together. “So almost always a cave would have a single species of Nesticus and then no other. They just refuse to live near each other.”
The Nesticus spiders that the team studied occupy a microhabitat in Appalachia and are specifically adapted to a unique home. A lot of these species are also considered micro-endemic.
“An endemic species is known from a particular region and only that region,” Milne says. “But a micro-endemic is known from a very, very narrow region of the world and only that region.”
Just how narrow that range can be is eye-opening.
“We know of three different species, at least, where they are only known from one cave in Appalachia and that’s it, in the entire world,” Milne says.
The isolation of cave habitats in Appalachia allows for this incredibly unique and specific evolution. Species can adapt relatively undisturbed. For example, Nesticus dykemanae, named in honor of the environmentalist Wilma Dykeman, lives only in a few locations near the headwaters of the West Prong of the Little Pigeon River.
“That’s why Appalachia is such a great place to study things,” Milne says. “We call them relict populations. Basically, they’ve been sitting there and adapted to their particular niche habitat for millions of years remaining seemingly unchanged.”
But even the deep caves of Appalachia may not be able to shelter these unique species from modern threats. Hedin and Milne note in their paper that several of these micro-endemics species deserve conservation attention.
“Because these micro-endemics are all cave adapted, they require really specific temperature and humidity requirements,” Milne says. “Temperature is usually a good bit cooler in caves and humidity is a lot higher. We’re talking close to 100% humidity within most of these caves.”
Milne explains that climate change is causing many of these caves to get warmer and deforestation is causing them to be drier. Identifying Nesticus species as worthy of protection might help preserve the caves they live in from development and excessive visitation by humans.
“Appalachia is such an old mountain range and there are so many unique habitats that don’t exist anywhere else in the US,” Milne says. “It’s almost like islands for these unique habitats, and so you have a lot of these species, spiders included, but not just spiders, that adapt to these Appalachian habitats, and they’re found nowhere else.”
Appalachia, with its unique geography and temperate climate, has a wide range of micro-habitats. One of the factors that makes Appalachia particularly abundant in endemic species is the age of the mountains. Life there has had millions of years to adapt to the unique habitat, according to Milne, and there are likely undiscovered species still waiting to be found in the hills and hollers.