Deep beneath the borderlands of Albania and Greece, a living curtain of silk hangs in total darkness. Draped across the walls of a sulfur-rich cavern, the web stretches over 100 square meters and shelters an estimated 110,000 spiders. Two species that normally hunt alone have built a shared fortress here, turning a hostile cave into a thriving metropolis.
Documented in the journal Subterranean Biology, the find marks the first recorded colonial web in a chemoautotrophic cave and the first time these two familiar spiders have been seen living cooperatively. It is a discovery that rewrites expectations about spider behavior and about how life knits itself together when sunlight disappears.
A cave that runs on chemical energy
This cavern, known informally as Sulfur Cave, is powered not by photosynthesis but by chemosynthesis: microbes convert hydrogen sulfide into energy-rich compounds, seeding the base of a food web independent of the surface. A warm, toxic stream around 26 °C courses through the chamber, perfuming the air with a sharp, rotten-egg tang and feeding mats of sulfur-oxidizing bacteria on the wet rock. In the gloom, white biofilms shimmer like frost, and small grazers arrive to feed.
Midge larvae and isopods scrape those microbial lawns, which in turn support swarms of adult midges, beetles, and centipedes. Close to the stream, the air teems with tiny midges at extraordinary densities, providing an inexhaustible buffet. The abundance of prey transforms the cave from a place of scarcity into one of surplus, with cascading effects on the predators that exploit it.
The architects of a mega-web
The web’s dominant residents are Tegenaria domestica, the common house spider, and Prinerigone vagans, a smaller sheet-web builder. On the surface, T. domestica is a solitary species, spinning private funnel webs under rocks and in basement corners. Here, thousands of those funnels merge into a thick, multilayered sheet that cloaks the limestone like a silken tapestry.
Perhaps most startling is what is missing: the antagonism that often defines spider society. Researchers observed no signs of the cannibalism or territorial skirmishes typical of crowded spider clusters. Instead, the two species live side by side, their overlapping webs forming a single, sprawling structure with an estimated surface area of 106 square meters. Roughly 69,000 T. domestica and 42,000 P. vagans share this space in relative peace.
Counting the uncountable
To reach those population estimates, the team identified spiders by morphology and DNA and performed systematic counts across measured sections of the colony, extrapolating to the web’s full area. They paired these surveys with stable isotope analyses to trace the colony’s energy sources. The chemical signatures of carbon and nitrogen in the spiders’ tissues pointed back to sulfur-oxidizing microbes rather than plants that rely on sunlight, confirming the cave’s closed, chemosynthetic economy.
The prey base was scrutinized as well. Adult midges, whose larvae graze the bacterial films at the water’s edge, were found at astonishing densities, explaining the near absence of food competition among the web’s residents. In a place where prey is effectively unlimited, the calculus of competition shifts, and so does behavior.
Evolution and adaptation in the dark
Genetic analyses suggest the cave spiders are not merely surface visitors. The colony’s T. domestica share distinct haplotypes compared with nearby outside populations, a hint of isolation and adaptation to subterranean life. Their microbiomes are simpler than those of surface-dwelling counterparts, reflecting the cave’s streamlined ecosystem.
Even their reproduction appears tuned to the cave’s rhythms. The T. domestica females in Sulfur Cave lay significantly fewer eggs per clutch than those above ground. In an environment with steady food and few predators, producing fewer, larger offspring can be a viable strategy, especially when living costs are higher in low-oxygen, chemically harsh conditions. The result is a population that is stable, abundant, and surprisingly cooperative.
Not the first giant web — but a unique one
Monster webs have made headlines before. In Texas in 2007, a gauzy megastructure spanned trees along a lakeside trail after a mass dispersal of long-jawed spiders. Industrial sites have occasionally hosted cavernous webs as well. Those events, however, were dominated by a single spider family, fueled by temporary prey booms, and tied to the surface world.
Sulfur Cave’s colony is different. It is the largest known persistent colonial web documented in a chemoautotrophic cave, an ecosystem that runs on chemical energy rather than sunlight. And it hosts two spider species that are ordinarily solitary, cohabiting peacefully at scale — a combination that has not been recorded before. In the chemical twilight beneath the borderlands, abundance appears to have bred tolerance.
What the mega-web reveals about life without sunlight
The colony offers a natural experiment in how resource abundance can reshape behavior. For spiders, social living is rare and usually arises in a few lineages with specialized traits. Here, two widespread, non-social species shifted toward a form of facultative coloniality, apparently because the costs of crowding were offset by the sheer volume of prey drifting through the web each day. When food is constant and competition fades, cooperation — or at least coexistence — can emerge.
There are broader lessons, too. Chemoautotrophic systems are analogs for life in other lightless realms, from deep-sea vents to subsurface aquifers and, potentially, extraterrestrial niches. Seeing a familiar house spider thriving in such a place underscores the plasticity of life and the speed with which ecology can rewrite behavior. The study adds a detailed case history to a growing body of work showing that subterranean ecosystems are not biological backwaters but mosaics of innovation.
Handle with care
For all its eerie allure, Sulfur Cave is no place for casual visits. Hydrogen sulfide can be deadly at high concentrations, and the colony’s survival likely depends on the cave’s fragile chemical balance. Disturbance — even from well-meaning curiosity — could ripple through the food web, thinning microbial films, draining the midge swarms, and ultimately collapsing the silk canopy that depends on them.
Safeguarding such sites is about more than protecting a curiosity. It is about preserving an entire self-contained world where chemistry shapes community, and where a surplus of food has dissolved the usual boundaries between hunters. The mega-web is a reminder that conservation must reach into the dark, not just bask in the light.