Researchers have discovered what may be the largest spider web complex ever recorded: a massive “mega-city” of arachnids housing tens of thousands of individuals inside a cave located on the border between Albania and Greece.

The structure, covering roughly 100 square meters, is home to approximately 69,000 common house spiders — also known as funnel weavers (Tegenaria domestica) — and 42,000 Prinerigone vagans, a species of sheet-weaving spider.

Although some social spider species are known to build communal webs inhabited by thousands, finding a structure of this size belonging to typically solitary species is extraordinary. Even more surprising: this is the first documented case of a cooperative web built by multiple different species.

Explorers first encountered this massive web during a subterranean fauna survey in 2022. They reported it to scientists, who conducted several follow-up visits to study the phenomenon in detail. The cave’s entrance lies in Greece, but its interior extends into Albania.

Researchers analyzed DNA samples to confirm which species had constructed the web. The most astonishing finding is that both species are normally solitary and do not share webs with other spiders. “What is most impressive in this cave is a large colonial web covering about 100 square meters of wall and housing 69,000 T. domestica and 42,000 P. vagans,” the study, published in Subterranean Biology, explains.

Never before had these surface-dwelling species been documented forming colonies. Moreover, molecular evidence showed that the cave population does not exchange individuals with surface populations.

Scientists found that certain microbes thriving in the cave ecosystem attract swarms of small mosquitoes, which become trapped in the massive web and serve as a steady food source for the spiders.

“The colonization of the Sulfur Cave by T. domestica is likely due to the abundance of food resources provided by dense swarms of chironomids that inhabit the site,” the researchers noted. Genetic analysis also revealed differences between these cave spiders and their surface relatives, suggesting they have developed unique adaptations to their subterranean environment.

Additionally, scientists believe that the absence of light in the cave may allow the smaller species, P. vagans, to coexist among funnel weavers without being detected or attacked.