In the depths of a sulfur cave on the border between Albania and Greece, a group of researchers has made a thrilling (and Guinness-worthy) discovery: the largest spider web in the world, a structure that extends over 100 square meters. It was created — literally — by two species of spiders that, in nature, live alone and cordially ignore each other: Tegenaria domesticabetter known as the common house spider, e Prinerigone vagans.
Yet, in this case, the rules of solitude have been broken: the two species have decided to coexist, building a multi-level silk city, glued to the damp, yellowish walls of the cave. According to scholars, around 69,000 specimens of Tegenaria domestica and 42,000 Prinerigone vagans live here: a true megalopolis of arachnids, never observed before.
How a colony of spiders is born: between toxic gases, darkness and midges
The discovery took place in 2022, during an expedition by the Czech Speleological Society, committed to surveying the underground fauna of the Balkan sulfur caves. When the researchers illuminated the wall of the cave, they found themselves in front of a landscape that seemed to come out of a science fiction film: a spider web the size of an apartment, intertwined in layers, populated by tens of thousands of constantly moving spiders.
To understand how this was possible, the team analyzed the DNA samples and the morphology of the specimens. Both species, the scholars explain, are normally solitary: they do not share territory or food. But in that cave the conditions are anything but normal: total darkness, toxic air full of hydrogen sulphide, absence of vegetation and no “surface” insects to hunt.
Yet, there is an ecosystem. And it works. Thanks to isotopic analyses, the researchers discovered that the entire food system of the cave is supported by microbes that live on sulfur. These tiny organisms feed chironomid larvae, small midges that hatch in the water and end up directly in the giant silk trap. The result is an endless buffet for the two species of spiders, who have stopped fighting each other to share the abundance.
An evolutionary adaptation: when hunger becomes cooperation
The study, published in the journal Subterranean Biologyalso tells another surprising fact: the spiders that live in the cave are genetically different from those found outside. In other words, they are evolving in isolation, adapting to an extreme environment where survival requires cooperation.
According to the authors, it is the first time that “facultative coloniality” has been observed in these species: a temporary social behavior, driven by abundant resources in a hostile place. In practice, when the world above is too harsh, even loners learn to coexist.
Our findings show a unique case of adaptation and cooperation in an underground habitat fueled by chemical processes, offering new perspectives on the evolution of spiders and Earth’s hidden ecosystems.
Beyond its record size, the discovery reminds us how fragile and extraordinary the natural balances that regulate life on Earth are. In an environment that would be lethal for us, two species of spiders have found a way to coexist, thrive and survive, transforming toxicity into a resource. It may not be an example of brotherly love, but it is a perfect ecological reminder: when nature cooperates, it always wins.
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