The Largest Spider Web

Spiders are arthropods that people encounter almost everywhere in daily life. A large proportion of spider species construct webs, and some of these structures can be remarkably large, with diameters exceeding one meter. But how large can a spider web actually become?

A study published in 2025 may offer a striking example. The research described a massive spider web discovered in 2022 by the Czech Speleological Society inside a sulfur cave. The web covers an area of approximately 106 square meters and may represent the largest spider web ever documented. The following sections explore how such an extraordinary structure came to exist.

The spider web described in the study（圖片來源：Marek Audy、Urák, I et al. (2025).，採用 CC BY 4.0 授權。）

Location

Natural caves form an environment completely different from the surface world. Deep inside caves, sunlight rarely penetrates, preventing plants from growing. As a result, cave ecosystems often develop biological communities that differ fundamentally from those found on the surface.

Among the many types of cave ecosystems, one particularly unusual form is based on hydrogen sulfide and oxygen chemistry. These systems are known as sulfidic subterranean ecosystems. In such environments, the primary producers are chemoautotrophic microorganisms. These microbes obtain energy by oxidizing hydrogen sulfide, allowing them to grow and produce organic matter that forms the base of the entire food web. This microbial production supports a wide range of organisms, including mollusks, arthropods, and even certain vertebrates.

Plan of Sulfur Cave; blue areas indicate sulfidic springs, and brown areas mark the location of the spider web（圖片來源：Urák, I et al. (2025). 改編自 Audy et al. (2022). and Sarbu et al. (2024). ，採用 CC BY 4.0 授權。）

The cave in which this giant spider web was discovered belongs to this type of sulfidic subterranean ecosystem. Located at the border between Albania and Greece, it hosts a remarkably rich biodiversity.

The Spider Web

Measurements, sampling, and analyses revealed that the web-covered area spans roughly 106 square meters. Rather than being a single uniform web, the structure consists of countless funnel-shaped webs interconnected into a vast network.

Two spider species inhabit this enormous web. The first is the house funnel weaver Tegenaria domestica, and the second is the much smaller sheet weaver Prinerigone vagans. Researchers estimated that the colony contains approximately 69,100 individuals of T. domestica and about 42,400 individuals of P. vagans, bringing the total spider population to roughly 110,000 individuals.

Tegenaria domestica（圖片來源：Urák, I et al. (2025). ，採用 CC BY 4.0 授權。）

Among these two species, T. domestica is responsible for constructing the web structure itself, whereas P. vagans merely inhabits the network. Initially, researchers expected that the much smaller P. vagans would be preyed upon by the larger funnel weavers. However, subsequent observations revealed no evidence of such predation.

The researchers proposed that several factors might explain this unusual coexistence. The cave environment is almost completely dark, which likely limits the visual detection abilities of T. domestica. Meanwhile, P. vagans is an ambush predator that remains nearly motionless unless prey approaches closely. Because of this behavior, the larger spiders may simply fail to detect the smaller species within the dense web structure, allowing the two to coexist in a rare state of peaceful association.

Prinerigone vagans（圖片來源：Urák, I et al. (2025). ，採用 CC BY 4.0 授權。）

Within the cave, the area covered by the spider web lies above a small stream. Large numbers of non-biting midges (chironomid flies) breed along this stream. The densest concentrations of these flies largely overlap with the area occupied by the web, strongly suggesting that these insects constitute the spiders' primary food source.

Molecular evidence indicates that the spider populations living in this cave no longer exchange individuals with nearby surface populations and have become an isolated group. The diversity of microorganisms associated with these cave spiders is also significantly lower than that observed in their surface counterparts.

Furthermore, surveys conducted repeatedly between 2023 and 2025 showed that the spider community maintains a high population density throughout the entire year. This suggests that their energy supply is closely linked to the chemoautotrophic microorganisms that utilize sulfur compounds within the cave ecosystem. Surface-derived organic resources alone would likely be insufficient to sustain such a large predator population, especially during colder seasons when external food inputs become scarce.

Conclusion

Like other sulfidic subterranean ecosystems sustained by chemoautotrophic microorganisms, the cave located on the border between Greece and Albania hosts an exceptionally rich and diverse community of invertebrates thriving in complete darkness.

The most remarkable discovery in this cave is a gigantic spider web covering approximately 106 square meters, inhabited by around 110,000 spiders. These two species normally occur on the surface and have never previously been observed forming such large colonies. Yet within the cave environment they have established an immense and densely populated community.

This study provides new insights into how surface-dwelling species can colonize sulfidic subterranean ecosystems and exploit the resources available within them.

Author: Bai Leng

Reference:

Urák, I., Vrenozi, B., Głąbiak, Z., Lecoquierre, N., Eiberger, C., Maraun, M., Ştefan, A., Flot, J.-F., Brad, T., Dainelli, L., Sârbu, Ș. M., Băncilă, R. I. (2025). An extraordinary colonial spider community in Sulfur Cave (Albania/Greece) sustained by chemoautotrophy. Subterranean Biology.