The size of a “cave spider” is a frequent subject of both fear and intense curiosity, largely fueled by images of extremely long-legged arachnids. The term “cave spider” is not a scientific classification but a descriptive umbrella term for any spider living within subterranean habitats. These environments can host species ranging from tiny, blind specialists to some of the largest spiders known on Earth. Accurately gauging their measurements requires distinguishing between the length of the body and the expansive reach of their legs.
What Groups Fall Under the Term Cave Spider
The spiders found in caves are categorized based on their dependency on the subterranean environment, which influences their size and form.
Troglobites
These are obligate cave-dwellers that cannot survive outside the cave ecosystem, often exhibiting the most profound physical changes.
Troglophiles
These spiders can complete their life cycles both inside and outside of caves, making use of the stable conditions.
Trogloxenes
These temporary visitors enter the cave for shelter but must return to the surface for food or other functions.
Spiders from at least 48 different families worldwide have been documented in subterranean habitats, demonstrating the diversity of the group. Notable groups often associated with large sizes include certain species from the Sparassidae family, commonly known as huntsman spiders. Other families, such as the Linyphiidae and Nesticidae, contain many true troglobites, though these tend to be smaller species adapted for deep-cave life.
Quantitative Measurements of Notable Species
The answer to how big cave spiders are is dominated by one specific species, the Giant Huntsman Spider (Heteropoda maxima). Discovered in a cave system in Laos, this spider holds the record for the largest leg span of any known spider species globally. Its maximum recorded leg span is an astounding 30 centimeters (approximately 11.8 inches), roughly the size of a standard dinner plate.
It is important to differentiate this leg span from the body length, which is a more accurate measure of the spider’s mass. The body (cephalothorax and abdomen combined) measures only about 4.6 centimeters (1.8 inches) long. This relatively small body size, combined with the extreme leg length, contributes to the species’ overall light weight; adults weigh around 167 grams (5.9 ounces). For comparison, while the Giant Huntsman has the longest leg span, the Goliath Birdeater tarantula (Theraphosa blondi) is considered the world’s heaviest spider.
Morphological Adaptations Influencing Visual Size
Many cave spiders appear much larger than their actual body mass suggests due to specialized evolutionary traits known as troglomorphy. One of the most significant adaptations is the extreme elongation of the appendages, which dramatically increases the spider’s perceived size. These long legs are an adaptation to the dark, allowing the spider to use highly sensitive hairs and slit sensilla to navigate and detect prey through vibrations.
This leg elongation is often coupled with a reduction in body thickness and a thinning of the outer cuticle, resulting in a lighter, more delicate structure. The lack of light in deep cave zones also leads to the loss of pigmentation, giving many troglobites a pale, almost translucent appearance. This combination of pale coloration and long, spindly legs creates a visually striking profile that exaggerates their size. Species that live closer to cave entrances, like the Giant Huntsman, may retain their eyes and some pigmentation because they are not fully committed to the perpetually dark zone.