The discovery matters more than it might first appear. Troglocoelotes is the only genus in the subfamily Coelotinae composed entirely of troglobitic species, meaning every known member lives its entire life in caves. The genus currently contains twelve described species, all known from the karst landscapes of Guangxi and Guizhou provinces in China. Yet only four of those species are represented by both sexes in scientific collections. The remaining eight, including T. doul until now, have been described solely from females, leaving taxonomists without the male anatomical features that are often essential for distinguishing closely related species.

Cave-adapted spiders like those in Troglocoelotes display a suite of striking morphological changes that reflect life in perpetual darkness. Pigmentation is reduced or lost entirely, giving the animals a pale, almost translucent appearance, and the eyes regress or disappear altogether. These spiders inhabit the deep zones of limestone caves, where they construct sheet-like webs in rock crevices, in shallow depressions on the cave floor, and beneath loose stones. Such extreme specialization makes each cave system a potential island of unique evolutionary history, which is why documenting every species, and every sex of every species, carries real conservation weight.

Shenlonggong Cave itself is a biological treasure trove. Located in Dacai Township within Huanjiang Maonan Autonomous County, the cave sits in a region characterized by extensive karst terrain riddled with limestone cavities. It serves as the type locality, the place from which a species was first formally described, for several troglobitic organisms, including the springtail Coecobrya draconis and the millipede Desmoxytes scolopendroides, alongside T. doul. The repeated emergence of new species from this single cave underscores how little is still known about subterranean biodiversity in southern China’s karst belts.

The original female specimen of T. doul was collected on 1 August 2023 and is housed at the Institute of Zoology of the Chinese Academy of Sciences. The new material consists of two adult males and one additional female, collected on 10 August 2026 at an elevation of 368 meters within the same cave system. All newly examined specimens are deposited in the Centre for Behavioral Ecology and Evolution at Hubei University in Wuhan, where the taxonomic work was carried out under stereomicroscopic examination and detailed photographic documentation.

The laboratory methods used in the study reflect the precision required for modern spider taxonomy. Male palps, the modified appendages that function as intromittent organs in spiders, were dissected and photographed after removal. The female epigyne, the external genital structure, was dissected from the body, immersed in a 0.1 milligram per milliliter solution of Protease K, heated in a water bath at 56 degrees Celsius, and stained with Amido black 10B to reveal internal sclerotized structures. Measurements were taken with a Leica M205 C stereomicroscope, and multifocal images were assembled using Helicon Focus software to produce fully sharp composite photographs of minute anatomical details.

The male of T. doul is a spider of modest dimensions, with a total body length of 7.19 millimeters. Its carapace, the hardened plate covering the front of the body, measures 4.19 millimeters long and 2.74 millimeters wide, while the opisthosoma, the rear body section, measures 3.12 by 2.05 millimeters. The eyes are vestigial, meaning they are reduced to nonfunctional remnants: the anterior median eyes are entirely absent, while the remaining eyes measure between 0.06 and 0.09 millimeters across. The legs are long and slender, with the fourth pair the longest at 19.94 millimeters, followed by the first, second, and third pairs in descending order. The chelicerae, the jaw-like structures used to grasp prey, bear three teeth on the front margin and two on the rear margin, and the body and legs are covered in long hairs.

In life, the male is colored in muted tones typical of cave-dwelling arachnids. The carapace is yellowish-brown, with a light brown head region and a pale yellow thoracic region marked by a black fovea, the groove that runs down the midline of the carapace. The chelicerae are dark brown, the endites and labium are brown with white distal portions, and the sternum is yellowish-brown. The legs are light brown, while the opisthosoma and spinnerets are grey, a color scheme that offers no camouflage in the lightless world the spider inhabits but reflects the loss of pigmentation that accompanies prolonged cave isolation.

The most taxonomically significant features lie in the male palp, whose intricate architecture serves as a species fingerprint among spiders. In T. doul, the patellar apophysis, a spur on the leg segment just above the tibia, is digitiform, meaning finger-shaped. The retrolateral tibial apophysis is trapezoidal and extends nearly two-thirds the length of the tibia, with its distal end slightly projecting beyond that segment. The cymbium, the spoon-shaped terminal structure of the palp, is more than twice as long as the bulb, the sperm-transfer organ it houses. The median apophysis is spoon-shaped and reaches almost half the length of the bulb, and the conductor, a sclerotized structure that guides the embolus during mating, is strongly curved ventrally with a blunt distal end and a triangular dorsal apophysis bearing three denticles at its tip. The embolus itself is thread-like and curved, arising from the embolic base at what taxonomists describe as the 8:30 o’clock position.

These features collectively distinguish the male of T. doul from all other male-known members of the genus. In every other congener for which males have been described, the cymbium is less than twice as long as the bulb, the median apophysis is only about one-third the length of the bulb, the conductor is straight and pointed rather than curved and blunt, and the embolus arises at the 6 o’clock position rather than 8:30. The patellar apophysis is pointed rather than finger-shaped in T. tortus, and the dorsal apophysis of the conductor is rounded and blunt in T. qixianensis and T. tortus, while it is absent altogether in T. liangensis and T. yumiganensis. The female of T. doul, meanwhile, resembles that of T. sinanensis in several respects but can be separated by the presence of a projection on the lateral part of the copulatory duct and by a spherical spermatheca that is narrower than the copulatory duct, features absent or differently shaped in its relative.

For now, T. doul remains known only from its type locality, Shenlonggong Cave, a restriction that is common among troglobitic spiders but that also renders them inherently vulnerable to any disturbance of their subterranean habitat. The description of the male completes the basic morphological portrait of the species and brings the tally of Troglocoelotes species known from both sexes to five out of twelve. As cave surveys continue across the karst regions of Guangxi and Guizhou, researchers expect that more of these pale, eyeless predators will come to light, each one a reminder of how much evolutionary diversity remains hidden in the world’s deepest recesses.

Subject of Research: First description of the male of the troglobitic cave spider Troglocoelotes doul from Shenlonggong Cave, Guangxi, China

Article Title: First description of the male of Troglocoelotes doul Lin & Li, 2024 ( Araneae , Agelenidae , Coelotinae )

Article References: Wang, J., Wei, M., Zhang, H., Liu, J., & Hu, C. (2026). First description of the male of Troglocoelotes doul Lin & Li, 2024 (Araneae, Agelenidae, Coelotinae). Biodiversity Data Journal, 14, Article e214835. https://doi.org/10.3897/bdj.14.e214835

Image Credits: AI Generated

DOI: 10.3897/BDJ.14.e214835

Keywords: Troglocoelotes doul, Coelotinae, Agelenidae, troglobitic spiders, cave biodiversity, karst caves, Guangxi China, spider taxonomy, male palp morphology, Shenlonggong Cave, species description, subterranean ecology