Palpigrades, sometimes called microwhip scorpions, remain among the least studied of all arachnid orders. They are minute, ranging from 0.7 to 2.0 millimetres in body length, generally rare, and confined to very specific habitats, usually damp soil, leaf litter, or caves. Only 138 species have been described worldwide, and most are known from just one or a few specimens. The first species, Eukoenenia mirabilis, was described from Europe in the late nineteenth century, and today 35 species are known from the continent, all within the family Eukoeneniidae. On the Iberian Peninsula and the Balearic Islands, eleven species had been recorded before this discovery, occupying a wide ecological range from endogean life under rocks and in soil to strictly subterranean existence in caves.

The new species was collected in December 2021 from Sima de Pla de les Llomes, a 220-metre-long epigenetic vadose cave developed in Middle Triassic dolomites at 570 metres above sea level. The cave is entered through an eleven-metre vertical shaft that opens into a labyrinth of crawl spaces, shafts and pseudo-galleries aligned along the region’s dominant north-south fracturing. The specimen was found in the last chamber on the southern side of the cave, in the deep zone where moisture condenses on the walls. That chamber is already famous among subterranean biologists: it also hosts three other troglobitic species of special interest, including the silverfish Coletinia longitibia, known exclusively from this single cave, the cicada Valenciolenda fadaforesta, and the dipluran Paratachycampa penyoensis.

The holotype, a single male specimen, was collected with a small brush, preserved in 70 percent ethanol, cleared in lactic acid and mounted on slides in Hoyer’s medium for microscopic study. Its body measures 1596 micrometres without the flagellum, and the detailed description records an intricate anatomy: a frontal organ with two pointed branches, chelicerae with nine serrated teeth, a precise arrangement of setae across the coxae of all four leg pairs, and a male genitalia structure with three lobes bearing characteristic setal patterns. The species name honours the municipality where the cave lies, and the type specimen is deposited in the Scientific Collections Centre of the University of Almeria.

What makes Eukoenenia serrana remarkable is not merely that it is new, but how strongly it displays the classic hallmarks of cave adaptation. It is large for a palpigrade, and its appendages, particularly the first pair of legs, are dramatically elongated. Yet it retains a diagnostic feature of its species group, the Eukoenenia mirabilis-berlesei group: a lateral organ on the prosoma with only a single blade. That combination of pronounced troglomorphy with a primitive sensory configuration turned out to be the key to the study’s most surprising finding.

To understand how cave life reshapes these animals, the researchers compiled morphometric data from published descriptions of all eight species in the E. mirabilis-berlesei group, comparing four species traditionally regarded as soil-dwelling, or endogean, with four cave-adapted, or hypogean, relatives. Using linear measurements and derived ratios, along with two-tailed Welch’s t-tests that account for unequal variances and sample sizes, they quantified the percentage of elongation for each character. The results were unambiguous: cave species show significantly larger values for nearly every appendage measurement, with elongation concentrated in the limbs rather than the body.

The numbers tell a vivid story. Tibia I, the tibia of the first leg, increased by roughly 78 percent in hypogean species when comparing group means, and by about 85 percent when comparing medians, among the highest values recorded for any measured character. Tibia IV followed closely with a mean increase of about 78 percent, while the basitarsus of leg IV lengthened by roughly 64 percent and the combined tarsus-tibia segment of leg I by about 69 percent. The palpal tibia, though still significantly longer in cave species, showed a more modest mean increase of about 61 percent. By contrast, overall body length and prosomal shield length increased more moderately, by roughly 19 to 24 percent, suggesting that cave adaptation in this group favours stretching the limbs far more than enlarging the body.

One ratio, however, stubbornly refused to change. The proportion between the basitarsus and tibia of leg IV remained statistically indistinguishable between endogean and hypogean species, indicating that when leg IV elongates in cave-dwelling palpigrades of this group, both segments lengthen in concert. This finding partially contradicts earlier work from the 1990s, which had reported differences in this ratio between soil and cave species across palpigrades more broadly. The new analysis suggests that within the E. mirabilis-berlesei group specifically, proportional elongation of leg IV segments is the rule, and that the ratio is not a reliable marker of cave adaptation for this lineage.

Perhaps the most intriguing result concerns the lateral organ, a sensory structure on the anterior prosomal margin whose number of blades has long been used as an index of subterranean adaptation. In highly troglomorphic Eukoenenia species elsewhere, the organ can carry up to 16 elements on each side, and an increase in blade number has been interpreted as a hallmark of advanced cave life. Yet Eukoenenia serrana, despite its dramatically elongated limbs and large body, retains just a single blade on each side, as does the closely related cave-adapted E. indalica from southern Spain. This retention implies that sensory and appendicular traits can evolve independently, and that multiplying the blades of the lateral organ is neither a prerequisite nor a necessary companion of limb elongation in cave-dwelling palpigrades.

The broader significance of the study lies in its demonstration that troglomorphic evolution is not a uniform package of changes but a mosaic, with different characters responding to distinct functional pressures and evolutionary pathways. Leg I in palpigrades functions more like a sensory antenna than a walking limb, held aloft to probe the environment, while leg IV contributes more to support and locomotion. The pronounced elongation of tibia I suggests that tactile exploration in dark, crevice-rich microhabitats may have imposed particularly strong selective pressure on this sensory appendage. As the twelfth palpigrade species known from the Iberian Peninsula and the fourth troglomorphic member of its species group, Eukoenenia serrana adds a new piece to a puzzle that is only beginning to come into focus, reminding scientists that even the tiniest and most obscure arachnids can hold profound lessons about how evolution sculpts life in the planet’s most isolated habitats.

Subject of Research: Morphological adaptation of palpigrade arachnids to subterranean cave environments

Article Title: Troglomorphic trends of palpigrades in the Eukoenenia mirabilis-berlesei group, and discovery of a new cave-dwelling species from Spain ( Palpigradi , Eukoeneniidae )

Article References: Barranco, P., Sánchez-Camacho, F. J., & Mayoral, J. (2026). Troglomorphic trends of palpigrades in the Eukoenenia mirabilis-berlesei group, and discovery of a new cave-dwelling species from Spain (Palpigradi, Eukoeneniidae). Subterranean Biology, 57, 117-138. https://doi.org/10.3897/subtbiol.57.202620

Image Credits: AI Generated

DOI: 10.3897/subtbiol.57.202620

Keywords: Palpigradi, Eukoenenia serrana, troglomorphy, cave adaptation, Subterranean Biology, Spain, morphometrics, arachnology, endogean species, hypogean species, lateral organ, new species

The family Leptonetidae, to which the new species belongs, comprises some 400 described species distributed across 22 genera worldwide. These spiders are quintessential denizens of cryptic habitats, occupying caves, leaf litter, soil crevices, and the spaces beneath stones. Many cave-dwelling leptonetids display classic troglomorphic features: elongated appendages, reduced pigmentation, and diminished or entirely absent eyes, all reflecting long evolutionary isolation from the sunlit surface world. What sets the family apart morphologically is a distinctive six-eyed arrangement, with four eyes clustered in a compact anterior group and two others positioned behind them, although in some troglophilic species even these vestigial structures have been lost altogether.

Identifying leptonetids is notoriously difficult. Their minute bodies, simplified female genital structures, and limited external morphological differentiation mean that taxonomists often depend on the finer details of male palpal anatomy, sometimes requiring high-resolution microscopy to resolve. The researchers behind the new description, led by István Urák, Serban Sarbu, and Tamás Szűts, relied on an integrative approach combining scanning electron microscopy, digital imaging, detailed drawings, and DNA barcoding to establish that their specimens represented something genuinely new. The holotype male was collected in November 2023 from Skordili’s Sulfur Cave, a system whose entrance lies in Greece but whose deeper galleries extend beneath the Albanian border.

The diagnostic feature that sets Cataleptoneta sulfur apart from all of its seven known congeners is remarkably specific: three setae on the male palpal tarsus whose bases are modified into a shape resembling a three-petaled tulip. Under the scanning electron microscope, these delicate structures are clearly visible on a retrolateral outgrowth of the tarsal depression, accompanied by a thin bent seta, a thick apical bent seta, and a basal straight seta. Such fine-scale morphological characters are precisely the kind of evidence needed to distinguish species in a family where gross anatomy offers few reliable markers. The male holotype measures just 1.26 millimeters in total length, with a pale, highly depigmented body, pale yellow carapace, and long slender legs arranged in the characteristic leptonetid formula.

Females of the new species are similar in coloration but slightly larger in body size with shorter legs. Their internal genitalia are simple, featuring a pair of spermathecae connected to weakly sclerotized, very elongate S-shaped sperm ducts with a characteristic inwardly directed medial twist. Both sexes possess six eyes, though the posterior lateral eyes are absent in some specimens, a further hint of the species’ deep adaptation to lightless environments. The spiders build simple, flat webs on cave walls and speleothems in moist areas, often close to sulfidic streams and lakes, and they feed on collembolans that graze on the rich chemoautotrophic sulfur-oxidizing biofilms coating the cave walls.

The biology of these caves is what makes the discovery truly extraordinary. Skordili’s Sulfur Cave is renowned for its exceptionally rich chemosynthesis-based subterranean community, an ecosystem powered not by photosynthesis but by microbes that derive energy from hydrogen sulfide. This invertebrate-driven food web supports an unusually dense array of life, including what researchers have described as an extraordinarily large spider web harboring dense colonies of spiders. The new leptonetid sits within this community alongside other small predators such as mites, pseudoscorpions, carabid beetles, and centipedes, all ultimately dependent on the same autochthonous sulfur-based food source. Because conditions in the caves remain constant year-round, males, females, and juveniles of the new species have been observed in every season, unaffected by the daily and seasonal fluctuations that govern life on the surface.

On the molecular front, the team obtained two COI mitochondrial barcode sequences, one from a Skordili’s Sulfur Cave specimen and one from Turtle Cave, each 642 base pairs long. The two sequences represent distinct haplotypes, one unique to each cave, with a p-distance of 0.008 indicating low but real genetic differentiation between the cave populations. Two independent species delimitation methods, ASAP and bPTP, both clearly recognized Cataleptoneta sulfur as a distinct species. The finding of two separate lineages within a single species, each confined to its own cave, illustrates how quickly populations can diverge when subterranean habitats act as evolutionary islands, cutting off gene flow between even geographically close systems.

The phylogenetic analysis also delivered surprises with implications beyond the new species itself. Using a maximum-likelihood approach on COI sequences, the researchers found that the genus Cataleptoneta, as currently circumscribed, is not monophyletic. In both the full and reduced datasets, only the genus Sulcia emerged as a natural group, while both Cataleptoneta and the related genus Barusia were rendered paraphyletic. This echoes the findings of a 2021 phylogenomic study by Ledford and colleagues, which used ultraconserved elements and reached a similar conclusion. The new species was placed in a basal position within the Paraleptoneta group in the larger analysis, and in a clade with Cataleptoneta lingulata and several Barusia specimens in the pruned dataset. Resolving these tangled relationships, the authors note, will require a well-sampled integrative study, since the paraphyly may stem from plesiomorphic characters being used as diagnostic traits.

With the addition of Cataleptoneta sulfur, the genus now includes nine species worldwide and eight in Europe, all concentrated in the eastern Mediterranean region. The distribution pattern suggests that further species likely await discovery in Mediterranean caves, making targeted surveys a priority for future research. The specimens were deposited in the zoological collection of the Department of Life Sciences at Sapientia Hungarian University of Transylvania in Romania, ensuring that the physical evidence underpinning the description remains available for future study.

Beyond its taxonomic novelty, the discovery underscores the value of leptonetids as models for studying speciation driven by long-term isolation and microhabitat stability. Their limited dispersal ability and high levels of local endemism mean that each cave system can harbor unique evolutionary lineages, yet these spiders are rarely encountered outside targeted surveys and remain an overlooked component of subterranean arthropod communities. As scientists continue to probe the chemosynthetic ecosystems of the Balkan karst, each new species described from these dark, sulfur-laced corridors adds a piece to the puzzle of how life persists, diversifies, and thrives in places where sunlight never reaches.

Subject of Research: Taxonomic description and molecular phylogenetics of a new cave-dwelling leptonetid spider species from sulfidic caves in Greece and Albania

Article Title: A new Cataleptoneta species ( Araneae , Leptonetidae ) from sulfidic caves in Greece and Albania

Article References: Urák, I., Ştefan, A., Motoc, R. M., Brad, T., Zsigmond, A.-R., Sarbu, S. M., & Szűts, T. (2026). A new Cataleptoneta species (Araneae, Leptonetidae) from sulfidic caves in Greece and Albania. ZooKeys, 1294, 89-107. https://doi.org/10.3897/zookeys.1294.202010

Image Credits: AI Generated

DOI: 10.3897/zookeys.1294.202010

Keywords: Cataleptoneta sulfur, Leptonetidae, cave spiders, sulfidic caves, Greece, Albania, COI barcoding, taxonomy, troglomorphism, chemosynthesis, new species, Vromoner Canyon