Corresponding author: Bernhard A. Huber (
Academic editor: O. Moldovan
Huber BA (2018) Cave-dwelling pholcid spiders (Araneae, Pholcidae): a review. Subterranean Biology 26: 89–106.
The colonization of subterranean habitats has occurred many times independently in many groups of animals (
Spider research has the potential to contribute significantly to these debates. Spiders are in several ways preadapted to life in caves. Many are nocturnal; most rely on mechanical and chemical rather than on visual stimuli; many groups are diverse in habitats that share certain characteristics with caves such as damp sheltered spaces or leaf litter; many spiders have a low energy demand and endure long periods of starvation; and most are generalist feeders (
The information summarized herein is largely taken from the taxonomic literature. In addition, data on 37 undescribed cave-dwelling species (eight of them troglomorphic) available in collections were added. The resulting table included all records of all species that have been reported from caves, i.e. also the epigean records of species that occasionally enter caves (‘accidentals’) or that are commonly found both outside and within caves. Excluded were ten synanthropic species, all of which are occasionally found in caves, in most cases presumably as a result of human introduction. The resulting table thus included only ‘native’, i.e. non-introduced species; it had 3100 entries. Table
Troglomorphic
Species | Troglomorphy | Reference |
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Long legs |
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Pale, eyeless, long legs |
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Eyeless, long legs |
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White, long legs |
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Small eyes |
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Pale, eyes essentially obsolete, scarcely visible as pale vestiges, long legs | ||
Pale, long legs, essentially eyeless, with trivial vestiges of eyes |
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Pale, long legs, eyes rudimentary, small, white |
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Pale, long legs, eyes evanescent, essentially obsolete, without pigment |
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Long legs |
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Pale, long legs, eyes evanescent, reduced to inconspicuous, unpigmented vestiges |
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Small eyes, long legs | ||
Pale, essentially eyeless, trivial vestiges of eyes, long legs |
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Pale, eyeless, long thin legs |
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Essentially eyeless, widely spaced corneal vestiges |
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Pale, long thin legs, eyes small |
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Posterior lateral eyes missing |
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Pale, eyes small |
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Long legs |
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Pale, long legs |
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Pale, eyes small |
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Small eyes, long legs | ||
Long legs, eyes evanescent, without pigment, essentially obsolete |
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Pale, small eyes | Unpublished | |
Essentially eyeless |
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Essentially eyeless, without external indication of lost eyes or with small white vestiges |
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Only two eyes, pale |
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Eyes absent |
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Pale, small eyes |
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Pale, eyeless | Unpublished | |
Pale, long legs |
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Pale, long legs |
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Pale | Unpublished | |
Pale, small eyes |
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Pale, eyeless |
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Whitish, eyeless |
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Pale, very small eyes without pigment | Unpublished | |
Eyeless |
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Eyeless |
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Eyeless |
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Pale, long legs |
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Eyeless |
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Pale, eyes obsolete or of moderate size, without pigment |
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Eyeless |
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Eyeless |
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Eyeless |
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Pale, rudimentary eyes without pigment |
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Whitish, long-legged, evanescent eyes without pigment |
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Eyeless |
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Pale, very small eyes |
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Pale, small eyes |
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Essentially eyeless, eyes present as small whitish vestiges |
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Pale, eyeless |
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Whitish, eyeless |
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Pallid coloration, small eyes |
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Pale, eyeless |
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Evanescent eyes, long legs |
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Pale, very long legs |
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Eyes very small |
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Almost entirely unpigmented, very long legs, small eyes |
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Small eyes, pale |
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Pale, reduced eyes |
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Pale, “eyes absent” [vestiges visible in figures] |
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“Total reduction of eyes” [vestiges visible in figure] |
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Pale, small eyes |
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Pale, small eyes |
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Pale, small eyes | Unpublished | |
Long legs |
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Pale |
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Pale, eyes reduced in size |
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Pale |
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Long legs |
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Pale |
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Long legs |
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Very small vestigial eyes |
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Eyeless | Unpublished | |
Pale, eyes with strongly reduced lenses |
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Pale, small eyes |
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Pale, small eyes |
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Long legs, small eyes |
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Paler than epigean |
Unpublished | |
Paler than epigean |
Unpublished | |
Long legs, small eyes | Unpublished | |
Almost unpigmented |
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Almost unpigmented |
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Almost unpigmented |
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†
Species were classified as troglomorphic (i.e. showing features associated with cave life) versus non-troglomorphic in order to avoid the unprovable designation of a species as troglobiont (obligatory cave dweller; as opposed to troglophile). In many cases, the classification of a species as troglomorphic or non-troglomorphic was unambiguous (eyes reduced or absent; very pale coloration compared to epigean relatives), but in several cases a clear distinction was not possible, especially when “long legs” and/or the loss of pigment were the only reported troglomorphisms. Some of the species in Table
A further source of noise in the data comes from the definition of cave. I largely use the anthropocentric definition of caves as “a natural opening in the Earth, large enough to admit a human being, and which some human beings choose to call a cave” (
Most published records do not provide coordinates, and even some published coordinates are obviously wrong. This introduces a further source of error, even though I made an effort to georeference as many caves as possible and to verify published coordinates. For this I used a variety of online databases as well as Google Earth and direct information from collectors.
A total of 473 native pholcid species (including 37 undescribed species) have been found in caves. This represents approximately 25% of the species currently known to exist (i.e. 1662 described species + ~300 undescribed species available in collections). Of these, 86 are here considered to be troglomorphic (Fig.
Main patterns in cave-dwelling pholcid spiders. Numbers in parentheses are usually species numbers (
Of the 473 pholcid species known from caves, 298 are known from caves only, i.e. have so far not been collected from epigean habitats; 64 species have the majority of records (i.e. 50–99%) from caves; 111 species have less that 50% of their records from caves (the ten synanthropic pholcid species would fall in this category too) (Fig.
Few cave pholcids seem to be relicts, based on the apparent absence of epigean close relatives (cf.
The most obvious and striking geographic pattern is the apparent dominance of Mexico. Of the 86 troglomorphic species, 39 (i.e. 45%) occur in Mexico, followed by Jamaica and the Canary Islands (7 species each), Galapagos Islands, Cuba, Madagascar, and Laos (4 species each). A more biologically meaningful comparison between regions rather than countries gives the same picture (Fig.
A second striking pattern is the prevalence of islands. Apart from Mexico, most troglomorphic species occur on islands: discounting the 40 Central American troglomorphs, 30 of the remaining 46 troglomorphic species (i.e. 65%) occur on islands. This apparent bias is even more pronounced when considering only the most strongly troglomorphic (i.e. eyeless) species: ten occur on islands, only two eyeless species are neither Mexican nor from an island (
Europe (incl. Turkey) appears like a hotspot in Fig.
Geographic distribution of 1000 caves in which native pholcid spider species have been found. Green: non-troglomorphic; red: troglomorphic.
Native species of
One of the most prevalent patterns in subterranean biology is that the large majority of troglomorphic animals belong to a relatively small number of major taxa (e.g.,
Part of an answer for this mysterious pattern may come from a recent study on diversification patterns within the family (J. Eberle, D. Dimitrov, A. Valdez-Mondragón, B.A. Huber, unpublished data). Shifts among different microhabitats such as leaf-litter, large sheltered spaces, and undersides of life leaves (i.e. green leaves in the vegetation as opposed to dead leaves in the leaf litter) were not homogeneously distributed among the family but concentrated in two of the five subfamilies:
A second common and equally poorly understood phenomenon is that widespread taxa have troglomorphic representatives in certain geographic regions only (e.g.,
Several factors are likely to contribute to the apparent uneven geographic distribution: (1) The uneven distribution of carbonate outcrop (see above); it has been argued before that the best predictor of subterranean species diversity is the availability of habitat expressed by the number of caves in a region (
Surprising and difficult to explain is the over-representation of troglomorphic species on islands. This does not seem to be a widespread pattern in subterranean animals; there is no good reason to assume that collectors and taxonomists have been biased towards working on island faunas; and the ages of the islands vary widely from a few million years (Galapagos, Canaries, Réunion) to >100 million years (Greater Antilles, Cape Verde, Madagascar).
Subterranean pholcid diversity is still poorly known in most parts of the world and it will clearly need massive long-time efforts by collectors and taxonomists to substantially change this. This section focuses instead on a few particular questions that seem relatively easy to answer within a limited period of time and with reasonable effort.
Examples of non-troglomorphic and troglomorphic pholcid spiders and their webs.
Pholcid spiders are common inhabitants of tropical and subtropical caves, and many species in several genera have evolved strong troglomorphisms. Despite of some ‘noise’ in the data (e.g., uncertainty in classifying some species as troglomorphic or not; uncertainty of exact geographic coordinates of some caves; uncertainty in taxonomic status of some ‘species’) a few general conclusions can be drawn:
● A total of 473 native pholcid species have been found in caves. This means that about 25% of the species currently known to exist are either occasionally or exclusively found in caves. Most cave-dwelling pholcids are not troglomorphic and thus presumably not obligate cave-dwellers but ‘troglophiles’.
● About 86 species of troglomorphic pholcid species have been found, including 21 eyeless species and 21 species with strongly reduced eyes. Troglomorphic pholcids exist in 20 of currently 77 extant genera, but
● Mexico is by far the richest country in terms of troglomorphic pholcids, followed by several islands (Greater Antilles, Galapagos, Canaries, Réunion, Cape Verde, Madagascar, Sulawesi, New Guinea) and mainland SE Asia. The apparent dominance of Mexico may partly be due to collectors’ and taxonomists’ biases.
● Native pholcid spiders have been found in 1000 caves. In most of these caves, only one pholcid species has been found, but two and more species (up to five) have been found in 91 caves.
● Most troglomorphic pholcids belong to two subfamilies (
I am grateful to colleagues for help with georeferencing localities: L.S. Carvalho, P. Oromí, M. Pavlek, P. Sprouse, and A. Váldez-Mondragón. I thank C. Hamilton and an anonymous reviewer for their constructive criticism and for their help in improving the manuscript. Financial support was provided by the German Research Foundation (DFG, project HU 980/11-1).