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. 1; Table 1). Troglomorphic species occur in 20 genera (of currently 77 genera), but a majority of them are representatives of just two genera: Metagonia Simon, 1893 and Anopsicus Chamberlin & Ivie, 1938 (Fig. 2). Together with three further genera (Pholcus Walckenaer, 1805; Psilochorus Simon, 1893; Spermophorides Wunderlich, 1992), they contain ~75% of all troglomorphic pholcid species. This bias is even more extreme when considering only the 42 ‘strongly’ troglomorphic species, i.e. those without eyes or with reduced eyes: 30 (71%) are in the genera Metagonia and Anopsicus (Fig. 3). Most troglomorphic pholcid species are in Pholcinae and Modisiminae (Fig. 4); four weakly troglomorphic species are in Smeringopinae and Arteminae; none in Ninetinae.

Figures 1–9. 

Main patterns in cave-dwelling pholcid spiders. Numbers in parentheses are usually species numbers (1–8), but cave numbers in (9). 1 Troglomorphic and non-troglomorphic cave-dwelling pholcids; a, eyes normal; b, eyes reduced in size and/or pigment; c, eyes absent 2 Generic assignment of the 86 troglomorphic pholcid species 3 Generic assignment of the 42 ‘strongly’ troglomorphic species 4 Subfamily assignment of the 86 troglomorphic species; S., Smeringopinae; A., Arteminae5 Known habitat of 473 cave-dwelling pholcids 6 Number of caves known to be inhabited by 298 species known from caves only 7 Geographic distribution of the 86 troglomorphic species 8 Geographic distribution of the 21 eyeless species 9 Number of species found in each of 1000 caves.

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. 5). Of the 298 species known from caves only, the large majority (234, i.e. 79%) are known from a single cave (Fig. 6). Among the other 64 species are some that appear fairly widely distributed, including six troglomorphic species that are known from 5–11 caves each. Two of these troglomorphic species occur in several caves but these caves are all close together (Metagonia potiguar Ferreira et al., 2011: five caves within 10 km; Aymaria jarmila (Gertsch & Peck, 1992): six caves within 15 km); the other four species cover large to very large areas (Metagonia tinaja Gertsch, 1971: nine caves within 50 km; Anopsicus quatoculus Gertsch, 1982: seven caves within 65 km; Anopsicus pearsei Chamberlin & Ivie, 1938: five caves within 190 km; Metagonia maya Chamberlin & Ivie, 1938: 11 caves within 260 km).

Few cave pholcids seem to be relicts, based on the apparent absence of epigean close relatives (cf. Grandcolas et al. 2014). Examples include (1) the four species of Metagonia on Cuba, Jamaica, and the Galapagos Islands, all of which are eyeless and have no epigean relatives on these islands (Gertsch 1986, Gertsch and Peck 1992, Pérez and Huber 1999); (2) an undescribed eyeless species on Réunion Island, possibly member of the East African genus Buitinga Huber, 2003 and with no known congener on Réunion (B.A. Huber, unpublished data); (3) the strongly troglomorphic Ossinissa justoi (Wunderlich, 1992) on El Hierro (Canary Islands), only representative of the genus, possibly related to two other troglomorphic species on Tenerife currently in Pholcus and not closely related to epigean Pholcus in the Canary Islands (Dimitrov and Ribera 2005, Huber 2011: 367).

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. 7): 40 species in Central America (following the United Nations geoscheme; https://unstats.un.org/unsd/methodology/m49/), followed by the Greater Antilles (11 species) and mainland SE Asia (9 species). The same pattern is observed when considering only the most strongly troglomorphic (i.e. eyeless) species: nine of 21 species (i.e. 43%) occur in Central America (all of them in Mexico) (Fig. 8).

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 (Metagonia diamantina Machado et al., 2011 from Brazil and Belisana khanensis Yao & Li, 2013 from Laos).

Europe (incl. Turkey) appears like a hotspot in Fig. 10, but this refers to caves inhabited by pholcids (see below) and not to species diversity of troglomorphic pholcid spiders. Only three troglomorphic pholcids are known from Europe: two putative species of Stygopholcus on the Balkan Peninsula and one species of Hoplopholcus in Turkey (Table 1).

Figure 10. 

Geographic distribution of 1000 caves in which native pholcid spider species have been found. Green: non-troglomorphic; red: troglomorphic.

Native species of Pholcidae have been collected from 1000 caves, most of which are located in three geographic regions: Central America and the Caribbean; Dinaric Alps and eastern Mediterranean; and mainland SE Asia including southern China (Fig. 10). In the large majority of these caves (909, i.e. 91%) a single representative of Pholcidae has been found (Fig. 9); 76 caves contained two species; only 15 caves were found to contain more than two species. Of the 91 caves with more than one species, 36 (i.e. 40%) occur in Mexico, followed by Puerto Rico (6 caves), Cuba (5 caves), Brazil, Hispaniola, Crete, and Thailand (4 caves each). The highest known diversity has been reported from Puong Cave, Ba Be National Park, Vietnam (5 species), followed by Cueva de la Capilla, Tamaulipas, Mexico (4 species) and Cueva Aguas Buenas, Puerto Rico (4 species).

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., Moldovan 2012, Christiansen 2012). Unsurprisingly, this pattern is also found in Pholcidae. Five of the currently recognized 77 extant genera contain 75% of the troglomorphic species. To some degree, this might simply result from certain genera being present in cave-rich regions: the genera Anopsicus and Metagonia (that contain the majority of troglomorphic pholcid species) happen to be present in cave-rich countries like Mexico, Jamaica, and Cuba. Genera endemic to, for example, South America may have fewer troglomorphs simply because South America’s carbonate outcrop area is much smaller than that of North America including Central America and the Caribbean (Ford and Williams 2007). However, the uneven systematic distribution remains when considering only cave-rich areas. Mexico, for example, is home so several species-rich pholcid genera in addition to Anopsicus and Metagonia, yet they contain very few troglomorphic species: while 38 of 92 Mexican species in Anopsicus, Metagonia, and Psilochorus, are troglomorphic (i.e. 41%), only three of the 76 species in the remaining seven native Mexican genera are troglomorphic (i.e. 4%).

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: Pholcinae and Modisiminae. This evolutionary ecological flexibility or ‘evolvability’ was thought to be among the main drivers of species diversification in those two subfamilies. It may also partly account for the uneven systematic distribution of troglomorphisms, where most troglomorphic pholcid species are in Pholcinae and Modisiminae (Fig. 4). This might explain why certain species-rich genera that are frequently found in caves, such as Physocyclus in Mexico or Trichocyclus Simon, 1908 in Australia have not evolved troglomorphic species: both belong to Arteminae. On the other hand, it does still not explain the few troglomorphs in some genera of Modisiminae and Pholcinae in cave-rich regions, such as Modisimus Simon, 1893 and Ixchela Huber, 2000 in Mexico, or Belisana Thorell, 1898, Khorata Huber, 2005, and Pholcus in SE Asia.

A second common and equally poorly understood phenomenon is that widespread taxa have troglomorphic representatives in certain geographic regions only (e.g., Deharveng et al. 2012). Pholcidae also follow this trend, with a remarkable concentration of troglomorphic taxa in Mexico, followed by mainland SE Asia and some islands, and very few troglomorphic species in mainland South America and mainland Africa, and none in Australia. In analogy to the argument above, this might simply result from the fact that certain taxa that are particularly preadapted to subterranean life happen to be present in certain regions and not in others. For example, Anopsicus has many troglomorphic species and happens to be restricted to Central America and the Greater Antilles. The case of Metagonia seems to contradict this idea. Metagonia is widely distributed from Argentina to the U.S.A., with similar numbers of species available from South America (incl. the Galapagos Islands) (58) and North America (incl. Central America and the Caribbean) (57). Troglomorphic species in Metagonia, however, are very unevenly distributed: only five species in South America versus 19 species in North America.

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 (Christman and Culver 2001, Culver et al. 2003). (2) The lower tendency for high energy caves to contain troglomorphic species (see Tropical versus temperate caves below). (3) Research biases. The latter also affects the uneven systematic distribution above, but is difficult to quantify. It includes collectors’ as well as taxonomists’ biases. India, for example, does not have a single record of a cave-dwelling pholcid, but this may simply result from the fact that India is among the most poorly studied countries regarding pholcid spiders in general. Mexican pholcids, on the other hand, have for many decades attracted the focused attention of collectors, and a single taxonomist (Willis J. Gertsch) has devoted a number of large papers to Mexican pholcids, with particular attention to cave-dwelling species (Gertsch 1971, 1973, 1977, 1982, 1986). Without Gertsch, Mexico would currently count only six troglomorphic pholcid species instead of 39. Some countries rich in troglomorphic pholcids may simply not have had the lucky combination of skilled and enthusiastic collectors and a dedicated taxonomist.

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).

Pholcidae have most of their species diversity in the tropics and subtropics and thus do not contribute directly to the debate about the apparent higher number of troglomorphic species in temperate rather than in tropical regions. The historical bias of speleology and taxonomy towards European and North American cave faunas is uncontested (cf. Moldovan 2012, Deharveng and Bedos 2012, Gallão and Bichuette 2018). Nevertheless, for certain taxa such as most insects the trend towards a relatively low diversity in tropical caves does not seem to disappear with a more comprehensive study of tropical faunas. On the other hand, spiders and arachnids in general, may not follow this pattern but, on the contrary, occupy in the tropics the predatory positions that are in temperate caves held by beetles and other arthropods (Reddell 2012). However, a comprehensive review of subterranean arachnids or just spiders has apparently never been attempted.

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.

Troglomorphisms. There is basically no information in the literature about troglomorphisms in pholcid spiders beyond simple qualitative remarks about those troglomorphisms that are easily observed (Figs 11–14): eye reduction, pigment loss, and leg elongation. Theory predicts that several additional traits may be affected. For example, egg size is expected to increase while egg number is expected to decrease; the concentration of sensilla is expected to increase; excitement (reaction to disturbance) is expected to decrease; and longevity and body size are expected to increase (Culver 1982, Hüppop 2012). For several species the classification as troglomorphic or not is quite ambiguous, and this introduces noise in the analyses. A quantification of troglomorphisms would clarify limits and probably reveal interesting patterns. For example, a simple plot of relative leg lengths (tibia 1/carapace width) in 55 species of Anopsicus (taken from Gertsch 1982) shows a slight increase in leg length in species found only in caves compared to epigean species, but a strong increase in species with reduced eyes or without eyes. Individual taxa offer additional possibilities to study particular troglomorphisms. For example, the South American Mesabolivar aurantiacus (Mello-Leitão, 1930) is sexually dimorphic in coloration, with males (in particular large males) having the third femora bright red rather than dark brown. In a recently discovered cave population in the Brazilian state Amazonas, the male third femora showed little or no difference to other (and female) femora (Huber 2018). The significance of this sexual dimorphism remains unknown. Another potential troglomorphism concerns web structure. Food shortage is considered a major selective pressure responsible for troglomorphisms (Hüppop 2012, Poulson 2012), and prey-capturing devices such as spider webs should in theory be affected. This has never been studied in any pholcid (nor, to my knowledge, in any spider), but the web of a recently discovered Brazilian Metagonia species with evanescent eyes was indeed very different from ‘usual’ webs of Metagonia and other pholcids (B.A. Huber, unpublished data): instead of representing a sheet with tumble lines (suitable for intercepting flying prey) it included an exceptionally high number of gum-foot lines (suitable for catching walking prey; cf. Japyassú and Macagnan 2004) (Figs 15–16).

Figures 11–16. 

Examples of non-troglomorphic and troglomorphic pholcid spiders and their webs. 11–12 Two undescribed species of Uthina from Bali and Sulawesi; note smaller eyes and paler color in cave-dwelling species (12). 13–14 Two undescribed species of Metagonia from Brazil (Pernambuco, Rio Grande do Norte); note evanescent eyes and pale coloration in cave-dwelling species (14). 15–16 ‘Typical’ pholcid web of epigean Metagonia bonaldoa 15 domed sheet with tumble lines above sheet; Brazil, Santa Catarina), and unusual web of cave-dwelling Metagonia potiguar 16 sheet attached to rock surface and many vertical gum-foot lines; Brazil, Rio Grande do Norte). Photos BAH.

Habitats of apparently cave-restricted species. Of the 473 pholcid species found in caves, 218 have only been found in caves yet do not show any obvious troglomorphisms. In theory, many of them might indeed be specialized to the cave entrance ecotone, but the more plausible hypothesis is that the actual habitat of most of these species is in fact the shallow subterranean habitat (cf. Deeleman-Reinhold and Deeleman 1980, Culver and Pipan 2009). Good candidates to test this hypothesis are (1) non-troglomorphic species with numerous cave-records but no records so far from outside caves (e.g., Modisimus boneti Gertsch, 1971; Metagonia blanda Gertsch, 1973; Hoplopholcus trakyaensis Demircan & Topçu, 2017; Uthina saiyokensis Yao & Li, 2016; Ixchela pecki (Gertsch, 1971); and Modisimus mitchelli Gertsch, 1971); and (2) troglomorphic species that are known from numerous caves each (see examples above).

The genusAnopsicus. In Central America and the Caribbean, the genus Anopsicus offers an ideal opportunity to study a large radiation that includes both epigean and hypogean species. Judging from the small ranges of epigean species, it seems likely that hypogean species have evolved repeatedly from epigean ancestors, but a phylogeny of the genus is not available.

The genusAymaria. On the Galapagos Islands, four nominal species of Aymaria Huber, 2000 are currently recognized, two of them epigean (on most or all islands), two hypogean and eyeless (on Santa Cruz and Floreana). The genitalia of all four ‘species’ appear essentially indistinguishable, and I have argued previously that only two species might be present, one epigean, one hypogean (Huber 2000). In the meantime, several species in a range of major taxa are known to include epigean and troglomorphic hypogean populations (e.g., Verovnik et al. 2004, Fong 2012, Gross 2012), and interspecific variation in troglomorphic traits has also been found in spiders (Shear 1978; further references in Mammola and Isaia 2017). As a consequence, the null hypothesis should be that only a single species of Aymaria occurs on the Galapagos Islands. Beyond species limits, Aymaria on Galapagos offers the opportunity to study a case where strong troglomorphism has evolved relatively rapidly, as judged from the age of the islands (~1–2.5 My for Santa Cruz and Floreana) and the apparent lack of genitalic differences.

Sympatric species. In 91 caves, two or more species of Pholcidae were found to coexist, but beyond anecdotal observations there is almost nothing known about interspecific competition and niche partitioning. The only quantitative data describe spatial segregation in Micropholcus piaui Huber et al., 2014 and Mesabolivar spinulosus (Mello-Leitão, 1939) in a cave in Brazil (by L.S. Carvalho, reported in Huber et al. 2014). Several alternatives to spatial segregation are plausible and have been reported in other spiders (Arnedo et al. 2007). Of particular interest are caves with multiple species of the same genus (e.g. Hanging Gardens Cave in California with three species of Psilochorus; Puong Cave in Vietnam with three species of Khorata).