Psilochorus Simon, 1893

Slowik, Jozef, 2009, A review of the cellar spider genus Psilochorus Simon 1893 in America north of Mexico (Araneae: Pholcidae), Zootaxa 2144, pp. 1-53 : 3-5

Psilochorus Simon, 1893

Diagnosis. Small pholcids (total length 1.2–3.8 mm), generally with long legs (femur I about 3– 6 x carapace length), globular abdomen, eight eyes, not on turret, carapace with thoracic grove; distinguished from other American genera by the combination of the following characters; a single cheliceral spur (lacking in Psilochorus acanthus Chamberlin & Ivie ), the presence of a pointed and upward projecting apophysis on the distal end of the male palpal femur, and the longer than wide procursus. It is easily distinguished from Physocyclus , which shares geographical distribution, but has a massive and complex procursus, larger body size and chelicerae with several to many cones instead of spurs.

Description. Small spiders, total length of 1.2–3.8 mm. Eyes eight, ALE and posterior eyes forming triads, AME smaller and located mesally between triads. Carapace ochre-yellow to red-brown. Deep Y indentation formed from cervical groove extending anteriorly around the lateral edges of the eye triads, often darkly shaded (Fig. 1). Sternum slightly darker than carapace. Male clypeus projecting anteriorly, unmodified. Chelicerae slightly darker than sternum, with prominent pro-marginal tooth and no stridulatory files. Male chelicerae with spur located anteriorly (Fig. 2). Palp femur not swollen, with pointed and upward projecting apophysis (pup) (Fig. 3). Procursus longer than wide, ending variably. Legs thin, tibia I length/diameter: 28–60, same color as carapace. Legs with setae scattered or patterned often with darker rings on each leg segment distally, dorsal trichobothrium (DT) at 5%–15%, retrolateral trichobothrium (RT) at 7%–25% (Fig. 4). Tarsi pseudosegmented (leg I with 14–27 segments) with three claws. Abdomen gray to blue, globular with 3–6 paired dark marks dorsally. Venter with scattered dark spots. Female with 2–3 epigynal plates varying in their shape.

Comments. Psilochorus has historically been a very vaguely defined genus, leaving it open to many additions that may not belong in the genus ( Huber 1998). All of the species in this paper closely conform to the generic type, Psilochorus pullulus ( Hentz, 1850) , therefore leaving little question to their taxonomic position. The pup apophysis is found in all North American species, as well as in all known Mexican and Brazilian species. It is interesting to note that the pup apophysis was neglected in both Chamberlin’s illustrations for P. californiae Chamberlin (Chamberlin 1919) and Simon's for P. pullulus ( Simon 1893) although present in both species. I have chosen to include the pup apophysis (Fig. 3) as a character of Psilochorus as it is found in all of the North American species as well as P. itaguyrussu Huber, Rheims & Brescovit, 2005 and P. ybytyriguara Huber, Rheims & Brescovit, 2005 from South America. It is not certain if any of the remaining South American species have the pup apophysis as they have been declared incertae sedis ( Huber 2000) and were not examined for this paper. The cheliceral spur of the males is also found in all species except for P. acanthus . Although Huber (1994) makes note of an enlarged femur III in males of P. simoni (Berland, 1911) it has only been seen as an occasional occurrence in other North American species looked at.

Natural history and reproduction. Species of Psilochorus have been collected commonly underneath rocks and logs in dry, xeric landscapes; the exception to this is P. h e s p e r u s Gertsch and Ivie of the Pacific Northwest and P. pullulus of the south east which seem to be found in more humid environments. Psilochorus imitatus Gertsch and Mulaik will make small messy webs underneath rocks, logs, and overhangs and appears to catch mainly small resting flies and collembola (Slowik pers. obs.). P. imitatus has also been observed using a Steatoda sp. web after the maker left; this allowed the spider to catch small crickets and larger flies which were not being caught in a known P. imitatus web. In this case it is not known if P. imitatus killed the prey caught in the Steatoda web or if the animals expired naturally; however, P. i m i t a t u s was observed feeding on the larger prey (Slowik pers. obs.). Many species of Psilochorus are known to be synanthropic. An interesting example is Psilochorus simoni which has been collected from non-synanthropic environments in the United States but is only collected from syanthropic environments in Europe, life history and natural history of the species is discussed by Huber (1994).

PLATE 1. Figure 1. Psilochorus coloradensis new species, female paratype dorsal view. Figures 2–4. Psilochorus imitatus Gertsch and Mulaik , 2. Male lateral view. 3. Lateral view of palp showing pup apophysis. 4. Tibia 1 showing placement of trichobothria.

Elevation may be helpful in identifying some like species. In the southwest, both P. u t a h e n s i s Chamberlin and P. imitatus will occur in close proximity but not in the same locale. The females of these two species are very similar and may be confused. In the southwest, P. utahensis is not found above 4500 ft, and P. imitatus not below 4500 ft. The cause of this distribution pattern is unknown as P. imitatus is found east of the Rocky Mountains in Nebraska down to 2500 ft. The only eastern species, P. pullulus , is found up to elevations of 1600 ft in Oklahoma. Again elevation may be helpful as females of P. pullulus and P. i m i t a t u s are similar and may be confused. The habitat factors limiting Psilochorus species elevational distributions are unknown.

Specimen locality maps are given for all species included in this paper as well as the distribution of Psilochorus in North America ( Fig. 187 ). These maps represent the known distributions for the species.

Specimens having been found well outside of the expected range for a species ( P. acantus found in New Mexico, P. rockefelleri Gertsch found in British Columbia and P. californiae found in Idaho) were most likely transported.

Reproduction of the genus was studied by Huber (1994) using Psilochorus simoni and is the only extensive copulation study of the genus. It has been suggested that a conformity of the male spur and female epigynum exists in Psilochorus and other Pholcids ( Kraus 1984, Huber 1994, Uhl 1994, Huber 1995, Huber & Eberhard 1997, Huber 2003, Huber 2005). The copulation position of P. imitatus was found to be very similar to that of P. s i m o n i (Slowik pers. obs.). Many females found in North America show modifications of the median epigynal plate and further study into the use of the male spur in regard to these modifications is necessary. In Belisana Thorell, 1898 , a Southeast Asian Pholcid genus, it has been found that the cheliceral spurs interaction with the epigynal plate acts to orientate the male for correct copulation (Huber 2005). Psilochorus acanthus , which lacks a spur, and species such as P. californiae and P. rockefelleri which have variable spur shapes, raises the question as to the real necessity of the spur for successful reproduction. Although it appears that spur shape plays a role in copulation it is hypothesized that the diversity of male spur shape is dictated by sexual selection by the female ( Huber 1999).

Identification Notes. Positive identification of Psilochorus specimens can be difficult as many of the animals are small and easily deformed when examined. For the male specimens, it is easiest to start with the general spur shape, but because of possible spur mutations, it is best to base the identification on the shape of the procursus combined with the bulb apophysis. Females are often more difficult since the differences are more subtle. When manipulated, the abdomens are often squeezed, altering the epigynal plates. Focus should be on the shape of the MEP and PEP and the shape of the apex or ridges and how the two plates come together. As with many other spider families, a confident identification may require duplicate males and females in pairs or from the same locale. The use of the apparent tubes found on the PEP is unknown and therefore they are referred to generally. The PEP of some species have dark marks on the anterior edge, P. imitatus for example, where the PEP tubes appear to overlap the pore plates when the epigynum is viewed externally, these have been abbreviated as dmPEP.

Specimen locality notes. The specimens examined information has been provided by either: the DMNS database, in which case the DMNS ZA. number is listed, or directly from the specimen label. DMNS specimens without a ZA. number had not been incorporated into the database at the time of this publication. Other abbreviations used not listed below exist and were taken directly from the specimen's label.

Abbreviations. AEP = Anterior epigynal plate; MEP = Median epigynal plate; PEP = Posterior epigynal plate; dmPEP = Dark mark of posterior epigynal plate; ba = Bulb apophysis; DT = Dorsal trichobothrium of tibia I; RT = Retrolateral trichobothrium of tibia I, PT = Prolateral trichobothrium of tibia I; ALE = Anterior lateral eyes; AME = Anterior median eyes; pup = Pointed and upward projecting; N P = National Park; S P = State Park; S W A = State Wildlife Area; O S = Open Space.

Nomina dubia. Psilochorus cornutus ( Keyserling, 1887) ; Neither type specimen for either the male described by Keysering nor the female described by Emerton (1902) could be located. Inquiries were made at all institutions listed above as well as the British Museum of Natural History, The Natural History Museum, London, and the Muséum National d'Histoire Naturelle, Paris. Neither description is sufficient enough to separate specimens of P. c o r n u t u s from other North American species. All eastern specimens labeled as P. cornutus examined were identified as P. pullulus ; however, without viewing the type specimens no synonymy is made.

Figures

FIGURE 187. Specimen localities of Psliochorus specimens examined.