Comprising 84 species parceled among 15 genera (Platnick 2012), the spider family Caponiidae is widely distributed but not currently known from Australia or Europe. Petrunkevitch (1939) divided the Caponiidae into two subfamilies, Caponiinae and Nopinae, the latter of which is characterized by subsegmented tarsi and has a New World distribution with 53 species described from North, South, and Central America and numerous species known from the Caribbean. While recent authors have questioned the monophyly of the Caponiinae, citing a lack of synapomorphies (Jiménez et al. 2011), the Nopinae is generally regarded as a valid group with subsegmented tarsi as a distinguishing feature. However, Platnick (1994) suggests that caponine taxa with fewer than eight eyes may be more closely related to nopines.

The genus Tarsonops, the subject of this paper, was erected by Chamberlin (1924) to accommodate the species, Nops sternalis, originally described by Banks (1898). At the time, Chamberlin (1924) also described, on the basis of female specimens, three additional species, Tarsonops clavis, Tarsonops sectipes, and Tarsonops systematicus, all collected in Mexico adjacent to the Gulf of California. He also provided a key to species based on female anatomy, with an emphasis on leg morphology. Subsequently, Gertsch (1935) published additional records for Tarsonops systematicus, collected in southern Texas, and Ubick (2005) reports that this species also occurs in California and Arizona and illustrates the male pedipalp (figure 18.10). Although numerous new species have been described in the family Caponiidae, including its subfamily Nopinae since the 1930s, no new species of Tarsonops have been described.

The primary purpose of this paper is to describe a new species of Tarsonops collected from Belize and to provide a key for the nopine genera. Unfortunately, this newly discovered species is known from only a single specimen. While a large of number of new species are described only from single specimens, greater than 1/6^th of all species (Lim et al. 2012), it is with some trepidation that we propose a new taxon on the basis of a single unique specimen. However, the morphological uniqueness of the species, the extension of the genus distribution, and recognition of important species level and morphological diversity serves as the impetus despite any misgivings. Moreover, it may very well be, given the combination/absence of characteristics (Table 1) for this species, that it may ultimately represent a new genus or species group, however, its palpal morphology closely resembles that described for Tarsonops systematicus by Ubick (2005). Although two of the nopine genera are monotypic (Nopsides Chamberlin, 1924 and Nyetnops Platnick & Lise, 2007) it is our opinion that the description of a new genus should be postponed until more material, including the female, and potential other species become available.

All measurements were taken with a Leica MZ16.5 stereomicroscope equipped with a 10× ocular and ocular micrometer scale. We measured the left appendage, usually in retrolateral view, using the highest magnification possible. Legs I-IV (femur, patella, tibia, metatarsus, tarsus) and palp article lengths (femur, patella, tibia, cymbium) given in order of proximal to distal. Illustrations were prepared using a Visionary Digital Imaging System (Ashland, VA). Photographs were recorded in multiple focal planes and assembled using the Zerene Stacker software package (Zerene Systems LLC, Richland, WA). The habitus illustration was constructed from whole body images that were bisected, copied, and reflected in Adobe Photoshop (Adobe Systems, Inc.) to produce a roughly symmetrical image (technique described in Bond 2012). Measurements in millimeters.

Although this species was taken from just inside a cave, it does not exhibit any obvious troglomorphies, and may be accidental in this habitat. The description of Tarsonops irataylori sp. n. extends the range of the genus 9 degrees east and 6.8 degrees south from the previously known range. A number of undescribed species of Tarsonops are known from collections in Mexico (Platnick, pers. comm. 31 October 2011).

Tarsonops irataylori sp. n. is the first species of Tarsonops described which lacks a ventral translucent keel on the anterior metatarsi and marked translucent extension of the membrane between the anterior metatarsi and tarsi. Chamberlin’s (1924) diagnoses of the genus does not list these characters, thus we have taken the conservative approach of placing the species in this genus. As discussed above, future studies in which more specimens are examined, may further warrant the establishment of a new genus to accommodate this somewhat unusual species.