Nesticus dilutus Gertsch

Hedin, Marshal & DELLINGER, Bob, 2005, Descriptions of a new species and previously unknown males of Nesticus (Araneae: Nesticidae) from caves in Eastern North America, with comments on species rarity, Zootaxa 904, pp. 1-19 : 10-12

Nesticus dilutus Gertsch 1984

Figs. 1, 13-14

Nesticus dilutus Gertsch 1984: 27, figs. 94-96.

Material Examined: Holotype. - Female, U.S.A: Tennessee: Rhea County, Grassy Creek Cave, south of Old Washington, coll. 5 October 1959, T.C. Barr (housed at AMNH).

Additional Type Locality Material Examined. Two males, ten females, U.S.A: Tennessee: Rhea County, Grassy Creek Cave, south of Old Washington, 35°31 ’40” N, 84°54 ’44” W, elev. ~ 210 meters, coll. 23 August 1992, M. Hedin & J. Hedin ( MCH male specimens # 1306, 1307 ; MCH female specimens # 1308 -1317).

Remarks. - Previously known only from the holotype female, collected in 1959 from Grassy Creek Cave, Rhea County, Tennessee.

Discussion. - Nesticus dilutus is a member of the close-knit N. tennesseensis species group, a clade with both morphological (Gertsch 1984) and molecular synapomorphies (Hedin 1997a, 1997b). Both male and female genitalia of N. dilutus closely resemble conditions found in the geographically variable N. tennesseensis (Petrunkevitch), a species with a relatively widespread distribution in northeastern Tennessee and southwestern Virginia. Given the extent of variation seen within N. tennesseensis (see Gertsch 1984), it might be hypothesized that N. dilutus is simply a southern geographical isolate of N. tennesseensis . We reject this hypothesis, and consider N. dilutus to be distinct at the species level, for the following reasons. First, DNA sequence data from both the mitochondrial and nuclear genomes suggests that N. dilutus forms a genetic clade exclusive of, and sister to, the N. tennesseensis genetic clade (Hedin 1997a, 1997b). These species are clearly close genetic relatives, but are both diagnosable at the DNA level. Second, as the southernmost representative of the species complex, separated from the nearest N. tennesseensis population by several hundred kilometers (see fig. 1 of Hedin 1997b), N. dilutus clearly occupies a unique and highly disjunct geographical position. This geographical separation is consistent with the evolutionary isolation of N. dilutus. Finally, the troglomorphic features of N. dilutus are more extreme than any such condition found in N. tennesseensis, again consistent with the evolutionary isolation of N. dilutus.

Diagnosis. - The troglomorphic features (pigmentation patterns, lack of median eyes, proportionately long legs) of N. dilutus separate this species from the closely related N. tennesseensis. Diagnostic DNA characters are reported in Hedin (1997a, 1997b).

Description of male from type locality ( MCH male specimen #1307) (Figs. 13 & 14; Table 1). Color of appendages and carapace uniform in coloration, between HTML colors “orange” (# FFA 500) and "dark orange (# FF 8 C 00). Abdomen more flesh-colored, tending towards "dark salmon" (# E 9967 A). Lenses of lateral eyes evident, without pigmentation. No external evidence of median eyes. Leg formula 1423. Leg I about 9 times longer than carapace width. Palpus with translucent dorsal paracymbial process that parallels the cymbium, diverging medially at an approximately 90 degree angle, anterior edge serrate. Short, blade-like, paradistal process underlies dorsal process. Distal end of paracymbium curved dorsally, with serrate edge. Ventral paracymbium with translucent basal flange. Tegular apophysis curving medially, then anteriorly, closely following median apophysis.

Redescription of female from type locality ( MCH female specimen #1316) (Table 1). - Gertsch (1984) describes the holotype female as "a possible troglobite with median eyes of both rows missing and lateral eyes evascent, near relative of N. tennesseensis", with "cephalothorax and appendages bright yellow, abdomen grayish". The redescribed female described is mostly consistent with this description, although the coloration is not "bright yellow". Color of carapace and legs similar to male, unpatterned abdomen lighter and creamy in coloration. Lenses of lateral eyes evident, but without pigmentation. No external evidence of median eyes. Leg formula 1423. Leg I about 8 times longer than carapace width. Epigynum as illustrated by Gertsch (1984), relatively squat (short from anterior to posterior), with broad median septum.

Variation. - Examination of additional material from the type locality (nine females and a single male) reveals no significant genitalic differences from the described or holotype specimens.

Natural History. Spiders were collected past the twilight zone in the vicinity of a cave stream, found in vertically-oriented webs spanning small vertical spaces. Spiders common, about 20 individuals observed in less than 15 minutes. Two females carrying eggsacs collected, with 41 and 20 eggs, respectively.

Distribution. - Known only from the type locality in east-central Tennessee, in the southern Appalachian Valley and Ridge geologic province (Fig. 1).

DNA sequences. - Mitochondrial 16S (Genbank accession number U 40507) and nuclear ITS ( AF 003790) DNA sequences were reported in Hedin (1997a).