Species account taken from "A Revision of Perdita (Xerophasma) Timberlake (Hymenoptera: Andrenidae) Author(s): TERRY GRISWOLD & WENSDAE MILLER Source: Zootaxa, Vol. 2517, (June 24, 2010), pp. 1-14"
Female. Length: 7.08.5 mm. Forewing length: 6.27 mm. Head, mesosoma dark brown to nearly black except: cream-colored labrum, clypeus, paraocular area below level of fovea, spot on vertex between and slightly dorsal to lateral ocelli (sometimes reduced to point), postgenal area along margin of eye, pronotum except spot laterally, 'U'-shaped markings on mesoscutum with base of 'U' at scuto-scutellar margin, arms of 'U' running longitudinally between parapsidal and admedial lines, scutellum, metanotum, tegula, propodeum laterally, legs. Costal vein, prestigma dark. Stigma slightly darkened, ventral margin distinctly darkened. T1-5 with subapical brown bands on otherwise caramel-colored terga. Frons finely reticulate. Mesoscutum, scutellum shiny, mesoscutum anteriorly and laterally slightly reticulate. Mesoscutum moderately densely, distinctly punctate, evenly covered with sparse long pale hair. Propodeal triangle somewhat shiny, finely reticulate.
Mandible simple, without preapical tooth. Labrum with apical margin obtusely angulate. Clypeal width dorsally slightly greater than width of each subantennal area. Frontal line strongly raised halfway to median ocellus then strongly depressed to median ocellus. Facial fovea dull, linear, densely pubescent, extending from middle of antennal socket nearly to level of median ocellus. Ocellar area slightly inflated. Median ocellar diameter 1/4 width of interocular distance at level of median ocellus. Length of vertex behind lateral ocellus less than ocellar diameter. Forecoxa with long, erect, slightly plumose, straight hair; mesepisternum ventrally with short, obliquely directed, straight hair. Length of marginal cell on wing margin twice apical width. Pygidial plate narrowly rounded.
Male. Length: 7.59.0 mm. Forewing length: 5.35.9 mm. Head, mesosoma, legs cream colored except: entire head slightly yellowed; dark brown on facial fovea, markings anterior to each of lateral ocelli, medially between lateral ocelli, line extending dorsally from median ocellus, marks sometimes forming V between hindocelli on vertex, occasionally as rings around ocelli, sometimes along dorsal margin of eye orbit; brown subapical band on T1 and sometimes T24. T2 lateral fovea forming fine, sometimes dark, line. Sculpture, punctation, pubescence as in female.
Clypeus dorsally about 1.5X width of subantennal area. Supraclypeal area not protuberant, without tuft of hair. Facial fovea pubescent, elliptical, minutely depressed, sometimes slightly tear-shaped with narrow end ventral, extending from level of ventral margin of median ocellus to slightly more than halfway to dorsal margin of antennal socket. Frontal line, ocellar diameter, forewing as in female. T7 with pygidial plate ill-defined, with apical tuft of hair protruding from beneath apical margin. S8 as in Fig. 18. Genitalia as in Figs. 8, 13.
Type material. Holotype female: USA NEVADA, Clark County, Mesquite, 36°48.98' N 114°04.22' W, 26 May 1998, Camissonia, T. Griswold, BBSL289459. Paratypes: NEVADA, Clark County: 2 females, 7 males, same data as holotype; 1 female, St. Thomas Gap, 36°24.35' N 114°5.62' W, 8 Jun 1998, Camissonia, T. Griswold; 2 females, 2 males, St. Thomas Gap, 36°24.45' N 114°05.58' W, 7 Jun 1998, pantrap, F. D. Parker; 2 females, Las Vegas Dunes Rec. Lds., 36°17.27' N 114°58.00' W, 22 May 1998, light blue pantrap, M. Andres, K. Receveur, K. Keen, C. Shultz; 1 female, 1 male, same except white pantrap; 1 male, same except dark blue pantrap; 1 female, SE of Overton, 36°31.32' N 114°26' W, 21 May 1998, yellow pantrap, C. Shultz, K. Receveur, K. Keen, M. Andres; 1 female, same except white pantrap; 2 females, same except light blue pantrap; 1 female, same except dark blue pantrap; 21 females, 13 males, Riverside, 1121 May 1983, F. D. & J. H. Parker; 21 females, 55 males, Mesquite, 25 May 1973, at dusk, Oenothera, G. Bohart; 6 females, 12 males, Glendale, 2 Jun 1973, P. F. Torchio; 12 females, 6 males, Toquap Wash, W Mesquite, 36°46'34"N 114°11'10"W, 24 May 2003, 19:30 h., Camissonia, D. Yanega; 2 males, Toquap Wash, 1 mi N Hwy I-15, 485 m, N36°46'39" W114°11'10", 25 May 2003, Camissonia, D. Yanega; 1 female, 3 males, Kaolin Wash, S Overton, 36°30'42"N 114°26'39"W, 25 May 2003, D. Yanega; 1 female, 2.16 mi SW Wechech Basin, 11S E755235 N4038290, 13 May 2005, flourescent yellow pantrap, R. Andrus, S. Higbee; 1 female, 0.4 mi E St. Thomas Gap, 11S E760620 N4032979, 12 May 2005, white pantrap, D. Allen, E. Ahlstrom, R. Andrus, S. Higbee; 2 males, 3.9 mi SSW Whitney Pocket, 11S E755045 N4039308, 26 May 2005, white pantrap, R. Andrus, S. Higbee; 1 male, same except flourescent yellow pantrap; 1 male, same except flourescent blue pantrap. Holotype deposited in the U.S. National Pollinating Insects Collection, Logan, Utah; paratypes in Logan and Riverside.
Diagnosis. Timberlake recognized this as a new species and gave it a manuscript name that remained unpublished at his death. Perdita vespertina differs from all other Xerophasma except P. celadona in the largely dark mesoscutum. The ocelli of P. vespertina are larger than in P. celadona. Females differ from P. celadona in the simple mandible, pale clypeus, and rounded rather than acute pygidial plate. Males differ in the absence of dark marks above the antennal sockets and the rounded rather than emarginate apex of S8. The scopa on the hindtibia is denser than in other Xerophasma.
Range. Apparently endemic to Clark County, Nevada in the eastern Mojave Desert (Fig. 27) where it was active at dusk. Known only from scattered low elevation, sandy localities from Las Vegas Dunes to Mesquite and St.Thomas Gap. Some specimens were labeled as collected on Oenothera but these were from plants now assigned to Camissonia (Parker, personal communication).
Variation. Some individuals lack the intercalated second submarginal cell on one or both wings. The incidence varies among populations. In the Riverside population (n = 36) 22% have intercalated submarginal cell absent or incomplete on one wing; 11% missing on both wings. The cell was absent in only one specimen (1 %) of the Mesquite sample (n = 82) and none of the Glendale sample (n = 17). Two females from Glendale, Nevada 1973 had the mesoscutum light, with the mesepisternum light dorsally but brown ventrally.
Etymology. From the Latin, vesper, meaning evening and west, in reference to the evening flight period and southwestern origin of the bee.