Reprinted with permission of the American Entomological Society from:
LaBerge, W. E. 1977. A revision of the bees of the genus Andrena of the Western Hemisphere. Part VIII. Subgenera Thysandrena, Dasyandrena, Psammandrena, Rhacandrena, Euandrena, Oxyandrena. Transactions of the American Entomological Society 103: 1-144.
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This western species is readily recognized by its densely tessellate terga and weak labral median crista in the females. The males are recognized by the more densely tessellate terga, more coarsely sculptured propodeum and the short, broad labral process. However, the males of obscuripostica are very similar to those of cristata and overlap of characters results in a certain degree of misdetermination. No clear-cut character distinguishing the species has as yet been found except in the female sex.
FEMALE. MEASUREMENTS AND RATIOS: N = 20; length, 9-11 mm; width, 3.0-3.5 mm; wing length, M = 3.76 ± 0.119 mm; FL/FW, M = 0.89 ± 0.005; FOVL/FOVW, M = 3.04 ± 0.049.
INTEGUMENTAL COLOR. — Black except as follows; mandible with at half or more rufescent; flagellum dark reddish-brown below; wing membranes moderately infumate, hyaline, veins dark reddish-brown to dark brown; tegula piceous; terga with apical areas usually translucent, rufescent or yellow; distitarsi dark rufescent to brown; tibial spurs pale.
STRUCTURE. — Antennal scape length equal to first four flagellar segments or slightly less; flagellar segment 1 equal in length to following two and one-fourth segments, segment 2 about as long as 3 or slightly shorter, segments 2 and 3 broader than long, 4-9 quadrate. Eyes narrow, each about four times as long as broad; inner margins converging slightly towards mandibles. Malar space short, linear. Mandible tridentate, superior margin with expanded lamella ending in third tooth basad of normal two teeth; inferior margin with small lamellate flap about one-third of distance from base; mandibles of normal length. Galea as in Thysandrena (see medionitens) but segmental ratio about as 1.0:0.8:0.8:0.8:0.7:0.6. Labial palpus as in medionitens but ratio about as 1.0:0.6:0.4:0.4. Labral process short usually two and one-half to three times as broad as long, feebly emarginate or entire, without transverse rugulae basally or with only one or two; labrum below process with a single weak median crista, shiny. Clypeus moderately rounded from side to side, with moderately deep punctures separated by half to one puncture width except sparse along narrow midline, smaller peripherally; surface shiny, unshagreened except basally. Supraclypeal area with minute but distinct, crowded punctures and fine shagreening. Genal area in profile distinctly broader than eye; surface moderately dulled by fine reticular shagreening and minute sparse punctures, near hypostomal carina often with short rugulae. Vertex above lateral ocellus equal to about one ocellar diameter, opaque, dulled by dense tessellation and scattered punctures. Face above antennal fossae with moderately coarse longitudinal rugae and interrugal punctures. Facial fovea broad, shallow, extending to just below a line at lower margins antennal fossae, separated from lateral ocellus by one ocellar diameter or more.
Pronotum normal, dulled by reticular shagreening, without obvious punctures. Mesoscutum dulled by fine tessellation and small obscure punctures separated mostly by one to two puncture widths. Scutellum similar but slightly shinier. Metanotum opaque, tessellate. Propodeum with dorsal enclosure finely areolate mediobasally to mostly so, coarsely tessellate; dorsal and posterior surfaces coarsely tessellate. with distinct punctures often forming short rugae radiating outward from suture of dorsal enclosure, occasionally rugatulopunctate: lateral surface evenly and coarsely tessellate with scattered obscure punctures. Mesepisternum finely tessellate with small punctures separated by one to three puncture widths, more crowded anteriorly and dorsally than posteroventrally. Wing venation and vannal lobe hind wing typical for subgenus.
Metasomal tergum 1 opaque, densely and coarsely tessellate with small punctures separated mostly by two to three puncture widths on disc and one to two puncture widths or less in apical areas. Terga 2-4 similar to tergum 1 but apical areas slightly shinier due to more coarse tessellation and more densely punctate, discs progressively more finely tessellate posteriorly. Pygidial plate V-shaped with rounded apex, internal raised triangle present. Sterna 2-5 moderately shiny, finely shagreened with small punctures separated mostly by one puncture width but sparse basally and absent in narrow apical rims.
VESTITURE. — Generally ochraceous but dull orange-yellow on thoracic dorsum and vertex except as follows: facial fovea dark brown; vertex and along inner margins of eyes dark brown; terga 5 and 6 with long basal hairs dark brown; tergum 4 often with a few scattered long dark hairs on disc apex hind femur, surrounding basitibial plate and often along posterior margin tibia dark brown. Terga 2-5 with well-formed apical pale fasciae of long decumbent pubescence often interrupted medially on terga 2 and 3, rarely on 4; tergum 1 with apical pale fascia at least at extreme sides and rarely almost complete; terga 1-4 with discal hairs more or less erect, long (especially on terga 1 and 2); terga 5 and 6 usually pale laterally. Propodeal corbiculum incomplete anteriorly with long simple internal hairs trochanteral flocculus complete, dense; tibial scopa of long crowded simple hairs (along midline of tibia piliferous punctures separated by half to one puncture width or less).
MALE. MEASUREMENTS AND RATIOS. — N = 20; length, 7-10 mm; width, 2.0-2.5 mm; wing length, M = 3.34 ± 0.125 mm; FL/FW, M = 0.92 ± 0.006; FS1/FS2, M = 1.10 ± 0.019.
INTEGUMENTAL COLOR. — Black except as follows: clypeus in ten percent or less of specimens with small apicomedian pale spot or band; mandible with apical fourth to third rufescent; flagellar segments 2-11 below dark brown; wing membranes hyaline, moderately to slightly infumate, veins dark reddish-brown to black; metasomal terga 2-5 with apical areas often slightly rufescent; sterna 2-5 with apical areas hyaline; distitarsi dark reddish-brown; tibial spurs pale.
STRUCTURE. — Antennae long, in repose reaching propodeum; scape length equals first two flagellar segments; flagellar segment 1 as long as segment 2 or longer (rarely slightly shorter), segment 2 usually slightly shorter than 3, segments 3-10 usually about one and one-half times as broad. Eyes each about three times as long as broad, inner margins converging towards mandibles. Malar space and galea as in female. Mandible short, not decussate, bidentate; superior lamella broad as in female inferior margin with small lamellate protuberance about a third of distance from base as in female but smaller. Maxillary palpus as in female but segmental ratio about as 1.0:1.0:1.0:1.0;0.7:0.8. Labial palpus as in female but ratio about as 1.0:0.7:0.6:0.7. Labral process short, usually about twice as broad as long, weakly emarginate apically, basal transverse sulcus with no transverse rugulae or only one or two, shiny; labrum apical to process shiny, without median crista. Clypeus as in female but median impunctate line absent and punctures separated mostly by half a puncture width or slightly more. Supraclypeal area and vertex as in female. Genal area in profile broader than eye, sculptured as in female but shagreening coarser and tending to be longitudinal. Face above antennal fossae as in female but rugae often coarser.
Pronotum as in female. Thoracic sculpturing as in female but scutellum often not at all shiny and propodeum often with more distinct and extensive areolate area in dorsal enclosure; dorsal and posterior surfaces of propodeum usually rugatulopunctate with short rugulae radiating outward from suture of dorsal enclosure. Wing venation as in female.
Metasomal terga 1-5 as in female terga 1-4 except as follows: apical areas without punctures; discs with punctures minute and often not visible tessellation weaker, especially in apical areas and discs of terga 3-5 where microscopic sculpturing becomes reticular shagreening rather than tessellation. Tergum 7 with distinct, narrow, vestigial pygidial plate which has narrow longitudinal groove in apical half. Sterna 2-5 reticularly shagreened with sparse minute punctures on discs. Sternum 6 flat, deeply emarginate apically. Terminalia as in Figures 77-78 (only subgenital plates figured, capsule as in cristata).
VESTITURE. — Generally ochraceous except as follows: vertex, along inner margins eyes and lateral genal surface with abundant dark brown to black scutellum often with long dark brown mixed with the pale; terga 5, 6 and 7, often tergum 4 with erect black discal hairs. Terga without pale apical fasciae or these reduced to extremely short and weak lateral patches; terga 1-4 with discal hairs long, erect or suberect (partly dark on tergum 4). Sterna 2-5 with subapical fimbriae of unusually long, straight, pale hairs, Sternum 6 without apical tuft or band.