Reprinted with permission of the American Entomological Society from:
LaBerge, W. E. 1980. A revision of the bees of the genus Andrena of the Western Hemisphere. Part X. Subgenus Andrena. Transactions of the American Entomological Society 106: 395-526.
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Andrena birtwelli is a distinctive summer bee of the western mountains; In structure it exhibits characters intermediate between those of the typical Andrena and the Cnemidandrena; Both sexes of birtwelli have short maxillary and labial palpi as in the Cnemidandrena; The female of birtwelli has short scopal hairs and cuneate hind tibiae, much as in the Scaphandrena and many Cnemidandrena, the clypeus is densely punctate (except in the narrow midline) and provided with stiff, simple, bristle-like, clypeal hairs, and terga 2 and 3 have the apical areas equal to half the tergal length or almost so; The male of birtwelli has a densely punctate clypeus, long first flagellar segment, long apical areas of terga 2 and 3 (equal to about one-third of each tergal length), and a flat sternum 6.
FEMALE. MEASUREMENTS AND RATIOS. — N = 20; length, 9-11 mm; width, 2.5-3.0 mm; wing length, M = 3.92±0.025 mm; FL/FW, M = 0.95±0.006; FOVL/ FOVW, M = 3.21 ±0.057.
INTEGUMENTAL COLOR. — Black except as follows: mandible with apical third to half rufescent; flagellum below reddish-brown to brown; tegula piceous; wing membranes hyaline, veins dark brown; distitarsi rufescent; tibial spurs testaceous; terga often with dark rufescent reflections.
STRUCTURE. — Antennal scape length equals first two and one-half flagellar segments; flagellar segment 1 slightly longer than segments 2 plus 3 (about as 10:9); segment 2 subequal to 3, about as broad as long; median segments slightly longer than broad; Eyes each about three and one-half times as long as broad, inner margins parallel; Mandibles short, when closed apex extends beyond midlabrum by one-fifth to one-fourth mandibular length; apex bidentate; base without tooth or angle near inferior articulation or tooth extremely small and indistinct; Malar space with minimum length equal to about one-fifth basal width of mandible or slightly longer; Galea pointed, outer-apical margin slightly concave; surface dulled by fine reticular shagreening; Maxillary palpus short, about as long as galea when stretched-out, segmental ratio about as 0.9:1.0:0.8:0.6:0.4:0.4; Labial palpus small, ratio about as 1.0:0.5:0.4:0.4; Labral process broad, basal width equals almost half width of labrum, short; emarginate apically with teeth often reflexed slightly; labrum apical to process convex, without sulcus or median crista, punctate near process; Clypeus evenly rounded from side to side, densely punctate except along narrow midline, punctures separated largely by half a puncture width or less, surface moderately dulled by extremely fine shagreening; Supraclypeal area slightly broader than long, dulled by minute crowded punctures and fine shagreening; Face above antennal fossae with fine, parallel, longitudinal rugulae reaching lateral ocelli, inconspicuous punctures and fine shagreening; Genal area broader than eye in profile (about as 3:2), with small rounded punctures separated by half to one puncture width, surface dulled by reticular shagreening except near eye margin; Facial fovea large, extending below a line at lower margins antennal fossae, separated from lateral ocellus by about one ocellar width or slightly less; Vertex above lateral ocellus equals one ocellar diameter or very slightly less (as 10:9), surface opaque, finely tessellate.
Pronotum with humeral angle and ridge distinct, ridge blunt; surface dulled by coarse reticular shagreening, moderately shiny posterior to ridge; Mesoscutum dulled by fine tessellation, punctures small, shallow, obscure, anteriorly separated by half to one puncture width; Scutellum similar but punctures crowded posteriorly and summit often moderately shiny; Propodeal dorsal enclosure densely tessellate with extremely short rugulae basomedially; dorsal and posterior surfaces densely tessellate with large shallow obscure punctures (punctures tessellate internally); lateral surface coarsely tessellate, small scattered punctures in upper half; Mesepisternum densely tessellate with large shallow punctured obscured by internal tessellation; Midbasitarsus about as broad medially as hind basitarsus; hind tibia broad, almost cuneate, viewed from inner surface about three times as long as apical width
Metasomal tergum 1 dull, densely tessellate, disc with small scattered punctures separated by one to two or three puncture widths, apical area shinier, impunctate; Tergum 2 with apical area about equal to basal area medially; basal area dull, tessellate, impunctate; apical area coarsely tessellate, often moderately shiny, impunctate; Terga 3 and 4 similar to 2 but apical area may be slightly shorter; Pygidial plate V-shaped with strongly marked, raised, internal triangle; Sterna 2-5 with apical areas finely tessellate, basal areas shagreened, moderately shiny to dull with abundant small punctures.
VESTITURE. — Ochraceous to pale ochraceous (dorsum of thorax often dark ochraceous) except as follows: clypeal bristle-like hairs often dark ochraceous apically to mostly brown; facial fovea with short hairs pale brown to dark brown; vertex and genal areas with few to many dark brown hairs; thoracic pleurae entirely pale to dark brown in lower half or more; propodeal hairs entirely pale to pale and dark mixed (corbicular hairs at least in part pale); terga and sternal hairs all pale to terga 2 to 6 with dark brown hairs and sterna 2 to 6 with brown hairs (tergum 2 usually with a few pale hairs basally in even darkest specimens); tibiae and tarsi (especially hind legs) brown to dark brown (darker in dark specimens), fore and middle tibiae with brown hairs at least apically in dark forms; Hind tibial scopal hairs short, medially along posterior margin half as long as breadth of tibia or less, shortest medially; hind basitarsus with outer surface hairs short apically, longer basally, bent towards apex of segment; trochanteral flocculus complete; propodeal corbiculum complete with several, long, weakly branched to simple, internal hairs; Terga 2-5 sparsely haired, without trace of apical pale fasciae.
MALE. MEASUREMENTS AND RATIOS. — N = 20; length, 7-9 mm; width, 1.5-2.5 mm; wing length, M = 3.37±0.178 mm; FL/FW, M = 0.95±0.007; FS1/FS2, M = 1.45 ±0.025.
INTEGUMENTAL COLOR. — Black except as follows: mandible with apical third to half rufescent; flagellar segments 2-11 brown below; wing membranes hyaline, veins reddish-brown to dark brown; tegula piceous; tarsi dark rufescent; tibial spurs testaceous; terga piceous, apical areas with rufescent reflections to moderately translucent especially along apical margins; sterna 2-6 with apical areas hyaline to reddish-brown.
STRUCTURE. — Antennae moderately long, reaching scutellum in repose; scape length equals first two flagellar segments or slightly less; flagellar segment 1 equal in length to one and one-third or more of segment 2 which is slightly longer than broad and shorter than segment 3; median segments (4-8) about one and one-third times as long as broad; Eyes each about three times as long as broad, inner margins parallel; Mandibles slightly decussate, when closed extending about one-third mandibular length beyond midlabrum; without inferior basal tooth or this reduced to a minute angle immediately before inferior mandibular articulation; Malar space and galea as in female; Maxillary palpus short as in female, segmental ratio about as 1.0:0.9:0.8:0.6:0.4:0.4; Labial palpus small, ratio about as 1.0:0.6:0.4:0.4; Labral process smaller than in female, distinctly bidentate and reflexed; labrum below process as in female; Clypeus relatively flat, densely punctate with small round punctures, interpunctural surface moderately shiny to shiny; Supraclypeal area, face above antennal fossae, genal area and vertex as in female.
Pronotum with dorsoventral ridge sharp, surface posterior to ridge shiny, unshagreened with few or no rugulae, surface anterior to ridge finely tessellate; Mesoscutum and scutellum as in female but scutellum never shiny; Propodeum sculptured as in female but dorsal enclosure often with basomedian third with short irregular rugulae; Mesepisterna as in female; Metasomal tergum 1 with basal area moderately shiny, reticularly shagreened with scattered punctures, apical area shiny to moderately shiny, unshagreened or with coarse, faint shagreening; Tergum 2 with apical area medially about one-third as long as basal area; basal area with small sparse punctures separated by two to four puncture widths, slightly shagreened, shiny to moderately so; apical area shiny, impunctate, shagreening extremely faint to absent; Terga 3-5 similar to 2; Tergum 7 without median pygidial area; Sterna 2-5 with basal areas with abundant punctures and fine shagreening dulling surfaces; Sternum 6 flat, not reflexed apically, without distinct apical emargination.
Terminalia as in figures 42-46.
VESTITURE. — Generally pale ochraceous except as follows: vertex and face along inner margins compound eyes usually with long dark brown hairs; tarsi with inner surfaces pale yellow to red; vestiture generally sparse and short but clypeal beard moderately dense, long; terga 2-5 without trace of apical pale fasciae; sterna 2-5 with extremely short, subapical fimbriae of suberect hairs.