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 moderate-sized bee is not closely related to any of the preceding species. The female of trizonata can be recognized by the long vestiture (similar to that of vierecki), the clypeal punctures being small, round, evenly spaced, the clypeus lacking an impunctate midline (if present, extremely narrow), and the tessellate sculpturing. The wing veins and tarsi are often red (as in vierecki) and the last two terga have white hairs. The male of trizonata is distinctive in that the apical margin of sternum 6 is reflexed (turned down) and provided with a transverse band or tuft of short, dense, white hairs. The clypeus of the male is provided with a dense beard of long white hairs completely hiding the surface (unless worn). The female of trizonata also has the clypeus abundantly clothed with long hairs which are often brown apically, but these are not so dense as to completely hide the surface.
FEMALE. MEASUREMENTS AND RATIOS. — N = 20; length, 8-10 mm width, 2.5-3.0 mm; wing length, M = 3.71 ± 0.126 mm; FL/FW, M = 0.96 ± 0.005; FOVL/FOVW, M = 2.89 ± 0.040.
INTEGUMENTAL COLOR. — Black except as follows: mandible with apical third to half rufescent; flagellum below brown to reddish-brown; wing veins and pterostigma usually red, rarely brownish-red; terga 1-4 with apical margins hyaline, narrowly rufescent basad of hyaline margins; distitarsi dark brown to red; hind basitarsus often red; hind femur rarely red.
STRUCTURE. — Antennal scape length equals first three and one-half flagellar segments; flagellar segment 1 equal in length to slightly more than segments 2 plus 3, segment 2 equal to 3 and shorter than 4, segments 2-4 shorter than broad, 5-9 about as broad as long. Eyes each about four times as long as broad, inner margins converging slightly towards mandibles. Malar space, mandible and galea as in medionitens. Maxillary palpus as in medionitens but segmental ratio about as 1.0:0.9:0.7:0.7:0.6:0.6. Labial palpus as in medionitens but ratio about as 1.0:0.5:0.5:0.5. Labral process usually entire, trapezoidal, or weakly emarginate apically; labrum below process with median crista, lateral cristae absent or extremely weak. Clypeus rounded from side to side, with small round discrete punctures separated mostly by half a puncture width or slightly more, slightly smaller basally; surface dulled or moderately dulled by fine reticulotransverse shagreening (coarser basally); impunctate midline absent or extremely narrow Supraclypeal area granular, with minute shallow crowded punctures and fine irregular shagreening. Genal area in profile about one and one-third times as broad as eye; surface dull, coarsely shagreened, punctures small and sparse. Vertex dull, tessellate, punctures small and sparse except above ocelli, above lateral ocellus taller than width of ocellus. Face above antennal fossae as in medionitens. Facial fovea as in medionitens but separated from lateral ocellus by slightly more than one ocellar diameter.
Thoracic form and sculpturing as in vierecki; dorsum of thorax usually dull, rarely moderately shiny posteromedially on mesoscutum; propodeum with dorsal area usually regularly tessellate. Wing venation as in medionitens.
Metasomal tergum 1 reticularly shagreened, punctures minute, not evident, surface dull to moderately shiny. Terga 2-4 similar to 1 but usually punctures more evident in apical areas and often slightly shinier apically. Pygidial plate and sterna as in medionitens.
VESTITURE. — Generally white to pale ochraceous except as follows: vertex with abundant dark brown; face along inner margins eyes dark brown; facial fovea usually entirely dark brown or brown in upper half; clypeus with brown hairs along apical margin and often in apical third. Thoracic dorsum with hairs long (as in vierecki or longer); metasomal terga 2-4 with apical fasciae composed of long, decumbent, white hairs (as in vierecki or longer), usually interrupted medially on tergum 3, always interrupted on tergum 2; terga 5 and 6 with pale hairs (usually white). Pollen-collecting hairs normal for subgenus but tibial scopal hairs relatively short. Clypeus with abundant hair throughout although not complete hiding surface (unless worn).
MALE. MEASUREMENTS AND RATIOS. — N = 20; length, 7-10 mm; width 1.5-2.5 mm; wing length, M = 3.27 ± 0.162 mm; FL/FW, M = 0.96 ± 0.004; FS1/FS2, M = 1.37 ± 0.031.
INTEGUMENTAL COLOR. — Black with same exceptions as female but terga 2-5 usually slightly more broadly hyaline apically; hind tibiae rarely rufescent.
STRUCTURE. — Antennae short, in repose not or barely reaching back to metanotum; scape length equals first two flagellar segments; flagellar segment 1 usually longer than 2, about as long as 3, segments 3-10 almost or quite one and one-half times as long as broad, segment 2 quadrate or slightly longer than broad. Eyes each about three and one-fourth times as long as broad or shorter; inner margins converging towards mandibles. Malar space, mandible and galea as in female. Maxillary palpus as in medionitens but segmental ratio as in female. Labial palpus as in medionitens but ratio about as 1.0:0.6:0.5:0.5. Labral process trapezoidal with thickened apical margin to weakly bidentate; labrum below process shiny.
Clypeus with punctures small, round, crowded, separated mostly by less than half a puncture width, impunctate midline usually absent. Supraclypeal area dulled by minute, confluent, shallow punctures and fine shagreening. Vertex as in female. Genal area and face as in medionitens.
Thoracic form and sculpturing as in female but dorsal area propodeum slightly more granular. Wing venation as in female.
Metasomal terga 2-5 as in female terga 2-4 but usually slightly shinier sterna 2-5 with sparse but distinct punctures except apically; sternum 6 with apical margin shallowly emarginate to straight, reflexed downwards. Terminalia as in Figures 67-71.
VESTITURE. — Generally white to pale ochraceous with dark brown to black hairs on vertex, along inner margins of eyes and often with brown along apical margin of clypeus. Hair long as in female; clypeus with thick beard of long white hairs completely hiding surface (unless worn). Terga 2-5 with apical pale bands interrupted medially. Sterna 2-5 with subapical fimbriae relatively short, complete.