Reprinted with permission of the American Entomological Society from:
Bouseman, J. K., LaBerge, W. E. 1978. A revision of the bees of the genus Andrena of the Western Hemisphere. Part IX. Subgenus Melandrena. Transactions of the American Entomological Society 104: 275-390.
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This striking Rocky Mountain bee is very similar to Andrena carlini at least superficially. The female of transnigra can be separated from that of carlini by the distinct band of black or dark brown hairs across the thorax between the tegulae and by the more densely punctate terga. The male of transnigra can be separated from that of carlini by the sixth sternum having a straight or slightly emarginate apical margin (not produced) and provided with abundant, short, brown to black hairs.
FEMALE. MEASUREMENTS AND RATIOS. — N = 20; length, 10-15 mm; width, 4-5 mm; wing length, M = 5.41 ± 0.115 mm; FL/FW, M = 0.91 ± 0.006; FOVL/ FOVW, M = 2.K7 ± 0.0.19.
INTEGUMENTAL COLOR. — Black except as follows: mandible with apical half rufescent; flagellum below brownish-black to dark reddish-brown; wing membranes moderately infumate, brown, veins black to dark reddish-brown; distitarsi slightly rufescent; tibial spurs piceous.
STRUCTURE. — Antennal scape as long as first three and one-half flagellar segments or longer; flagellar segment I as long as segments 2 plus 3 plus half of 4; segments 2 and 3 equal in length and shorter than 4, about as broad as long; segments 4-10 slightly longer than broad. Eyes each about four times as long as broad, inner margins parallel. Mandible and galea as in carlini. Maxillary palpus as in carlini but segmental ratio about as 0.9:1.0:0.9:0.8:0.7:0.7. Labial palpus as in carlini but ratio about as 1.0:0.7:0.5:0.5. Labral process entire, usually trapezoidal with rounded apical angles; labrum below process sulcate as in carlini but sulcus less shiny, occasionally with minute longitudinal cristae on either side of strong median crista. Clypeus shiny with small round punctures separated mostly by about half a puncture width or slightly more, occasionally with impunctate midline but usually without. Supraclypeal area and face above antennal fossae as in carlini. Facial fovea as in carlini. Vertex above lateral ocellus equals about two ocellar diameters, surface dulled by abundant small punctures and dense tessellation. Genal area in profile almost twice as broad as eye, surface dulled by reticular shagreening and minute obscure sparse punctures, occasionally shiny near compound eye.
Pronotum, mesoscutum, scutellum and metanotum as in carlini but dorsal surface outside of enclosure and posterior surface with punctures more abundant and lateral surface tessellate without vermiculate rugulae posteriorly. Mesepisternum and wing venation as in carlini.
Terga 1-4 with apical areas equal to less than half median length of each tergum (about as 2:5); tergum 1 with basal area punctures small, separated largely by two to three puncture widths; terga 2-4 with basal area punctures usually more crowded, especially laterally, separated by one to two puncture widths; apical areas with punctures absent or sparse, shallow and obscure; surfaces moderately dulled by reticular shagreening. Pygidial plate V-shaped with acute apex, internal raised area absent. Sterna as in carlini.
VESTITURE. — Black except dorsum of thorax pale ochraceous to white with a conspicuous transverse band of black hairs between tegulae; face often with some pale hairs mixed with dark near antennal fossae and on occiput; hind femora usually with some long dorsal hairs pale; trochanteral flocculus often brown and rarely ochraceous.
MALE. MEASUREMENTS AND RATIOS. — N = 20; length, 7-10 mm; width, 2.5-3.5 mm; wing length, M = 4.39 ± 0.156 mm; FL/FW, M = 0.84 ± 0.007; FS1/FS2,M = 1.17 ± 0.019.
INTEGUMENTAL COLOR. — Black except as follows: mandible with apical third rufescent; flagellar segments dark brown to reddish-brown below; wing membranes (lightly infumate, yellowish, veins dark brown to reddish-brown; terga 1-4 often with apical areas dark rufescent; sterna 2-5 usually with apical areas translucent; tarsi often dark rufescent; tibial spurs dark reddish-brown to black.
STRUCTURE. — Antennae as in carlini. Eyes each about three and one-half times as long as broad, inner margins parallel. Mandibles and galea as in carlini. Maxillary palpus as in carlini but segmental ratio about as 1.0:1.0:0.8:0.8:0.6:0.6. Labial palpus as in carlini but ratio about as 1.0:0.6:0.6:0.5. Labral process and labrum as in carlini. Clypeus moderately long, densely punctate, punctures separated largely by half a puncture width, surface moderately dulled by shagreening. Supraclypeal area and face above antennal fossae as in carlini. Vertex above lateral ocellus equals one and one-half to two ocellar diameters, sculptured as in carlini. Genal area in profile equals one and one-half to two times width of eye, sculptured as in female.
Thoracic form and sculpturing and wing venation as in carlini but propodeal dorsal enclosure often almost entirely roughened by weak, irregular rugulae.
Metasomal terga 2-5 with basal areas with punctures minute, obscure, separated largely by two to three puncture widths or more; apical areas impunctate or virtually so; surfaces moderately shiny, with weak reticular shagreening. Tergum 7 with pygidial area not evident. Sterna 2-5 sculptured as in carlini. Sternum 6 with apical margin straight or slightly concave, not produced as in carlini; surface without raised triangular area; punctate in apical third.
Terminalia as in figures 44-47; note the following: gonostylus blunt, sternum 7 with emargination deep and lobes bluntly rounded, sternum 8 with rounded apex (as in carlini in lateral view).
VESTITURE. — White to pale ochraceous except as follows: vertex, face alongside of compound eyes, and genal areas with abundant long black hairs; mesoscutum occasionally with a few long dark hairs posteromedially; metepipleuron, posterior margin of mesepipleuron and anterior part of propodeal lateral surface often with long black hairs; terga 3-7 with hairs black, often tergum 2 with hairs black in part; sternal vestiture dark brown to black; tibiae and tarsi with black hairs, occasionally fore and middle tibiae pale; hind femur often with dark hairs at least ventrally.