Vertex height of 1 ocellar diameter. Image also shows 1 OD distance from the fovea.
Short vertex, < 1 OD.
Tall vertex, nearly 2OD.
This set of pages is intended as a companion to the DiscoverLife key to Andrena females. The DL key is not for the faint of heart; it contains 100 trait descriptors, and 521 taxa, likely close to the US total. Though daunting, DL is intended for the non-specialist, people that don't have large collections or fluency in the technical language of taxonomists. The key is mainly in plain English. It offers figures and images to explain characters. It is interactive - a user can winnow down the list of possible matches using only characters they can interpret with some confidence.
However: A new user will struggle, until they have spent a hundred hours thinking hard about the characters, looking up the occasional difficult term, and getting it wrong too many times. At least, that has been my experience. What the key - any key - needs is really clear photos that show what a trait condition looks like under a scope. I've been adding these to the DL key, and I repurpose them here, so that I can show them at hi-res [click images to enlarge], and with explanatory comment.
Navigation:
Vertex | Antenna | Fovea| Clypeus | Malar space | Labral process | Pronotum | Scutum | Propodeum | Wing | Corbicula | Pygidium | Tibia | Scopa | Spurs|
Examples using the DL Guide: A. wilkella (MI) |A. wilkella male (MI) | A. placata (MI) |A. imitatrix (MI) | A. carlini (MI) | A. crataegi (MI) | A. salicifloris (OR) | A. nasonii (MI) | A. chlorogaster (OR) | A. carolina male (MI) | A. auricoma (OR) | A. astragali (OR)
Related pages: Glossary of bee terminology | Andrena subgenera of Pacific Northwest | Photo Blog
Vertex height (or length) is the distance between a lateral ocellus and the posterior edge of the head. The good thing about this character is that it is always visible, though it is difficult to gauge accurately, even with a reticle.
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Vertex height of 1 ocellar diameter. Image also shows 1 OD distance from the fovea.
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Short vertex, < 1 OD.
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Tall vertex, nearly 2OD.
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2) Antenna, particularly first vs second+third flagellar segments |
The first flagellar segment here is about equal in length to the second+third.
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F1 longer than F1+F2.
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Super important as characters for female Andrena. Tricky to assess for degraded specimens, where facial hairs obscure the fovea, or in poor light. Move the light, move the specimen, trim the hairs. If the fovea are hairless ... perhaps you have a Panurginus.
Fovea dark; length to lower level of antennal socket.
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Fovea light; width about equal to antennal socket;length nearly to top of clypeus.
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Fovea narrow, less than width of antennal socket.
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Fovea narrow, less than width of antennal socket, AND distant by greater than its own width from the eye margin.
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Fovea moderately impressed; <1 ocellar diameter from ocellus.
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Fovea is shallow. Dark above, pale below.
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Fovea is deeply impressed; color: pale.
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There are two key traits here. Is the surface texture shiny, roughened, or a both? Is there a an impunctate medial space?
Surface texture completely dulled.
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The surface texture here transitions from relatively smooth medially to somewhat tesselate ("tiled") laterally. Punctation is sparse.
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Surface texture shiny medially, dull at edges; distinct impunctate medial band.
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Shiny; impunctate band; punctures coarse and deep.
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In the DL key, the short malar spaces are described in relation to the eye rim. This the literal rim, not including the band of darker ommatidia. Large malar spaces are described in terms of L x W.
Short: less than eye rim
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Short, but greater than eye rim.
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Long, about 1/5 as long as broad.
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Long, about 1/3 as long as broad.
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The characters in DiscoverLife concern length (compared to the labrum) and shape of the margin. The tricky calls here are deciding the boundaries between weakly, emarginate, and bidentate. DL indicates 194 species that might have an entire process. But just 44 that are scored for only having this condition. My suspicion is that DL scores are broad in order to allow for misinterpretation (rather than reflecting within-species variation).
Entire, broadly truncate; long
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Entire; short
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Emarginate, but not bidentate.
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Emarginate. Sam Droege tip: viewed frontally this will appear entire; tipped back, the indentation is visible.
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Bidentate.
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Entire, strap-like (A. barbilabris)
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There are two important characters, particularly useful in discriminating subgenera. The humeral angle is akin to the shoulder when viewed from in front. The pronotal ridge is a "ridge extending down from humeral angle." These pronotal characters are difficult to evaluate, often obscured from clear view by the head and hairs. Removing heads of low quality specimens is a good approach to understanding this character.
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Photo at right: Andrena nivalis, specimen determined by LeBerge. The humeral angle is akin to shoulder when viewed from in front. From the side, as in this image, it is difficult to judge, but the angle is absent (or weak). There is a discernible crease on the pronotum in this image. It is NOT a pronotal ridge - it does not extend down from the humeral angle. Note from Joel Gardner, who explained this to me: [The lateral sulcus] is present on almost all species that lack a pronotal ridge, Scaphandrena being the main exception (the absence of the lateral sulcus is a good way to recognize certain Scaphandrena). Most species that have a pronotal ridge lack a lateral sulcus, "Tylandrena" (in the classical sense) being the main exception. |
Where to look for pronotum features.
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This is a pronounced humeral angle.
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Pronotum lacking a humeral angle.
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Pronotal ridge absent.
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Pronotal ridge present. As noted, taxa with this feature will usually lack a lateral sulcus.
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Pronotum with distinct ridge AND "deeply impressed diagonal suture."
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The DL key asks you to assess pit density "between the parapsidal lines, except midline," and surface texture.
Pits 1-3 widths apart. Shows minimal shiny area posteriorally (left).
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Pits ~3 widths apart; surface roughened/"dull."
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Pits > 3 widths apart. Rough/tessalate and shinier areas both evident.
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9) Propodeum: Posterior thorax, facing the abdomen, with a roughly triangular enclosure posterior to the scutellum |
Caption for Image
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Triangle 100% sculptured. Posterior and lateral borders with definite ridge or carina. A. sigmundi.
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Also 100% sculptured, but carina absent. Surface outside of triangle is rugose. A. crataegi.
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Again, 100% in triangle. No carina. Texture outside triangle "Coarsely and irregularly roughened."
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Sculpturing limited to about 10-20%, on the dorsal edge of triangle. No carina. Surface outside triangle is "areolate."
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Sculpturing about 10%. A. nivalis.
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The less common traits - short r-vein or 2 submarginal cells - will narrow possibilities considerably. The vein m-cu trait is less useful - many Andrena species in DL are scored very broadly for this trait.
80% of species have 3 submarginal cells.
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Less common: 2 submarginal cells.
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Upper arrow is the r vein, which is long - length = 4 vein widths. Lower arrow is the position at which vein m-cu meets the second submarginal cell. In this case, ~3/4 of the way distal to the base of the cell.
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Compare to left; r vein is short, < 4 vein widths. Vein m-cu meets second SMC ~2-3 of the way distal to the base of the cell.
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11) Corbicula: Pollen carrying area lateral to the propodeum. |
Complete corbicula
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Complete corbicula, with anterior fringe of hairs, and bare interior.
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Hairless interior.
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An incomplete corbicula, lacking an anterior fringe. Typically these will also have internal hairs.
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Left: weak anterior fringe; Right: internal hairs.
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The pygidial plate is a sort of spatula with which bees prepare brood cells. DL differentiates two conditions, "with an internal plate or ridge," or "unmodified." Modified plates appear to take many forms, are no doubt useful ID clues, beyond DL.
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The curly hairs: a complete flocculus. A. barbilabris.
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Complete flocculus.
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An incomplete flocculus, hairs straighter, and short.
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Tricky, because hairs tend to obscure the actual edges of the underlying integument. Best to view with back-light.
Narrow, nearly parallel-sided, scarcely broader at apex than at midpoint.
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Normal, broader at apex than at midpoint, but at most 1.5 times as wide as basitarsus.
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Cuneate, apex at least 1.5 times as wide as basitarsus.
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Hairs simple and unbranched
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Weakly branched throughout.
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Plumose, with many branches.
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These might deviate in several ways from the typical straight or gently curved condition. DL presents 5 options, that require interpreting "slight but obviously," "sometimes twisted," "distinctly." In the absence of clarity on this, I can only follow DL: if I have a known species, I will assume that it is correctly scored.
Normal, straight.
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"Strongly bent at least in outer half AND broadened and flattened near base"
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A variation on "Strongly bent at least in outer half AND broadened and flattened near base"
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Twisted spur of A. barbilabris.
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Hooked.
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December 2024 - I am working through the collection of the Rufus Isaacs Berry Crops Entomology Lab at Michigan State University. I am using these as tests - can I find and interpret the above traits? What else jumps out from a detailed image?
Easy calls, based on 1st 3 images at right:
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Notable: All light/tawny-orange hairs. Dense, orange scutal hairs. Malar area short. Gena narrow. Vertex> 1ocd. Long unbranched tibial hairs. Triangle roughened, not rugose.
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There is no path in DL that clearly excludes species other than A. wilkella. So I looked at the primary literature: A Revision of the Bees of the Genus Andrena of the Western Hemisphere. Part XIII. Subgenera Simandrena and Taeniandrena (LaBerge, 1989)
"A large, readily recognized bee from North
eastern North America. The female has slightly elongated punctures
on the flattened clypeus, highly punctate metasomal terga, roug
ened but not rugulate propodeal enclosure ..."
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Clypeus notably flat. Punctures elongate (I think). Clincher: "Labral process tra
ezoidal, slightly constricted before apex, emarginate apically, surface basad of apical teeth with fine curved rugulae, shiny;"
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Easy calls:
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Notable: Tergae, scutum, head densely hairy.
T2: A. wilkella is scored as having dense pits, separated by less than one pit diameter. I would have called the very shallow pits here "indistinct." 
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There
Two specimens that strike me as having the same vibe.
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Discussion: I disagree with DL about this ID, based on examination of several specimens determined by J.S. Ascher. The DL scoring is accurate, but the matching algorithm is off. This kind of mistake is surprisingly uncommon in my experience, given the complexity of a key for 521 species. |
Easy calls:
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Evidence against A. wilkella: this specimen with plumose tibial scopa (simple in wilkella); absent impuncate center of clypeus (present in wilkella).
A. placata feature: "Terga 5 and 6 hairs usually dark brown." 
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Above: Medial impunctate band of clypeus absent; A. wilkella would have this condition.
Below: Distinctly bidentate labral process. 
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Above:Highly plumose tibial scopa; simple in wilkella.
Below:Of the three options, only A. placata has the middle leg basitatsus "expanded medially, broader than hind basitarsus."" 
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Easy calls, based on 1st 3 images at right:
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Note on image: not clear here that clypeus is shiny in center. But it is.
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Fovea broader than antennal socket: excludes A. alleghaniensis. Also: alleghaniensis with short scutal hairs.
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Above: Labral process emarginate [alleghaniensis bidentate, morrisonella entire or weak].
Below: "Propodeal corbicula hairs in anterior half mostly or entirely branched [simple in morrisonella]. 
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Easy calls, based on 2 images at right:
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Above: Vertex=2ocd. Excludes milwaukeensis and thaspii.
Below: Very short malar space. Only matches to A. vicina. 
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Easy calls:
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A. amphiloba does not match. We would expect dark hairs on the tibia, a normal tibial spur, and a fovea like Trachandrena.
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Above: "Noted for coarse propodeum and curved and thickened inner hind tibial spur."
Below: Distinctive carinate ridge of hind femur--a crataegi thing. 
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Easy calls:
All matches are subgenus Trachandrena, characterized coarse sculpturing, an entirely rugose, enclosed propodeum, and fovea narrowed at the level of the antennal socket. DL does not unamnbiguously resolve species. So I consulted A Revision of the Bees of the Genus Andrena of the Western Hemisphere. Part VI. Subgenus Trachandrena (LaBerge). A. fuscicauda doesn't match this specimen in several respects. Differentiating miranda and salicifloris is harder. |
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miranda vs salicifloris:
A. salcifloris and A. miranda split on a final couplet. A. salicifloris: mesoscutum with anterior third with punctures crowded, separated
by mere ridges; vertex above lateral ocellus equals about one ocellar diameter.
A. miranda: Mesoscutum with anterior third with punctures discrete, separated by half to almost one puncture width; vertex above lateral ocellus usually distinctly longer than one ocellar diameter. My specimen appears to have crowded punctures AND a long vertex. To quote LaBerge: "There is no doubt that misidentification of such specimens will continue and, indeed, it is often impossible to place some in the correct species with any confidence." 
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Above: Marrow lower fovea of Trachandrena.
Below: Scutum of another specimen. 
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Easy calls:
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Above: Propodeal triangle 100% sculptured; note posterior view of corbicula.
Below: Cuneate hind tibia. 
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Above: Tubercle on the posterolateral edge of the mesepisternum: a nasonii thing.
Below: Same feature on male A. nasonii. 
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Easy calls:
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Easy calls:
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Quadrangle longer than broad. Tawny hairs. Clypeus shiny-dull, punctate except midline. Decussate mandibles. Max palp exceeds galea by 2 segments.
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Cheek 2X as wide as eye. From DL: With distinct angle of 90 degrees or more along lower rear margin breaking the curve from top to bottom.
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Above, from DL: UNCOMMON: With longitudinal rugulae in basal half, tessellate or smooth at apex
Below, from DL: DL: weak tooth not projecting below level of mandible articulation. 
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Easy calls:
DL has no images of bucculenta or lutehirta, so overall appearance is unhelpful. A. lutehirta is scored as having a long vertex (1.5-2od), whereas this specimen has vertex less than one ocellar diameter. This specimen has 3 SMC, whereas A. bucculenta is scored as having 2SMC, a condition that excludes the subgenus of the other two options. This leaves a diagnosis of A. auricoma. |
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Above:
An 88 year old specimen from OSU. Note cuneate tibia.
Below: The sculpturing here is 0% sculptured, although the surface texture is borderline. The label here indicates Euandrena. But Ptilandrena is the updated classification. 
Cautionary notes: Scoring for western Andrena species is often incomplete. The LaBerge subgenus key would seem to indicate Ptilandrena as having highly plumose tibial scopae, whereas this specimen has simple hairs. However LaBerge himself contradicts this: I have images of a specimen, with det. by LaBerge, of another Ptilandrena (A. nigrihirta) with tibial scopa very similar to this specimen, with simple hairs. |
Above: Slightly emarginate labral process.
Below: Note super-shiny tergae, sculpturing of frons. 
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| Shortcut: Andrena astragali forages exclusively on death camas, which is visited by no other bee. |
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