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Andrena deconstructed: A photo guide to the characters of the DiscoverLife Andrena key
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, and with explanatory comment.
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, and with explanatory comment.
1) Vertex - the top of the head, above the ocelli
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What 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. Pictures help.
Below: Vertex height of 1 ocellar diameter. The photo also shows about 1 OD distance from the fovea. 
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Above: short vertex, < 1 OD.
Below: tall vertex, nearly 2 OD. 
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2) Antenna, particularly first vs second+third flagellar segments
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3) Fovea: the areas of depressed hairs medial to the eye
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.
Above: Fovea dark; length to lower level of antennal socket
Below: Fovea narrow, less than width of antennal socket, AND distant by greater than its own width from the eye margin. 
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Above:
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Below: Fovea is shallow. Dark above, pale below
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Below: Fovea is deeply impressed. Color: pale
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Right.
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Above: fovea very narrow, at eye margin.
Below: fovea distant from eye margin. |
4) Clypeus: the face below the antennal sockets.
Below: Note the surface texture, which transitions from relatively smooth medially to somewhat tesselate ("tiled") laterally. Punctation is sparse.
Above: A clearer distinction between a shiny lower and medial area, and a textured, dull periphery. There is a narrow but distinct vertical, inpunctate band.
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Above: Completely shiny, edge-edge.
Below: Completely dull. 
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5) Malar space: the distance between the eye rim and mandible base.
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.
Below: Clockwise - Increasing malar space.
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6) Labral Process: a projection immediately below the clypeus.
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. This trait is apparently variable within species; DL indicates 194 species that might have an entire process. But just 44 that are scored for only having this condition.
Entire margin. Process long.
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Entire margin. Extremely long, strap-like. A. barbilabris.
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Emarginate.
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Emarginate - approaching bidentate.
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Bidentate.
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7) Pronotum: Surface between mesepisternum and head; "neck."
The humeral angle is akin to shoulder when viewed from in front. A difficult character, often obscured from clear view by the head and hairs. Removing heads of low quality specimens is a good approach to understanding this character.
Below: Andrena nivalis, specimen determined by LeBerge, an absolutely authoritative taxonomist. The humeral angle is akin to shoulder when viewed from in front. From the side in this image, it is difficult to judge, but should be weak or absent in nivalis. The pronotal ridge is a "ridge extending down from humeral angle." There is a discernible crease on the pronotum in this image, but is perhaps not a pronotal ridge - DL indicates that this is not a feature of nivalis.
Below: Andrena nivalis, specimen determined by LeBerge, an absolutely authoritative taxonomist. The humeral angle is akin to shoulder when viewed from in front. From the side in this image, it is difficult to judge, but should be weak or absent in nivalis. The pronotal ridge is a "ridge extending down from humeral angle." There is a discernible crease on the pronotum in this image, but is perhaps not a pronotal ridge - DL indicates that this is not a feature of nivalis.
Above: This is a pronounced humeral angle.
Below: Similar view of a pronotum that lacks a humeral angle. The margin, between arrows, is rounded. 
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Above: No pronotal ridge.
Below: Pronotal ridge clearly defined. 
Below: One more feature of pronotum with a distinct ridge is the presence or absence of a "deeply impressed diagonal suture."
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8) Scutum: Dorsal area between wings
The DL key asks you to assess pit density "between the parapsidal lines, except midline," and surface texture.
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Above: Pits > 3 widths apart. Some tessellation and shinier areas both evident. A. nuda.
Below: A. angustitarsata. Exhibits both shininess, and surface roughened, which usually equates to "dull." Huh. Detail is interesting - I would call this aereolate rather than tesselate. |
Above: Pits 1-3 widths apart. Shows minimal shiny area posteriorally.
Below: Pits 1 width apart. Large shiny area ... not sure if tesselation here would be called "weak." |
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9) Propodeum: Posterior thorax, facing the abdomen, with a roughly triangular enclosure posterior to the scutellum.
Above: Highly sculptured triangle--100%. Posterior and lateral borders with definite ridge or carina. A, sigmundi
Below: Also 100% sculptured. Carina absent. Surface outside of triangle is "Dulled by tessellation or shagreening." A. crataegi 
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Below: Again, 100% in triangle. No carina. Texture outside triangle "Coarsely and irregularly roughened."
Above: Sculpturing limited to about 10-20%, on the dorsal edge of triangle. No carina. Surface outside triangle is "areolate."
Below: Same score as above ... perhaps. A. nivalis 
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10) Wing.
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How many submarginal cells? Below: 3, most common state.
Below: 2 features here. 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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Below: 2 submarginal cells.
Below: 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.
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Photos above, and right: Complete corbicula, characterized by an anterior fringe of hairs, and bare interior.
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Photos below, and right: Incomplete corbicula, anterior fringe absent, internal hairs present.
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12) Pygidial plate: Hairless protrusion from tergum 6.
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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13. Hind leg: Tibial scopa, hind spur, and trochanteral flocculus
Shape of the tibia. Tricky, because hairs tend to obscure the actual edges of the underlying integument.
Left to right, below: 1) Narrow, nearly parallel-sided, scarcely broader at apex than at midpoint; 2) Normal, broader at apex than at midpoint, but at most 1.5 times as wide as basitarsus; 3) Cuneate, apex at least 1.5 times as wide as basitarsus.
Left to right, below: 1) Narrow, nearly parallel-sided, scarcely broader at apex than at midpoint; 2) Normal, broader at apex than at midpoint, but at most 1.5 times as wide as basitarsus; 3) Cuneate, apex at least 1.5 times as wide as basitarsus.
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Scopal hair formation.
Left to right, below: 1) Hairs simple and unbranched IN CENTER of outside face of tibia narrow, nearly parallel-sided, scarcely broader at apex than at midpoint; 2) Weakly branched throughout; 3) Plumose, with many branches.
Left to right, below: 1) Hairs simple and unbranched IN CENTER of outside face of tibia narrow, nearly parallel-sided, scarcely broader at apex than at midpoint; 2) Weakly branched throughout; 3) Plumose, with many branches.
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Hind tibial spurs. 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.
Above: What I will reference as nearly straight, "normal."
Below: "Strongly bent at least in outer half AND broadened and flattened near base" seems to fit. The only choice that mentions flattened near base. 
Below: DL refers to "spurs." Does a given condition apply to both hind tibia spurs? Apparently; this photo is the second of the spurs on this A. prunorum specimen.
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Above: A variation on "Strongly bent at least in outer half AND broadened and flattened near base"
Below: Yet again, DL assigns "Strongly bent at least in outer half AND broadened and flattened near base." I don't see the flattening. Bent for sure. 
Below: "With apex sharply hooked."
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Trochanteral flocculus. Yet another structure for the accumulation of pollen.
Below left to right: 1) Clearly "Complete, with long, curled hairs except sometimes at extreme base"; 2) Weaker flocculus; 3) Clearly "Incomplete, with short, relatively straight, hairs in basal half or more of flocculus"
Below left to right: 1) Clearly "Complete, with long, curled hairs except sometimes at extreme base"; 2) Weaker flocculus; 3) Clearly "Incomplete, with short, relatively straight, hairs in basal half or more of flocculus"
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Version 26 Dec 2022. All text and photos are mine; I grant permission for any non-commercial use.
