​Andrena deconstructed: 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 [click images to enlarge], and with explanatory comment.


Vertex | Antenna | Fovea| Clypeus | Malar space | Labral process | Pronotum | Scutum | Propodeum | Wing | Corbicula | Pygidium | Tibia | Scopa  | Spurs

1) Vertex - the top of the head, above the ocelli

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.

Vertex height of 1 ocellar diameter. Image also shows 1 OD distance from the fovea.
Short vertex, < 1 OD.
Tall vertex, nearly 2OD.

2) Antenna, particularly first vs second+third flagellar segments

The first flagellar segment here is about equal in length to the second+third.
F1 longer than F1+F2.

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. If the fovea are hairless ... perhaps you have a Panurginus.

Fovea dark; length to lower level of antennal socket.
Fovea light; width about equal to antennal socket;length nearly to top of clypeus.
Fovea narrow, less than width of antennal socket.
Fovea narrow, less than width of antennal socket, AND distant by greater than its own width from the eye margin.
Fovea moderately impressed; <1 ocellar diameter from ocellus.
Fovea is shallow. Dark above, pale below.
Fovea is deeply impressed; color: pale.

4) Clypeus - the face below the antennal sockets

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.
The surface texture here transitions from relatively smooth medially to somewhat tesselate ("tiled") laterally. Punctation is sparse.
Surface texture shiny medially, dull at edges; distinct impunctate medial band.
Shiny; impunctate band; punctures coarse and deep.

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.

Short: less than eye rim
Short, but greater than eye rim.
Long, about 1/5 as long as broad.
Long, about 1/3 as long as broad.

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. 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
Entire; short
Emarginate, but not bidentate.
Entire, strap-like (A. barbilabris)

Labrum, crista and sulcus

If you are well beyond DiscoverLife, and puzzling over the primary literature, you will encounter features of the labrum. This lies immediately beneath the labral process. It may feature a median crista (ridge), and/or sulcus (trough), and additional ornamentation (lamellae). These are difficult to interpret, as the labrum is usually obscured by overlying hairs.

Median crista of labrum. 

7) Pronotum: Surface between mesepisternum and head; "neck."

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.

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.
This is a pronounced humeral angle.
Pronotum lacking a humeral angle.
Pronotal ridge absent.
Pronotal ridge present. As noted, taxa with this feature will usually lack a lateral sulcus.
Pronotum with distinct ridge AND "deeply impressed diagonal suture."

8) Scutum: Dorsal area between wings

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).
Pits ~3 widths apart; surface roughened/"dull."
Pits > 3 widths apart. Rough/tessalate and shinier areas both evident.

9) Propodeum: Posterior thorax, facing the abdomen, with a roughly triangular enclosure posterior to the scutellum

Caption for Image
Triangle 100% sculptured. Posterior and lateral borders with definite ridge or carina. A. sigmundi.
Also 100% sculptured, but carina absent. Surface outside of triangle is rugose. A. crataegi.
Again, 100% in triangle. No carina. Texture outside triangle "Coarsely and irregularly roughened."
Sculpturing limited to about 10-20%, on the dorsal edge of triangle. No carina. Surface outside triangle is "areolate."
Sculpturing about 10%. A. nivalis.

10) Wing.

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.
Less common: 3 submarginal cells.
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.
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.

11) Corbicula: Pollen carrying area lateral to the propodeum.

Complete corbicula
Complete corbicula, with anterior fringe of hairs, and bare interior.
Hairless interior.
An incomplete corbicula, lacking an anterior fringe. Typically these will also have internal hairs.
Left: weak anterior fringe; Right: internal hairs.

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.

13) Hind leg, trochanteral flocculus.

The curly hairs: a complete flocculus. A. barbilabris.
Complete flocculus.
An incomplete flocculus, hairs straighter, and short.

14) Hind tibia: shape

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.
Normal, broader at apex than at midpoint, but at most 1.5 times as wide as basitarsus.
Cuneate, apex at least 1.5 times as wide as basitarsus.

14) Hind tibia: scopal hair formation

Hairs simple and unbranched
Weakly branched throughout.
Plumose, with many branches.

15) 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.

Normal, straight.
"Strongly bent at least in outer half AND broadened and flattened near base"
A variation on "Strongly bent at least in outer half AND broadened and flattened near base"
Twisted spur of A. barbilabris.

Version 24 Jan 2024. All text and photos David Cappaert, with the advice of Cody Blackketter and Marisa Fisher of Quamash EcoResearch. I grant permission for any non-commercial use of text and images. See also: Key to the Andrena subgenera (females) of the Pacific Northwest. cappaert@comcast.net

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