Leopoldius

1.a. Snout distinctly shorter than the head, fleshy; antennae: 3rd segment bare -> 2

1.b. Snout about as long as the head, stiff by chitin; antennae: 3rd segment dorsally with small bristles; coxae black; frons browhish yellow with black markings. 10-14 mm. D. -> Leopoldius (Abrachyglossum) capitatum Loew

2.a. Scutellum yellow; femora yellow or yellow with a black patch covering less than 1/3rd of the femur; front border of wing more or less hyaline. Female: theka yellow; at least coxa 1 yellow -> 3

2.b. Scutellum black; femora yellow with a large black patch or ring, dorsally covering 1/2 of the length or more (femora may be almost black); front border of wing strongly blackened; all coxae black; abdomen shining, with black hairs. Female: theka black, clearly protruding in side view. 10-11 mm. B, D, central Europe. -> Leopoldius calceatum Rondani

3.a. Frons with a broad yellow stripe from eye to eye, which is as yellow as the other yellow parts of the head, sharply bordered with the black parts. Only females can be identified. -> 4

3.b. Frons black, if yellow is present above antennal implant, it is darker than the other yellow parts of the head. -> 6

Females, theca at the ventral side of the body. From left to right (or top to bottom on small screens): L. diadematus and L. valvatus with a small theka and L. coronatus with a wide theca. After De Bree et al. (2014).

4.a. Female: in ventral view, theca narrow, about half the abdominal width (figure above) -> 5

4.b. Female: in ventral view, theca as wide as the abdomen, with only a few bristles in the middle (figure above), compare to L. signatus as the only other Leopoldius with wide, yellow theca). Male and female (approximate): Frons: yellow stripe smaller than black stripe, front border of black stripe straight. 12-13 mm. NL, B, D. -> Leopoldius coronatus Rondani

5.a. Female: abdomen from below: theca uniformly filled with bristles, leaving only the base free, anal segment rounded and about as long as wide; abdomen in side view: theca hardly protruding. Males and females (approximate): frons: yellow stripe as wide as black one, front border of black stripe with two yellow incisions. 12-14 mm. D. -> Leopoldius diadematum Rondani

5.b. Female: abdomen from below: theca with a dense circular bristlefield in the tip half, anal segment elongated and almost twice as long as wide; abdomen in side view: theca protruding for about 1/3rd of the abdominal height.  12 mm. NL, D, central and southwest Europe -> Leopoldius valvatus Kröber (= Leopoldius cabrilsensis Carles-Tolra, see Stuke et al, 2010)

6.a. Snout blackish; tergite 3 and 4: yellow bands without incisions, tergite 2: yellow band with one incision. Female: theca elongate, about half the width of the abdomen in ventral view with the side margins bent inwards. 10-12 mm. NL, B, D. -> Leopoldius brevirostris Germar (= Leopoldius macrus Seguy, see Clements (2000))

6.b. Snout yellowish; tergite 3 and 4: yellow bands with one (males) to three (females) incisions, tergite 2 with one incision. Female: theca wide, about as wide as the abdomen in ventral view, with a dense britle field in the top half. 10-11 mm. NL, B, D. -> Leopoldius signatus Wied. in Meigen

Theca of L.eopoldius brevirostris.

Leopoldius signatus, female, abdomen (from Kröber, 1925)