Key to the East Palaearctic species of Sympycnus Loew (males)
1 Hind tarsus without long spine, long bristles or erect setae (Fig. 23), rarely with elongate hairs dorsally................. 2
- Hind tarsus with long bristles (Figs 16, 42) or erect setae (Figs 3, 10) on some tarsomeres............................ 6
2 At least fore femur entirely and hind femur partly black....................................................... 3
- Fore and hind femora yellow, sometimes brown or black at apex or at base........................................ 4
3 Fore tarsus without claws; hind tarsomere 3 without erect setae...................................................................................................... S. taimyricus Negrobov, Barkalov, Selivanova & Grichanov
- Fore tarsus with 1 claw (Fig. 33); hind tarsomere 3 (Fig. 34) with short erect setae, sometimes hardly visible............................................................................... S. stackelbergi sp. nov. [see also couplet 13]
4 Fore and mid tarsomeres 3–5 with long hairs dorsally, longer on tarsomere 5; mid basitarsus with dorsal row of long bristles......................................................................... S. longipilosus Tang, Wang & Yang
- Fore and mid tarsi without remarkably long hairs and bristles (Figs 2, 14, 27, 33)................................... 5
5 Hind basitarsus shorter than tarsomere 4 (Fig. 23); fore basitarsus with elongate setae at base and at apex (Fig. 22)............................................................................................. S. simplicipes Becker
- Hind basitarsus longer than tarsomere 4; fore basitarsus with short setulae (Fig. 27)............... S. simplicitarsis Becker
6 Hind tarsomere 3 with two long thin bristles at base, without long spine at apex (Fig. 42)............................. 7
- Hind tarsomere 3 without two long thin bristles at base, with or without long bristle at apex........................... 8
7 Hind femur pale yellow, with apical 1/3 to 2/3 brown to dark brown; mid tibia usually with posteroventral bristle; hind tarsom- ere 3 distinctly shorter than tarsomere 2; apicoventral lobe of hypandrium subtriangular and pointed at apex (see Pollet et al. 2015, fig. 8).......................................................................... S. pulicarius (Fallén)
- Hind femur entirely pale yellow; mid tibia without posteroventral bristle; hind tarsomere 3 nearly as long as tarsomere 2 (Fig. 42); apicoventral lobe of hypandrium semi-rounded (Fig. 43)................................... S. yakutensis sp. nov.
8 Hind tarsomere 3 with long spine or thick bristle at apex (Fig. 16)............................................... 9
- Hind tarsomere 3 without long spine or thick bristle......................................................... 13
9 Mid femur with almost full posteroventral row of strong bristles, about as long as femur diameter; fore tibia without anterodor- sal row of setae (Fig. 14); fore tarsus without claws (Fig. 15)....................................... S. leleji sp. nov.
- Mid femur with at most 2–3 posteroventral setae at apex; fore tibia with or without anterodorsal row of setae; fore tarsus with or without claws..................................................................................... 10
10 Fore tibia slightly, but distinctly thickened, without anterodorsal row of setae, with rows of long hairs, about as long as tibia diameter.............................................. S. gorodkovi Negrobov, Barkalov, Selivanova & Grichanov
- Fore tibia simple, usually with anterodorsal row of setae (Figs 21, 33), with short hairs............................. 11
11 Hind tarsomere 3 subequal in length to tarsomere 4, with only 1 apical spine...................................................................................................... S. olejnicheki Negrobov, Barkalov & Selivanova
- Hind tarsomere 3 with 1 short bristle at middle in addition to long apical bristle; tarsomere 4 1.5–1.8 times longer than tarsom- ere 3............................................................................................... 12
12 Apical bristle on hind tarsomere 3 about as long as tarsomere 3; fore and hind femora partly black.... S. seticosta Negrobov
- Apical bristle on hind tarsomere 3 about 2 times as long as tarsomere 3; all femora yellow............. S. vadimi Negrobov
13 Only hind tarsomere 3 with erect setae (Fig. 34); other segments without modified setae........... S. stackelbergi sp. nov.
- At least hind tarsomeres 3 and 4 with long bristles or erect setae............................................... 14
14 Hind tarsomere 3 enlarged, 1.5 times wider than tarsomere 4 of same tarsus.................... S. changaicus Negrobov
- Hind tarsomere 3 indistinctly wider than tarsomere 4......................................... S. ferganicus sp. nov.