Key to the world species of Parathalassius

This key treats all 15 species of Parathalassius currently known, including the 12 species comprising the newly expanded Nearctic fauna, and the three described Palaearctic species. This key may be used to identify both sexes of each species, except P. dilatus sp. nov., for which females are unknown. Females of most species can be identified using this key, except for those of P. abela sp. nov. and P. candidatus, which are apparently indistinguishable morphologically.

1 Males, abdomen ending in moderate to large sized globular genital capsule (hypopygium) lateroflexed to right and inverted into cavity formed by pregenitalic segments (Figs 68–71)......................................................2

- Females, abdomen tapered posteriorly with apical segments partially retracted into segment 5 (Figs 127–132)...........16

2 Tibiae and tarsi pale yellow to yellowish-brown (Figs 5–6).....................................................3

- Tibiae dark, tarsi dark at least apically.....................................................................5

3 Head with golden-yellow setae on gena and postgena (Figs 29–30); wing with dark apical spot (Fig. 62); Nearctic................................................................................................. P. midas sp. nov.

- Head with pale setae on gena and postgena; wing without dark apical spot, entirely hyaline; Palaearctic.................4

4 Antenna with postpedicel subtriangular, 1.45–1.5X longer than wide (cf. Figs 38–39); western Palaearctic.... P. blasigii Mik

- Antenna with postpedicel short-subtriangular, 1.1X longer than wide (cf. Fig. 45); eastern Palaearctic...................................................................................................... P. maritimus Shamshev

5 Head and thorax with dark setae; eastern Palaearctic........................................... P. ulrichi Shamshev

- Head and thorax with pale setae; Nearctic.................................................................. 6

6 Thorax with 2 notopleural setae, smaller sized species........................................................7

- Thorax with 3 or 4 notopleural setae, medium to larger sized species............................................13

7 Wing darkened at least towards apex (Figs 59–60)............................................................8

- Wing entirely hyaline (Figs 57, 65, 67)....................................................................10

8 Foreleg with tarsomere 1 dilated, tarsomere 2 slightly expanded (Figs 48–49)......................... P. dilatus sp. nov.

- Foreleg with tarsomeres 1–2 narrow, not dilated.............................................................9

9 Wing broadly infuscate with base of M2 usually appearing indistinct (Fig. 59)...................... P. infuscatus sp. nov.

- Wing darkened only towards apex with base of M2 distinct (Fig. 60)............................... P. melanderi Cole

10 Eyes with ommatrichia of nearly uniform length, without dense reflective mat of long thickened hairs on lower third (Fig. 36).................................................................................... P. uniformus sp. nov.

- Eyes with ommatrichia of lower third distinctly longer and thickened, forming dense reflective mat of long whitish hairs (Figs 25, 34)............................................................................................. 11

11 Antenna with postpedicel short-subtriangular, 1.25–1.4X longer than wide (Fig. 45); hind tarsomere 1 with long erect setae dorsally; hypopygium compact, with parts somewhat concealed.................................... P. socali sp. nov.

- Antenna with postpedicel subtriangular, 1.5–1.6X longer than wide (Fig. 39); hind tarsomere 1 without long erect setae dor- sally; hypopygium larger, with parts more exposed (cf. Figs 70–71)............................................. 12

12 Hypopygium with right cercus truncate at apex, apicomedial margin short and blunt (Fig. 121); phallus with pointed dentiform process near apex (Figs 103–104); dorsal lobe of left surstylus with apex broadly rounded (Figs 73–74); central California, from Mendocino County south to San Luis Obispo County (Fig. 143)............................ P. aldrichi Melander

- Hypopygium with right cercus pointed at apex, apicomedial projection long and narrow (Fig. 122); phallus with pointed denti- form process more basal at 3/4 length (Figs 105–106); dorsal lobe of left surstylus with apex narrow and digitiform (Fig. 72); British Columbia, south through Washington and Oregon to Humboldt County in northern California (Fig. 153)................................................................................................ P. wheeleri sp. nov.

13 Head with golden-yellow pruinosity on face and clypeus, not concolourous with greyish-white frons and vertex (Figs 1, 3, 23)........................................................................................ P. abela sp. nov.

- Head with face and clypeus mostly concolourous with greyish-white frons and vertex (Figs 26, 33, 35).................14

14 Hind tibia with short anterodorsal row of erect comb-like setae (Fig. 51); hind tarsus with tarsomeres 2–4 bearing several short thick perpendicular peg-like setae dorsally (Fig. 54)........................................... P. sinclairi sp. nov.

- Hind tibia with relatively long anterodorsal setae (Figs 50, 52); hind tarsus with tarsomeres 2–4 bearing short thin or thick angled setae dorsally, setae not perpendicular and peg-like (Fig. 53)............................................. 15

15 Hind tarsomere 2 distinctly longer than tarsomere 3 (Fig. 53); hind tibia with anterodorsal setae mostly longer than width of tibia, evenly curved towards apex of tibia (Fig. 50); antenna with postpedicel subtriangular (Fig. 40); wing with vein CuA+CuP ending close to wing margin, slightly curved in most specimens (Fig. 58).............. P. candidatus Melander

- Hind tarsomeres 2 and 3 nearly subequal in length (Fig. 55); hind tibia with anterodorsal setae mostly subequal or equal to width of tibia, barely curved towards apex of tibia (Fig. 52); antenna with postpedicel elongate-conical (Fig. 46); wing with vein CuA+CuP ending far from wing margin, straight (Fig. 66)................................... P. susanae sp. nov.

16 Wing broadly infuscate with base of M2 usually appearing indistinct (cf. Fig. 59)................... P. infuscatus sp. nov.

- Wing entirely hyaline, with at most slight infuscate shading along veins near apex of cell dm, base of M2 distinct (Figs 61, 63)................................................................................................... 17

17 Tibiae and tarsi pale yellow to yellowish-brown (Figs 5–6)....................................................18

- Tibiae dark, tarsi dark at least apically....................................................................20

18 Terminalia prominent with long pointed hemitergites (divided tergite 10) that each bear 2 acanthophorite spines (Figs 127– 128); sternite 8 with apex distinctly bifurcate (Figs 129, 134); Nearctic.............................. P. midas sp. nov.

- Terminalia small with short broad hemitergites (divided tergite 10) that each bear 2 acanthophorite spines (cf. Figs 130–131); sternite 8 with apex truncate or weakly emarginate (cf. Figs 133, 139); Palaearctic.................................19

19 Antenna with postpedicel subtriangular, 1.45–1.5X longer than wide (cf. Figs 38–39); western Palaearctic.... P. blasigii Mik

- Antenna with postpedicel short-subtriangular, 1.1X longer than wide (cf. Fig. 45); eastern Palaearctic...................................................................................................... P. maritimus Shamshev

20 Head and thorax with dark setae; eastern Palaearctic........................................... P. ulrichi Shamshev

- Head and thorax with pale setae (Figs 24, 37); Nearctic......................................................21

21 Thorax with 2 notopleural setae, smaller sized species.......................................................22

- Thorax with 3 or 4 notopleural setae, medium to larger sized species............................................26

22 Wing with slight infuscate shading along veins near apex of cell dm (most specimens) (Fig. 61); hind tarsomere 2 usually pale on basal half; sternite 8 without longitudinal ridges on apical third or half (Fig. 136)................... P. melanderi Cole

- Wing entirely hyaline, without infuscate shading along veins (cf. Figs 57, 65, 67); hind tarsomere 2 usually entirely darkened (rarely pale basally); sternite 8 with or without longitudinal ridges on apical third or half............................23

23 Sternite 8 with longitudinal ridges absent or barely apparent (Fig. 139), at most slightly developed laterally along apical margin and not present medially................................................................... P. socali sp. nov.

- Sternite 8 with longitudinal ridges on apical third or half, extended across sternite including medial margin (Figs 138, 140)...................................................................................................... 24

24 Sternite 8 with longitudinal ridges on apical third, with weak basal bulge, apex emarginate (Fig. 138)... P. uniformus sp. nov.

- Sternite 8 with longitudinal ridges on apical half, with distinct basal bulge, apex truncate (Fig. 140)...................25

25 Central California, from Mendocino County south to San Luis Obispo County (Fig. 143)............. P. aldrichi Melander

- British Columbia, south through Washington and Oregon to Humboldt County in northern California (Fig. 153)................................................................................................ P. wheeleri sp. nov.

26 Sternite 8 with apical quarter rugose (Fig. 137)............................................... P. sinclairi sp. nov.

- Sternite 8 without apical rugosity, apical half with longitudinal ridges (Figs 133, 135, 141)..........................27

27 Hind tarsomeres 2 and 3 nearly subequal in length (cf. Fig. 55); wing with vein CuA+CuP ending far from wing margin, straight (cf. Fig. 66); sternite 8 not distinctly narrowed apically (Fig. 141)........................... P. susanae sp. nov.

- Hind tarsomere 2 distinctly longer than tarsomere 3 (cf. Fig. 53); wing with vein CuA+CuP ending close to wing margin, slightly curved or straight (cf. Figs 56, 58); sternite 8 widened near base, distinctly narrowed apically (Figs 133, 135)......................................................................... P. abela sp. nov. or P. candidatus Melander