Artificial key to the genus Arthrorhaphis .

— The morphologically poorly characterised “ Arthrorhaphis septentrionalis ” (Fig. 8F) and Arthrorhaphis sp. 1 are not included in the key. Characters for the parasitic species have been compiled from Obermayer (1994), Santesson & Tønsberg (1994), Kocourková & Van den Boom (2005), and Etayo (2017).

1. Lichenised thallus absent; species strictly lichenicolous .....2

1. Lichenised thallus present; lichenicolous stages present or absent.........................................................................7

2. On the thallus of foliose or squamulose lichens.............3

2. On the thallus of crustose lichens...................................6

3. Causing aeruginose discolouration of the host thallus; on squamules and podetia of Cladonia spp ....................... ................................................... A. aeruginosa (Fig. 6A)

3. Aeruginose discolouration of the host thallus absent; on other lichens...................................................................4

4. Asci predominantly 4-spored; hymenium 110–150 μm; on Phyllobaeis imbricata ......................... A. phyllobaeis

4. Asci predominantly 6- or 8-spored; hymenium ≥150 μm; on other lichens..............................................................5

5. Asci predominantly 6-spored; hymenium 200–250 μm; on Arctoparmelia incurva ... A. arctoparmeliae (Fig. 6B)

5. Asci 8-spored; hymenium 150–180 μm; on Melanohalea olivacea ........................................ A. olivaceae (Fig. 6C)

6. Hymenium (except epihymenium) ± clear; ascospores 3.5–4.5(–5) μm wide, 10–15-septate; on Dibaeis baeomyces ................................................ A. muddii (Fig. 6D)

6. Hymenium strongly inspersed; ascospores 2.5–3.5 (–4.5) μm wide, 7–9(13)-septate; on Baeomyces rufus .... ............................................................ A. grisea (Fig. 6E)

7. Ascospores of the alpina, jungens or vacillans type; Ca-oxalate crystals in the medulla usually present..............8

7. Ascospores of the citrinella type; Ca-oxalate crystals absent in the medulla or beneath the soredia ( A. citrinella s.l.)................................................................................ 10

8. Ascospores (vacillans type); soredia absent..... A. vacillans

8. Ascospores of the alpina or jungens type; soredia present or absent.........................................................................9

9. Ascospores alpina type ..................................................... ........................... A. alpina var. alpina s.l. (Fig. 8A,C,E)

9. Ascospores jungens type .................................................. ............................ A. alpina var. jungens s.l. (Fig. 8B,D)

10. Soredia or granules absent; thallus distinctly bullate-areolate; parasitic stages absent...........................................11

10. Soredia or granules present (occasionally indistinct); thallus areolae present or absent; parasitic stages present or absent...........................................................................12

11. Thallus surface ± smooth and shiny; areolae folded in ridges and with central cavity or resting in umbrella-like fashion on black hyphal strands; Australasia ................... .............................................. A. catolechioides (Fig. 7A)

11. Thallus surface ± verrucose, matt to slightly shiny; areolae convex to distinctly bullate, not folded in ridges; pale yellow medulla present; East Asia ... A. bullata (Fig. 7F)

12. Thallus forming small compact colonies on saxicolous bryophytes and cyanobacteria; thallus surface entirely disintegrated into finely granular to farinose soredia; parasitic stages absent; East Asia and Scandinavia .......... .................................................... A. farinosa (Fig. 7D,E)

12. Thallus otherwise; parasitic stages present or absent........13

13. Thallus entirely of small loose to compact aggregations of granular soredia on saxicolous bryophytes; parasitic stages absent; Europe and Iceland .................................... ................................................... A. citrinella (Fig. 7B,C)

13. Thallus of discrete to confluent areolae on soil, terricolous bryophytes and plant remains, or parasitic on Baeomyces spp. (rarely on other terricolous lichens); areolae breaking into soredia, rarely completely disintegrated or esorediate; widespread in the Northern Hemisphere........ ........................................................ A. vulgaris (Fig. 6F)