Key to sections and species of Ypsilopus Summerh.

1a Leaves spread along the stem, flower spur at least 7.5 cm long, column at least 5 mm long sect. Barombiella:2
1b Leaves arranged in a fan, flower spur less than 5 cm long, column less than 2 mm sect. Ypsilopus:3
2a Column longer than 2 cm Ypsilopus schliebenii
2b Column less than 2 cm long Ypsilopus amaniensis
3a Inflorescence 1-2-flowered Ypsilopus viridiflorus
3b Inflorescence 3- to many-flowered 4
4a Plant pendent Ypsilopus longifolius
4b Plant erect 5
5a Leaves shorter than 7 cm, flowers subcampanulate Ypsilopus leedalii
5b Leaves longer than 10 cm, flowers not subcampanulate 6
6a Lip obscurely 3-lobed, sub-rhombic to ovate, acute Ypsilopus erectus
6b Lip clearly 3-lobed with spreading side lobes and an acute, triangular median lobe 7
7a Spur more than 3 cm long, flowers bright yellow Ypsilopus citrinus
7b Spur less than 3 cm long, often much shorter, flowers white fading to orange-yellow 8
8a Leaves spotted dark green on a paler background in both fresh and dried material Ypsilopus tanneri
8b Leaves unspotted 9
9a Spur inflated toward the apex Ypsilopus sarcodanthus
9b Spur slender, not inflated 10
10a Flowers with tepals 5-7.5 mm long, side lobes erose or bifid Ypsilopus tricuspis
10b Flowers with tepals 8-12 mm long, side lobes slender and acute Ypsilopus furcistipes


Excluded species
Ypsilopus liae Delep. & J.-P.Lebel, as it lacks the characteristic Y-shaped stipes of Ypsilopus, and had been described before as Diaphananthe arbonnieri Geerinck, is actually a species of Rhipidoglossum as firstly pointed out by Fischer & al. (2011).
References:
Molecular phylogeny and taxonomic synopsis of the angraecoid genus Ypsilopus (Orchidaceae, Vandeae) TAXON 68 (3) June 2019: 455–470; WCSP (2020). 'World Checklist of Selected Plant Families. Facilitated by the Royal Botanic Gardens, Kew. Published on the Internet; http://wcsp.science.kew.org/ Retrieved 31 October 2020.'