Abstract. Delforge, P. - Contribution to the knowledge of the Orchids of the island of Kos (Dodecanese, Greece) [Natural. belges 90 (Orchid. 22) (2009): 49-232]. After a presentation of the geology, the geomorphology, the the vegetation, and the human impact over the landscape of the island of Kos, a review of the past and present studies and research about its orchids is made. It appears that 51 orchids species were known for Kos before 2009 (Table 1). Research in Kos in February, March, April, and May 2009 has revealed 53 orchid species for the island, bringing their number up to 58 (Table 1). Among the 58 species known at the present for the island, 1 is doubtful and probably extinct (Dactylorhiza iberica), 2 are very likely extinct (Cephalanthera epipactoides, Himantoglossum comperianum), 2, not seen in 2009, are represented by few individuals probably sporadic (Ophrys polycratis, Serapias patmia). Three species, Ophrys cretensis, O. omegaifera and Paludorchis palustris var. elegans are represented by only one or few individuals in flowers in 2009, growing on one single site, and 7 others are present in only 1‑2/269 squares of 1 km × 1 km: Epipactis densifolia, Ophrys basilissa, O. bombyliflora, O. cerastes, O. dodekanensis, O. ptolemaea (sp. nova here described), and Orchis provincialis. Discussion is made for each species observed or listed in the island of Kos, following the order used in Delforge (2006a, 2009).

The genus Cephalanthera is represented by C. epipactoides (probably extinct). The genus Epipactis is represented by E. densifolia (extremely local, critically endangered by overgrazing and the harnessing of springs). Limodorum abortivum is very local, Spiranthes spiralis, which flowers only in Octobre, known from few places and extremely local.

The genus Dactylorhiza is represented with certainty only by D. romana (very local, first mention for Kos). The genus Serapias in the island comprises S. parviflora (rather widespread), S. bergonii (local), S. vomeracea (very local), S. patmia (extremely local, not seen in 2009), S. orientalis (widespread, with 5 varieties: var. orientalis, var. carica, var. monantha, var. sennii, and var. spaethiae var. nova here described) All the Anacamptis pyramidalis seen in 2009 belong to A. pyramidalis var. brachystachys, rather widespread.

The former genus Orchis s.l. is represented by 12 species belonging to 6 genus, (1) Paludorchis with P. laxiflora (very local) and P. palustris var. elegans, extremely local and nearly extinct owing to the destruction and the overgrazing of the wet zones around the Alykes lagoon, while populations of 10,000 individuals flowered in this area in 1985, (2) Herorchis with H. picta var. caucasica (rather widespread); (3) Vermeulenia with V. collina and V. papilionacea var. heroica (both rather widespread), (4) Anteriorchis with A. fragrans (rather widespread), and A. sancta (very widespread, by far the most widespread orchid of the island), (5) Orchis s. str. with O. anatolica (very local), O. provincialis (extremely local, first mention for Kos), and O. italica (very local), (6) Neotinea with N. lactea (extremely local), and N. maculata (local).

The genus Himatoglossum is represented by H. comperianum (1 individual seen in 1985 only, probably extinct) and H. robertianum (local).

The genus Ophrys is represented by 32 species. The sectio Pseudophrys on Kos comprises 13 species which belong to 6 species groups: (1) the Ophrys iricolor group with O. iricolor (rather widespread), (2) the O. attaviria group with O. attaviria (very local, with 2 varieties: var. attaviria and var. cesmeensis), O. parosica (local, with 2 varieties: var. parosica and var. phaseliana), and O. pelinaea (local), and O. blitopertha (local), (3) the O. subfusca group with O. cinereophila (local), (4) the O. lutea group with O. sicula (rather widespread, polytypic) and O. phryganae (rather widespread, polytypic), and (6) the O. omegaifera group with O. omegaifera (extremely local and very rare), O. basilissa (extremely local and very rare), O. sitiaca (very local), O. polycratis (extremely local, not seen in 2009), and O. meropes (very local, sp. nova here described).

The section Ophrys (= Euophrys Godfery nom. nudum) on Samos comprises 19 species which belong to 8 species groups: (1) the Ophrys speculum group with O. speculum ( widespread), (2) the O. tenthredinifera group with O. bombyliflora (extremely local, endangered by the destruction of wet biotopes), the large-flowered O. leochroma (widespread) and O. villosa (very local), (3) the Ophrys apifera group with O. apifera, very local, (4) the O. bornmuelleri group with O. heterochila (local) and O. lyciensis (very local), (5) the O. oestrifera group with O. cerastes (extremely local), O. cornutula (very local), and O. dodekanensis (extremely local, first mention for Kos), (6) the O. heldreichii group with O. calypsus (very local), O. colossaea (very local), O. hippocratis (very local, early-flowering, sp. nova here described), O. homeri (very local), and O. ptolemaea (extremely local, sp. nova here described), (7) the O. umbilicata group with O. umbilicata ( widespread), and (8) the O. mammosa group with O. ferrum-equinum (extremely local, with varied flowers: f. ferrum-equinum and f. labiosa), O. mammosa (very local), and O. cretensis (extremely local, first mention for Kos and the eastern Aegean).

After carefull examination of “critical” plants precisely located in the field as well as photographs, published or not, it appears that numerous old or recent mentions of orchids, made for Kos, are questionable or erroneous. As a result, the following species must be deleted from the list of the orchids of the island of Kos: Anteriorchis coriophora (s. str.), Epipactis helleborine, Herorchis morio (s. str.), Ophrys candica, O. cornuta, O. ‘fuciflora-holoserica’, O. fusca (s. str.), O. lesbis, O. leucadica, O. (holoserica subsp. vel var.) maxima, O. oestrifera, O. parvula, O. regis-ferdinandii, O. scolopax, O. tenthredinifera (s. str.), and Serapias politisii.

A list of 53 species and 9 hybrids, and 58 distribution maps, based on the presence in 269 utmwgs84 squares of 1 km ę 1 km, are provided as well as a list of 566 sites prospected during the spring 2009, from February 27 to May 28.

Key-Words: Flora of Greece, flora of Aegean, flora of Kos, Dodecanese; Orchidaceae, Ophrys hippocratis P. Delforge spec. nova, Ophrys meropes P. Delforge spec. nova, Ophrys ptolemaea P. Delforge spec. nova, Serapias orientalis var. spaethiae P. Delforge var. nova.