A new species, Corybas papillatus, is described and illustrated from peninsular Thailand. The new species is easily recognized through a combination of the following characters: the purplish flower, the rounded apex of the dorsal sepal, the outer surface of dorsal sepal covered with irregular papillae in the upper half, the lateral sepals adnate laterally at the base to the connate petals, the V-shaped throat, the labellum bearing short hairs, dentate to erose labellum margins, and well-developed conical spurs. A key to the species of Corybas in Thailand is presented.

Helmet orchid, Khao Luang National Park, montane forest, Thai-Malay Peninsula

Corybas Salisb. is a genus of terrestrial orchids comprising about 120 species, and is widely distributed from India, South China, peninsular Thailand, the Malesian region, to New Zealand and the Western Pacific Islands (Dransfield et al. 1986; Pridgeon et al. 2001; Pedersen 2011; Tandang et al. 2020). The members of the genus are easily recognized by being small in size with a single cordate leaf and 1–2 underground tubers, dorsal sepal and labellum which together form a tube with expanded mouth, lateral sepals and petals often antenna-like, and labellum usually bearing two spurs (Dransfield et al. 1986). Only two Corybas species are currently known from Thailand, namely Corybas ecarinatus Anker & Seidenf. (Anker and Seidenfaden 2001; Pedersen 2011), and C. geminigibbus J.J. Sm. (Chantanaorrapint and Chantanaorrapint 2016).

During a recent visit to Khao Luang Mountain, Khao Luang National Park, by staff of BKF herbarium, an interesting taxon of the genus Corybas was collected with a unique combination of characters that did not match any of the known species. It is therefore described here as a species new to science.

This study is based on material collected during July 2018 from Khao Luang National Park, Nakhon Si Thammarat province, southern Thailand. Specimens were preserved in alcohol (70% ethanol) and deposited in BKF herbarium. Morphological characters were studied using a stereo microscope Olympus SZX7 and the distinctive characters of the species were illustrated with the aid of an Olympus drawing tube. Measurements were taken from spirit material. The specimen details were compared in detail with original drawings and descriptions given in the protologues of Corybas species in the Malaysian region (e.g. Dransfield et al. 1986; Anker and Seidenfaden 2001; Tandang et al. 2020).

Corybas papillatus is most similar to C. villosus, which is endemic to Peninsular Malaysia (Dransfield et al. 1986; Go et al. 2015). These two species share several common features, viz. flower coloration being purplish, the dorsal surface of dorsal sepal covered with irregular papillae in the upper half, the hairiness of the labellum, the V-shaped throat, irregularly dentate to erose labellum margins, and well-developed conical spurs. Corybas papillatus is distinguished from C. villosus by lacking dorsal sepal keel (strongly keeled in C. villosus) and the adnate lateral sepals and connate petals (all free in C. villosus).

Corybas papillatus might be confused with C. ridleyanus Schltr., another endemic to Peninsular Malaysia (Dransfield et al. 1986; Go et al. 2015), which also has purplish flowers and reddish-purple labellum excepted for a zone below the margin, and a V-shaped throat. Corybas ridleyanus, however, differs from C. papillatus by the truncate apex of the dorsal sepal and the free lateral sepals and petals.

Together with the recent discovery of a new species and new records of orchids from peninsular Thailand, especially in the Nakorn Si Thammarat mountain range (e.g. Ormerod et al. 2012; Tetsana et al. 2014; Chantanaorrapint and Chantanaorrapint 2016; Chantanaorrapint et al. 2017) it is clear that peninsular Thailand is an important region for orchid diversity and that further new species records can be expected to be found from many unexplored areas in this part of the country.

There are now three species of Corybas known from Thailand. A key to distinguish these is given below.

We would like to thank the staff of BKF, C. Hemrat and S. Jirakorn for their help in the field; and A. Schuiteman (K) for his useful comments relating to this new species. We would also like to thank anonymous reviewers for their valuable comments and suggestions to improve the manuscript. This work was supported by the Prince of Songkla University annual government expenditure under the Plant Genetic Conservation Project Year 2018 (no. SCI600107S) under the Royal initiative of Her Royal Highness Princess Maha Chakri Sirindhorn.