Monopodial orchid with elongated and somewhat bent stem with two rows of leaves, sheathing at the base. Lateral inflorescence, with brownish raceme of rather large flowers (3 – 5 cm in diameter). Differs from closest genes Arachnis and Esmeralda mainly by spurred lip. Pollinia 4, grouped into two bodies. Sepals are free.

We would like to note that this plant has been found by us twice at diverse locations –
not only differing by altitude (more than 1 000m), but also climatic conditions, arising out of almost constant temperature of tropical lowland forest and mountainous forest with two rather distinctive seasons.
Seidenfaden, Smitinand put A. siamense as endemic in Thailand, this should be corrected.
Spirit samples of flowers of our plants were determined in 2002 by Prof. Hendrik Pedersen from Copenhagen since the difference of colour and shape of sepals (see photos) in both plants and lack of literature led us to wrong determination.