Bijdr. (1825) 365
Monopodial epiphytic or terrestrial plants. Stem usually much elongated, sometimes climbing, with many leaves arranged in two rows. Leaves sheathing at the base, glabrous, deciduous, duplicate, leathery. Inflorescence lateral, a raceme or a panicle. Flowers medium-sized to rather large, usually showy. Sepals free. Petals free, fairly similar to the lateral sepals. Lip not spurred, but somewhat sac-shaped at the base, hinged to the column. Column-foot absent. Pollinia 4, about equal-sized, arranged in 2 bodies, solid, caudicles absent, stipe short and broad, viscidium present.
Southeast Asia, Malaysia, Indonesia, the Philippines, Papua New Guinea, east to the Solomon Islands. About 13 species; in New Guinea 2 species.
Epiphytes in lowland forest.
Species from the western part of the distribution range are the well-known 'scorpion orchids', widely cultivated and extensively used in hybridisation. Among similarly popular related genera (e.g. Vanda, Renanthera, Papilionanthe) members of the genus Arachnis are distinguished by the lip being elastically hinged to the base of a footless column and by lacking a true spur. The genera Armodorum and Esmeralda, which do not occur in New Guinea, appear to be the nearest relatives of Arachnis.