Acianthus is a small genus of 9 known species, 8 endemic in Australia and 1 in New Zealand, from the orchid family (Orchidaceae). Acianthus was once circumscribed to include about twice as many species, but the additional species have now been redistributed to other genera, such as Acianthella.
The name is derived from the Greek words achis (point) and anthos (flower), referring to the pointed sepals. Common Australian names include "Pixie-caps," "Gnat Orchid," and "Mosquito Orchid." Acianthus species occur in Australia and New Zealand, in small groups in forests on decaying litter, or occasionally on partially decayed logs or in association with bracken fern.
These terrestrial orchids have small, globular, subterranean tubers from which the flower stems arise. They form dense vegetative colonies, in sheltered forest or heathland, and are often found underneath shrubs and bracken. There is a single, heart-shaped, clasping leaf, often with purple markings, near the base, held horizontally above the soil. The flowers often resemble mosquitoes. These orchids produce a terminal raceme, arising from near the center of a solitary leaf, bearing pinkish or greenish flowers with a long spur. There are small flowers (4 to 5 mm), blending in with the surroundings. They are pollinated by small gnats seeking the nectar from the unlobed labellum.