Acianthus
Acianthus, commonly known as mosquito orchids, is a genus of about twelve species of plants in the orchid family, Orchidaceae. Mosquito orchids are terrestrial herbs with a single, heart-shaped, usually ground-hugging leaf and one to many small, green, pinkish or purplish flowers on a fleshy stalk. They are found in New Caledonia, Australia and New Zealand.
Description
Orchids in the genus Acianthus are terrestrial, perennial, deciduous, sympodial herbs with a single egg-shaped, heart-shaped or lobed leaf at the base. They have small, roughly spherical, underground tubers from which the flower stems arise. Lacking true roots, they have root-like stolons which develop "daughter" tubers at their ends. These orchids spend the dry, summer months dormant until, following late-summer or autumn rains, the leaf appears. The leaf is glabrous, sometimes ground-hugging, more usually held above the ground and is often purplish-red on the lower surface. Sometimes the leaves of plants with flowers are different from those lacking them. The leaves of all Australian species are very similar, making them hard to identify to species level in the absence of flowers.Flowers appear in the cooler months, usually in autumn, winter or spring, There are one to many resupinate small, green, pinkish or purplish flowers in diameter. The flowers are held on an upright, narrow but fleshy stalk, blend in with their surrounding and often resemble mosquitoes. The sepals are longer than the petals and usually have a long, thin extension on their end. The dorsal sepal is broader than the lateral ones and sometimes forms a hood over the column. The lateral sepals project forward beneath the labellum and the petals spread widely or curve backwards against the ovary. The labellum is heart-shaped and has a prominent callus. The fruit is a thin-walled, glabrous capsule, containing a large number of winged seeds.
Taxonomy and naming
The genus was first formally described by Robert Brown in 1810 in Prodromus Florae Novae Hollandiae. He described three species but did not nominate a type species. Orchids in this genus are closely related to those in the genus Caladenia.The genus name is derived from the Greek ake or akis, "a point, needle" and anthos "flower" referring to the pointed perianth and the acuminate floral segments.
Species
The following is a list of Acianthus species accepted by Australian Plant Census as at March 2025, apart from A. sinclairii that is accepted by the New Zealand [Plant Conservation Network]:- Acianthus apprimus D.L.Jones – early mosquito orchid
- Acianthus borealis D.L.Jones – northern mosquito orchid
- Acianthus caudatus R.Br. – mayfly orchid
- Acianthus collinus D.L.Jones – hooded mosquito orchid
- Acianthus cuneatus D.L.Jones & L.M.Copel.
- Acianthus exiguus D.L.Jones – tiny mosquito orchid
- Acianthus exsertus R.Br. – gnat orchid, large mosquito orchid
- Acianthus fornicatus R.Br. – pixie-caps
- Acianthus ledwardii Rupp – Ledward's mosquito orchid
- Acianthus pusillus D.L.Jones – small mosquito orchid
- Acianthus saxatilis D.L.Jones & M.A.Clem.
- Acianthus scopulus D.L.Jones
- Acianthus sinclairii Hook.f. – heart-leaved orchid, pixie cap
Distribution and habitat
Of the Australian species of Acianthus, 9 are found in South Wales">South Island">South Wales, 6 in Queensland, 3 in Victoria and 2 in South Australia. The sole New Zealand example, A. sinclairii occurs on both North and South Islands, as well as on Raoul, Stewart, Chatham and Three Kings Islands.Mosquito orchids species grow in small groups in forests on decaying litter, occasionally on partially decayed logs. They sometimes form dense vegetative colonies, in sheltered forest or heathland, and are often found underneath shrubs and bracken.