Acianthus exiguus, commonly known as tiny mosquito orchid,^[2] is a flowering plant in the orchid family Orchidaceae and is endemic to New South Wales in Australia. It is a terrestrial herb with a single, heart-shaped leaf and up to five translucent greenish-white flowers with pinkish markings and is found growing in forests on the north coast of the state.

Quick Facts Tiny mosquito orchid, Scientific classification ...

Tiny mosquito orchid
Acianthus exiguus near Repton
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Angiosperms
Clade:	Monocots
Order:	Asparagales
Family:	Orchidaceae
Subfamily:	Orchidoideae
Tribe:	Diurideae
Genus:	Acianthus
Species:	A. exiguus
Binomial name
Acianthus exiguus D.L.Jones[1]

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Acianthus exiguus is a terrestrial, perennial, deciduous, sympodial herb with a single heart-shaped, glabrous, dark green leaf which is light reddish-purple on its lower surface. The leaf is 18–30 mm (0.7–1 in) long, 15–20 mm (0.6–0.8 in) wide on a stalk 2–5 cm (0.8–2 in) high.^[2][3][4][5]

There are up to five flowers well-spaced on a thin raceme 6–10 cm (2–4 in) tall, each flower 5–7 mm (0.2–0.3 in) long. The dorsal sepal is oval to elliptic in shape, 4–5 mm (0.2–0.2 in) long, 3–3.5 mm (0.12–0.14 in) wide, translucent greenish-white with faint red markings and forms a hood over the column. The central vein extends about 1.5 mm (0.06 in) beyond the end of the labellum. The lateral sepals are 6–7 mm (0.2–0.3 in) long, about 0.7 mm (0.03 in) wide, linear to lance-shaped, often have a curved tip, are similar in colour to the dorsal sepal and project forwards below the labellum. The petals are a similar colour and are about 2.0–2.5 mm (0.08–0.1 in) long, narrow egg-shaped to lance-shaped and usually spread widely. The labellum is pink to pinkish-mauve, 3.5–4.0 mm (0.14–0.16 in) wide, about 2 mm (0.08 in) wide, with the edges not rolled and lacking teeth. The thick, fleshy callus covering most of the central area is green and has many small pimple-like papillae on the outer half. Flowering occurs from May to June.^[2][3][4][5]

This species is distinguished from other mosquito orchids by its small, greenish, semi-erect flowers and narrow, purplish labellum.^[3][4]

Acianthus exiguus was first formally described by David Jones in 1991 and the description was published in Australian Orchid Research from specimens found near Wardell.^[6][3] The specific epithet (exiguus) is a Latin word meaning "small", "little" or "petty".^[3][7] In a 1995 paper published in the journal Allertonia, Paul Kores claimed that A. exiguus and 3 other species are not distinct from A. fornicatus and should be regarded as a synonyms.^[8] That position has been adopted by Plants of the World Online.^[9]

The flowers of this species, unlike those of other mosquito orchids, are self-pollinating.^[10]

This orchid grows in localised populations between Repton and Wardell in the North Coast region of New South Wales.^[3] It grows in sandy soil and clay loam in tall, moist forest and rainforest.^[2][3][4][5]

Acianthus exiguus has a ROTAP rating of 3RC-, indicating that it is uncommon.^[5]