Arachnorchis is a terrestrial genus in the Orchidaceae family characterized by tubers partially enclosed in a fibrous sheath. The about 200 species are endemic to Australia and are primarily found in southern areas but the distribution ranges from coast to inland areas and into the mountains. Although most species grow in well-drained soil, some are found in soils wet or water logged in winter during their growing season. They are found in a variety of forested and shrubby habitats. Formerly included in Caladenia and still considered that genus by many, this genus was created in 2001 by David Jones and Mark Clement when they split Caladenia into numerous genera. The type species is Arachnorchis patersonii. Species in this genus are often referred to as Spider Orchids because of their spidery flowers that are green, white, pink, red, or yellow. The flowers in this group have long tapering segments, usually clubbed or trichome covered sepals, or if without these features at least the flowers not pink.
More information on this genus can be found here.
The Species Orchid Society of Western Australia hosts a lot of information online about Australian orchids here.
The Australian terrestrial orchids are notoriously challenging to grow, although some enthusiasts are starting to achieve success by cultivating the symbiotic fungus that many of the orchids require in order to grow. Very careful fertilization is required to keep the fungus and orchid in balance. There's a good discussion of the relationship here.
Arachnorchis falcata syn Caladenia falcata known as the Fringed Mantis Orchid in native to Western Australia where it grows in well-drained soil in woodland, shrubland and on granite outcrops, often under she-oaks. Flowers are greenish to greenish-yellow with red stripes and markings. Petals curve backwards while the lower sepals curve forward. The lip is green and fringed with maroon calli and apex. Photo by Bob Rutemoeller labeled as this species at a flower show in Albany September 2007.
Arachnorchis hirta syn. Caladenia hirta is endemic to Western Australia. This orchid is often referred to as the Sugar Candy Orchid. It is widespread in the southwest in open forests and heathlands of the coast, ranges, and the wheat belt. The one to three large flowers are white or pink.
Arachnorchis hirta ssp. rosea syn Caladenia hirta ssp. rosea is a common inland clumping subspecies with bright pink flowers. Photo by Mary Sue Ittner labeled as this taxa at a flower show in Albany September 2007.
Arachnorchis longicauda syn Caladenia longicauda known as the Tufting White Spider Orchid is found in Western Australia. It is found in open grass or low shrubby areas in woodland, usually in well drained soil. It has white flowers with drooping segments, broad at the base and narrowing abruptly to a long slender tip. The lip has a marginal fringe of long teeth and rows of calli. There are many subspecies. The one pictured below was seen in the Stirling Range National Park in September 2007 and could be subsp. eminens which is found there. You can’t see the calli since there is a green spider on the lip. We saw it on a bank next to the road and it was late in the day and hard to get close enough to it for a very clear picture. Photos by Mary Sue Ittner.
Arachnorchis longiclavata syn Caladenia longiclavata is endemic to the southwest of Australia. It grows to 40 cm. tall with an erect stalk. Flowers are green, gold, and deep red. Because the segments are wide at the base contracting to a clubbed tip this orchid is often referred to as the Clubbed Spider Orchid. There are many variations in this species. Photo by Mary Sue Ittner labeled as this species at a flower show in Albany September 2007.
Arachnorchis reticulata syn Caladenia reticulata This species is found in well-drained soils in shrubby forest in southeastern Australia and is commonly known as the Veined Spider Orchid. The 1 to 3 5 cm. flowers are yellow with red veins to red and grow on a wiry hairy scape to 30 cm. tall. The spreading segments are reminiscent of a spider’s legs. Photos taken by Bob Rutemoeller and Mary Sue Ittner on the Great Ocean Walk near Apollo Bay in heathland.
Arachnorchis tentaculata syn. Caladenia tentaculata (Eastern Mantis Orchid) grows in open forest, woodland, and heathland in Victoria and South Australia. It has flowers about 10 cm. across that are greenish with crimson stripes and a delicately balanced green white and maroon lip with fringed margins, a maroon apex and maroon calli. It blooms from October to November and is very similar to Arachnorchis dilatata which is smaller and blooms later. Photos below taken by Bob Rutemoeller and Mary Sue Ittner October 2007 in the Grampians.