Orchids have evolved some of the most extraordinary and unique shapes imaginable. It is almost hard to imagine that they are indeed flowers, such is the extent of their specialisation. Botanically, there are petals and sepals like any other flower, but they are so highly modified that they loose their definition in the shape of the flower as a whole. Many of these highly specialised modifications have arisen out of unusual pollination strategies, often closely linked to one or few specific insects. Here, in no particular order, I take a walk through some of the main groups of Australian terrestrial orchids.
Caladenia, the spider orchids
Spider orchids are a group of several hundred species of terrestrial orchids, all endemic to Australia. There are many different types of Caladenia orchids, but here I focus on one group, the spider orchids. Many spider orchids have a close relationship to thynnine wasps - in most cases it is one species of orchid pollinated exclusively by males of only one species of wasp. The orchid attracts the male wasps through chemical cues that mimic female wasp pheromones. The male is thereby duped into attempting to mate with the labellum, the central 'lip', which resembles the female wasp. The yellow pollen is positioned above the labellum, so that when the wasp tries to mate with the labellum it receives a package of pollen on its back. Becuase of their trickery, these orchids are known as 'sexually deceptive' - they lure the wasps with false promises of females and in return offer nothing.
Pterostylis, the greenhoods
Greenhoods come in many different shapes and sizes, but can they can broadly be separated into those with an open hood (top) and those with a more closed hood (bottom). Like the spider orchids, many species of greenhood are thought to achieve pollination through sexual deception of insects. Instead of attracting wasps, greenhoods attract fungus gnats, a type of small fly. One of the key features of greenhoods is that the labellum is highly touch sensitive. When the labellum is disturbed by a fly landing on it (or a human knocking it!) it flips upwards or backwards, trapping the gnat inside the hood of the orchid. From here, the poor insect must crawl through a narrow passage which is specially designed to take the insect past the pollen sacs. When the gnat finally escapes, it has often received some pollen on its back, which it can then transport to another flower.
Starting in 2017, I will be doing my PhD at the Australian National University studying the pollination of greenhoods. Follow here.
Diuris, the donkey orchids
Diuris are commonly known as Donkey Orchids or Doubletails. Most species are bright yellow or orange, while a few are a beautiful mauve or pink or white. Many Diuris species look confusingly similar to each other, and appear to be pollinated by a range of different small insects such as bees and beetles. It is possible that the bright colours and overall shape may be mimicing other similar flowers such as native peas. This has not yet been confirmed.
Corunastylis, the midge orchids
Midge orchids are truly tiny. Most species are not much bigger than a twig. They are mostly summer-autumn flowering orchids, and appear to be pollinated by correspondingly tiny chloropid flies. It is unclear how the orchids attract the flies, and whether each orchid species attracts one or several species of fly.
Corybas & Corysanthes, the helmet orchids
Helmet orchids are distinctive, small orchids with one round or heart-shaped leaf. Little is known about the pollination of helmet orchids. There is anecdotal evidence that they are pollinated by fungus gnats but there is no concrete scientific evidence to confirm this or explain the mechanism by which they achieve pollination.
Caleana, the flying duck orchids
Of all the unusual shapes in Australian orchids, the Flying Duck Orchid is surely one of the most bizarre and amazing. The labellum is shaped like a duck's head and presumably mimics the shape of a female sawfly. The male sawflies, the pollinators, are duped into attempting to mate with the 'duck's head'. When a male lands on the duck's head, it flips downwards, trapping the sawfly against the column and thereby transferring pollen onto the sawfly.
Dipodium, the hyacinth orchids
Hyancinth orchids are a group of tall, colourful orchids. Most species have multi-flowered stalks and bright pink (or yellow) flowers. They are also among the tallest terrestrial orchids and can grow to over a meter tall. This combination of size and bright colours make them quite easy to spot - they are one of the few terrestrial orchids visible while driving. There is evidence to suggest they are pollinated by native bees - it is possible that they are trying to visually attract pollinators with their bright pink colours.
Chiloglottis, the bird orchids
Chiloglottis is group of orchids (variously called bird, wasp and ant orchids) that are mainly restricted to eastern Australia. All species are pollinated by a different species of thynnine wasp, the males of which are attracted to the orchid by chemical cues mimicking the pheromones of the female wasp. Furthermore, the calli on the surface of the labellum are thought to resemble a female wasp which may also help to stimulate the male wasp to attempt mating with the flower. Recent work on the Large Bird Orchid has found several cryptic taxa which look identical but attract a different species of wasp. By using different pollinators and thus ensuring there is no sharing of genetic information, these cryptic taxa are on the road to becoming new species.
Thelymitra, the sun orchids
Sun orchids are a large genus of similar-looking orchids distributed throughout Australia, New Zealand and parts of SE Asia. Most Australian species are purple, although some are pink or orange. This may be part of a strategy to mimic native lilies and irises, tricking native bees into visiting the flowers to collect pollen. Sun orchids are so-called because of their tendency to only open flowers on hot, sunny days. On cool days and at night the flowers often remain closed and in particularly cool seasons they may not even open at all. Many species are self-pollinating.
Calochilus, the beard orchids
Beard orchids are fabulously weird plants. The labellum is covered in long thin hairs, which gives rise to the name. Some beard orchids are appear to be pollinated by scoliid wasps, although more research is needed to confirm the pollination mechanism. Many beard orchids self-pollinate themselves if they are not externally pollinated within a few days.
Cryptostylis, the tongue orchids
Cryptostylis is a genus of orchid spread throughout southern and eastern Asia and Australasia. In Australia, there are 5 species. Despite dramatic differences in floral shape, each species is pollinated by the same ichneumonid wasp Lissopimpla excelsa. Attraction of the wasp is achieved through chemical scents which are thought to mimic the sex pheromones of the female wasps.
Prasophyllum, the leek orchids
Prasophyllum is a large genus of orchids found in Australia and New Zealand. Most species have a relatively similar flower shape, differing in the colour or shape of the petals or sepals. Many species are scented and produce nectar as a reward for a variety of pollinating insects.
Thynninorchis, the elbow orchids
Thynninorchis, together with Arthrochilus, are two closely related genera commonly known as elbow orchids. The labellum is highly modified and (somewhat) resembles a female thynnine wasp. Similar to other sexually deceptive orchids, the labellum releases a scent which mimics the sex pheromone of the female wasp, thereby attracting the male. When the male arrives, it grasps the labellum and attempts to fly away with it. However, the labellum is delicately hinged (the 'elbow'), so when the wasp tries to fly away with the labellum it is instead guided by the arc of the labellum to a small receptacle which houses the pollinia, thereby pollinating the orchid.