Since plants are immobile they need help from outside to mate. Orchids always rely on animals - mostly insects - to achieve exchange of dna to create offspring. True, a large number of orchids have self-pollination as backup but this is not considered the most optimal way of reproduction for plants. The most succesfull method is that of reward: an insect is attracted with the promis of food. One third of the orchid species apply "food deceptive techniques", promissing food but not providing any to the visitor - who leaves with the pollen. This way the plant achieves pollination without wasting expensive energy on meals for the insects.
There is however another technique of attracting insects without paying up: sexual deception. Male insects are attracted by a floral scent that imitates the olfactory cue or sex pheromone, often topped of with an imitation of the female in shape, color and touch. The movements of the excited insect take longer than an average visit to a flower that provides nectar and the frustrated male often revisits the orchid a number of times in succesion.
A number of terrestrial orchid species reproduce via this mechanism. Most well known are Ophrys, Cryptostylis, Drakaea, Caladenia, Chiloglottis, Geoblasta, Arthrochilus, Calochilus, Calochilus, Leporella and Spiculaea … The large majority of terrestrial orchid genera that use pseudo copulation are found in Australia in the tribe Diurideae, the largest genus with pseudo-copulated species is Ophrys from Europe. Sexual deceit for orchid pollination is not bound to a single continent: it occurs in South America (Geoblasta pennicillata) and South Africa (Disa - at least 2 species) as well.
Pseudo copulation pollination was first described in 1916 and 1917 by Pouyanne and Correvon 1)2) for the relationship he remarked between Campsoscolia ciliata and Ophrys speculum in Algeria. The French publication went unnoticed until Godfrey confirmed it in 1925, causing an increase in interest for the subject ('pouyannian mimicry'). This was followed by a string of publications in Australia from 1927 to 1938 where Edith Coleman described her extensive research in the pollination of Cryptostylis spp. by male Lissopimpla excelsa (was L. semipunctata).
Sex pheromones are an effective way of attracting the male insects from a large distance - insect odour receptors are extremely sensitive. Most genera produce a cocktail of chemical compounds which have the same effect on the insect, but they're not the same chemical as produced by the female, they just smell the same or trigger the same receptors. There is one species which mimics and releases a single volatile sex-pheromone identical to the female insect: Chiloglottis trapeziformis produces 2-ethyl-5-propylcyclohexan-1,3-dione just as the female thynnine wasp Neozeleboria cryptoides3)
The mechanisms behind pseudo-copulation can be classified in two ways:

• Passive : the flower's structure is built in such a way that the male moves into the correct position because the flower's imitation of the female directs it to. Any movement is achieved by the insect's behaviour
• Active : the flower contains a spring-mechanism that forces the insect into the desired position.

Below are a number of examples of Pseudo copulation. You can click them to get the full-size image or to start the video.

Species from the genus Ophrys attract Bees, Bumble-bees and Wasps. The flowers attract the male insects via a pheromone and the labellum emulates the color and hairs of the female. In most cases the insect "mates" with the flower with the head oriented towards the pollinea and the abdomen along the labellum - dubbed "head pollination". Sometimes one can observe different attempts where the male tries the opposite position. So in most cases, the pollinea get attached to the head of the insect. In Ophrys species of the fusca and lutea complex group we observe "abdomen pollination", the head of the insect is pointing away from the pollinea which get stuck to the insect's abdomen. Putting flowers from these two different approaches next to each other shows that the head-pollinated flowers are tubular and often elongated, while the abdomen pollinated flowers have a planar labellum.

Abundances are given as mean +- s.e.m.(n:10); P values were calculated using the Mann-Whitney U-test.
Extract from Orchid pollination by sexual swindle, Nature, Vol 399, 3 June 1999

Pseudo-copulation
Andrena nigroaenea male (Hym. Andrenidae)
on Ophrys lupercalis
Image © Nicolas J. Vereecken, used with kind permission.

Pseudo-copulation
unknown bee male
on Ophrys lutea ssp. sicula
Image © Jan Gaisler
used with kind permission.

Pseudo-copulation
Eucera elongatula male (Hym. Apidae)
on Ophrys scolopax
Image © Nicolas J. Vereecken, used with kind permission.

More Images

Ophrys pollination by photographer Nicolas J. Vereecken.

Head vs Abdomen-pollinators on Günther Blaich's site

Nicolas J. Vereecken, who is doing his PhD project on "Pollinator-mediated selection, reproductive isolation, and the evolution of floral traits in the genus Ophrys"

Atlas Hymenoptera

La pollinisation de l’Ophrys arachnitiformis (Orchidaceae) par les mâles de Colletes cunicularius (L.) (Hymenoptera, Colletidae) dans les Pyrénées-Atlantiques (France).
Vereecken N & Genoud D. 2007.
OSMIA 1: 20-22. PDF

Cuticular hydrocarbons as source of the sex pheromone in Colletes cunicularius (Hymenoptera: Colletidae) and the key to its mimicry by the sexually deceptive orchid Ophrys exaltata
Mant JG, Brändli C, Vereecken NJ, Schulz C, Francke W & Schiestl FP. 2005. (Orchidaceae).
Journal of Chemical Ecology 31 (8): 1765-1787.

Mate choice and odour preferences in the solitary bee Colletes cunicularius (L.) (Hymenoptera, Colletidae), the exclusive pollinator of the (sexually deceptive) orchid Ophrys exaltata (Orchidaceae).
Vereecken NJ. 2004.
Travail de fin d’études, Faculté Universitaire des Sciences Agronomiques de Gemboux (100p).

Without a doubt the most impressive example of sexually deceptive plants. The flowers are pollinated by the male thynnine wasp, but things are a bit more complex than with for example Ophrys. The female wasp is wingless, as a larva she burrowed into the ground and later emerges from her underground pupa stage to mate. Immediately after digging her way out she searches for the stem of a plant to climb to a certain height, and waits. She produces a pheromone that attracts the males from miles away. The male doesn't just mate on the spot: he holds on to her and takes off to mate in mid-air, after which he returns her safely to the surface.
This behaviour is exploited by Drakaea and Thynninorchis: their labellum full of calluses has evolved into the shape, colour and touch of the female and the flower (which is actually upside-down) stands solitary on a long narrow stem … imitating a waiting female. The male is attracted by the imitated pheromones - produced by the osmophores - , takes hold of the labellum and tries to take of. Doing this, the male rotates the labellum over the hinge and knocks himself with the back of the thorax against the pollinea that stick on. You might think that this is a scenario that requires good aiming, but the male holds the labellum (female) in a very particular way and doesn't give up after being knocked around a couple of times. The next time he visits a flower he's fooled again but this time delivers the pollen to the stigma.

Drakaea livida

Image © Ramon
Used with kind permission

The image on the left shows the imitation of a Thynnine female waiting on a stem. The real insect chooses a stem in a clear area and climbs to the top, wafting her perfume to attract a male. You can see why the Drakaea bears a solitary flower on a long stem that arises from a ground-hugging leaf.

Drakaea glyptodon
Image © Ramon
Used with kind permission

The imitation goes further than the production of pheromones, on the left you can clearly identify what would be the pilose thorax, abdomen and head of the real insect.

Drakaea glyptodon
Image © Ramon
Used with kind permission

This is an example of the hinge in its "activated" form. In this position, the male would be knocked against the stigma/pollinea. In contrary to Caleana, Drakaea does not use a spring mechanism to catapult the insect in the desired position but relies on its urge to hold on and take of.

Drakaea livida
Image © Ramon
Used with kind permission

A different Drakaea species: a different wasp species: a differently colored female.

Drakaea livida
Image © Ramon
Used with kind permission

Used to be united with Drakaea for obvious reasons. The hinge of this genus is connected directly at the perianth. The modified labellum hangs below the stigma/pollinea and can only move up, gravity pulls it into its resting position.
In Drakaea the auricles and wings of the column are broadly and bluntly triangular, and are situated near the base of the column. In Spiculaea they are actually long-triangular in shape, being placed towards the base of the column.

Aptly named "the duck orchid" for the striking resemblence of the labellum with a duck's head and the rest of the flower looking like the body in full flight, wings down. The flower is actually upside-down in comparison with normal orchids. The leaves of these plants are long and narrow, making them hard to find among the native grasses. Caleana uses a spring mechanism that traps the insect in a pouch, the only way out is by passing the pollinea and stigma. The excited male lands on the labellum (the duck's bill) and this triggers the relaxation of two hinges (labellum-lamina and lamina-perianth), swinging the insect upside-down with the back of its thorax into the pouch containing stigma and pollinea. After the mechanism has been activated it takes a number of hours up to a day to reset.

Caleana major
© John Varigos
Image used with kind permission

Paracaleana nigrita
Image © Ramon
Used with kind permission

Paracaleana nigrita
Image © Ramon
Used with kind permission
The flower on the right has been triggered and the labellum is returning to its upright position. The left flower shows a look into the pouch, with the pollinea visible as yellow bulbills in the bottom.

Paracaleana nigrita
Image © Ramon
Used with kind permission

Paracaleana nigrita
Image © Ramon
Used with kind permission

Crypostylis subulata
© John Varigos, image used with kind permission

The bizar looking flower of this genus is an open invitiation for closer examination. That was probably what Edith Coleman must have thought when she started her extensive studies on this genus, spawning a series on papers that kickstarted the story of pseudo-copulated pollination in Australia. All Cryptostylis species are pollinated by male ichneumon wasps Lissopimpla excelsa 4). The same species has also been reported "pseudocopulating" with the flowers of Caladenia dilatata var. rhomboidiformis 5).

Male Lissopimpla excelsa tries to copulate with Cryptostylis subulata
© Densie Clyne

After backing into the stigma, the wasp attempts to copulate with the flower by bending its body into an arch, with the base of the lip of the flower held by the claspers of the wasp. The upper side of the apex of the abdomen comes in contact with the viscidium, and the pollinea detach from the flower.

References
Schiestl, F.P., and Peakall, R. 2005. Two orchids attract different pollinators with the same floral odour compound: ecological and evolutionary implications. Funct. Ecol. 19:674-680.

Schiestl, F.P., Peakall, R., Mant, J.G., Ibarra, F., Schulz, C., Franke, S., and Francke, W. 2003a. The chemistry of sexual deception in an orchid-wasp pollination system. Science. 302:437-438.

It's not difficult to see that this genus used to be merged with Drakaea. The labellum attracts the male Thynoid wasp who holds on to it, convinced it's a female waiting to be fertilised. The insect's attempts to take of with the fake female to mate in mid-air results in repetitive bumping against the pollinea/stigma. The image below clearly shows the hinge that loosely connects the labellum to the rest of the flower.

Thynninorchis huntianus
© John Varigos
Image used with kind permission

There are at least 2 species in this genus that use sexual deception, namely and Disa atricapilla (Black-Haired Disa) and D. bivalvata. Male wasps are the pollinators of these species.

Disa bivalvata illustration
© by William Wesley & Son
Courtesy of the Swiss Orchid Foundation at the Herbarium Jany Renz

D. bivalvata is pollinated by Hemipepsis hilaris, a spider-hunting wasp

Disa bivalvata topview
© by P. Linder
Courtesy of the Swiss Orchid Foundation at the Herbarium Jany Renz

Disa atricapilla illustration
© by William Wesley & Son
Courtesy of the Swiss Orchid Foundation at the Herbarium Jany Renz

D. atricapilla is pollinated by Podalonia canescens

Disa atricapilla inflorescence
© by P. Linder
Courtesy of the Swiss Orchid Foundation at the Herbarium Jany Renz

Images

Pollinia and sticky viscidium of Disa atricapilla.
Disa atricapilla images
Podalonia canescens with pollinaria of Disa atricapilla.

References

WHITEHEAD, V. B. & STEINER, K. E. 1991. Males of Podalonia canescens (Sphecidae) pollinating the orchid Disa atricapilla. Proceedings 8th Congress, Entomological Society of southern Africa, Bloemfontein, 1-4 July.

The flowers are pollinated by scolliid wasps of the genus Campsomeris. The female wasp has wings, and after reading the texts on the other genera above you're probably asking what the story behind these two is. That's exactly what we're asking ourselves … nobody really knows.

Left: Calochilus paludosus © Nuytsia@Tas
Below Campsomeris sp. female (Scoliidae) © Marshal Hedin, under Creative Commons Attribution ShareAlike 2.5

Leporella fimbriata
© Kris Kopicki

A different approach to pseudo-copulation: species from Leporella are pollinated by male bull-ants. Peakall et al. (1987) reported male Myrmecia urens ants carrying pollinia from Leporella fimbriata. It's unclear whether this is truly a case of pseudo-copulation as these insects generally visit different sources of nutrition in succession before returning to the nest. 6)

Geoblasta pennicillata pollination by Campsomeris bistrimacula
from First confirmed case of pseudocopulation in terrestrial orchids of South America
in: Flora 201 (2006) 365–369
© 2006 Elsevier GmbH

(A) Close-up of a flower in lateral view.
The labellum showing an insect-like structure, with black and yellow hair-like projections at the rims and glossy purplish black plates.
(B) Close-up of a flower in dorsal view.
(C) A flower and its pollinator.
The male wasp is settled on the labellum attempting to copulate, while introducing its metasoma below the column.
(D) A male wasp of Campsomeris bistrimacula carrying a pollinium of Geoblasta pennicillata on the dorsal metasoma

Pseudo copultaion in Geoblasta was suspected before but it was not until 2006 that it was published 7). The monotypic species is actually pollinated by the same male wasp genus as Calochilus campestris in Australia, suggesting common ancestry of this mechanism.

Pollinator attraction in a sexually deceptive orchid by means of unconventional chemicals
by Manfred Ayasse, Florian P Schiestl, Hannes F Paulus, Fernando Ibarra, and Wittko Francke
download PDF

First confirmed case of pseudocopulation in terrestrial orchids of South America: Pollination of Geoblasta pennicillata (Orchidaceae) by Campsomeris bistrimacula (Hymenoptera, Scoliidae)
by Liliana Cioteka, Pablo Giorgisa, Santiago Benitez-Vieyrab and Andrea A. Cocucci
download PDF

Adams, P.B., Lawson, S.D., 1993. Pollination in Australian orchids: a critical assessment of the literature 1882–1992. Austral. J. Bot. 41, 553–575.

Blanco, M.A., Barboza, G., 2005. Pseudocopulatory pollination in Lepanthes (Orchidaceae: Pleurothallidinae) by fungus gnats. Ann. Bot. 95, 763–772.

Borg-Karlson, A.-K., 1990. Chemical and ethological studies of pollination in the genus Ophrys (Orchidaceae). Phytochemistry 29, 1359–1387.

Clayton, S., Aizen, M., 1996. Effects of pollinia removal and insertion on flower longevity in Chloraea alpina (Orchidaceae). Evol. Ecol. 10, 653–660.

Correa, M.N., 1968. Rehabilitacion del genero Geoblasta Barb Rodr. Rev. Museo La Plata n.s. 11, 69–74.

Dafni, A., 1986. Floral mimicry—mutualism and unidirectional exploitation of insects by plants. In: Southwood, T.R.E., Juniper, B.E. (Eds.), The Plant Surface and Insects. Arnold, London, pp. 81–90.

Dafni, A., 1987. Pollination in Orchis and related genera: evolution from reward to deception. In: Arditti, J. (Ed.), Orchid Biology, Reviews and Perspectives, vol. 4. Cornell University Press, Ithaca, pp. 79–104.

Dodson, C.H., 1962. The importance of pollination in the evolution of the orchids of tropical America. Am. Orchid Soc. Bull. 31, 525–534 (641–649, 731–735).

Dressler, R.L., 1993. Phylogeny and Classification of the Orchid Family. Dioscorides Press, Portland, Oregon.

Flanagan, N.S., Peakall, R., Clements, M.A., Otero, J.T. (2007) Identification of the endangered Australian orchid Microtis angusii using allele-specific PCR assay. Conservation Genetics, In press (DOI 10.1007/s0592-006-9198-6)

Flanagan, N.S., Peakall, R., Clements, M.A., Otero, J.T., (2006) Conservation of taxonomically difficult species: the case of the Australian orchid, Microtis angusii . Conservation Genetics 7, 847-859 (DOI 10.1007/s10592-006-91119-8.)

Flanagan, N.S., Ebert, D., Porter, C., Rossetto, M., and Peakall, R. (2006) Microsatellite markers for evolutionary studies in the sexually deceptive orchid genus Chiloglottis , Molecular Ecology Notes 6, 123-126 (DOI 10.1111/j.1471-8286.2005.001161.x)

Flanagan, N.S., Peakall, R., Clements, M.A., and Otero, J.T. (2007) Molecular genetic diagnosis of the 'taxonomically difficult' Australian endangered orchid, Microtis angusii: An evaluation of the utility of DNA barcoding. Lankesterian a 7, In press.

Fordham, F., 1946. Pollination of Calochilus campestris. Victorian Nat. 62, 199–201.

Jones, D.L., Gray, B., 1974. Pollination of Calochilus holtzei F. Muell. Am. Orchid Soc. Bull. 43, 604–606.

Kerr, W.E., Lopez, C.R., 1963. Biologia da reproducao de Trigona (Plebeia) Droryana F. Smith. Rev. Bras. Biol. 22, 335–341.

Kores, P.J., Molvray, M., Weston, P.H., Hopper, S.D., Brown, A.P., Cameron, K.M., Chase, M., 2001. A phylogenetic analysis of Diurideae (Orchidaceae) based on plastid DNA sequence data. Am. J. Bot. 88, 1903–1914.

Lehnebach, C.A., Riveros, M., 2003. Pollination biology of the Chilean endemic orchid Chloraea lamellata. Biodivers. Conserv. 12, 1741–1751.

Mant, J.G., Schiestl, F.P., Peakall, R., and Weston, P.H. (2002) A phylogenetic study of pollinator conservatism among sexually deceptive orchids. Evolution 56, 888-898.

Mant, J., Bower, C.C., Weston, P.H. and Peakall, R. (2005) Phylogeography of pollinator-specific sexually deceptive Chiloglottis taxa (Orchidaceae): evidence for sympatric divergence? Molecular Ecology 14, 3067-3076

Mant, J., Peakall, R., and Weston, P.H. (2005) Specific pollinator attraction and the diversification of sexually deceptive Chiloglottis (Orchidaceae). Plant Systematics and Evolution , 253, 185-200

Mant, J., Peakall, R., and Schiestl, F.P. (2005) Does selection on floral odor promote differentiation among populations and species of the sexually deceptive orchids, Ophrys. Evolution 59, 1449-1463.

Peakall, R. (2007) Orchid speciation - confronting the challenges. Molecular Ecology , In press. Invited News and Views Commentary

Peakall, R., and Schiestl, F.P. (2004) A mark-recapture study of male Colletes cunicularius bees: implications for pollination by sexual deception. Behavioral Ecology and Sociobiology 56, 579-584.

Peakall, R., Jones, L., Bower, C. C. and Mackey, B. G. (2002) Bioclimatic assessment of the geographic and climatic limits to hybridisation in a sexually deceptive orchid system. Australian Journal of Botany 50, 21-30.

Peakall, R., Bower, C. C., Logan, A. E., and Nicol H. I. (1997) Confirmation of the hybrid origin of Chiloglottis X pescottiana R. S. Rogers (Orchidaceae: Diurideae). 1. Genetic and morphometric analysis. Australian Journal of Botany 45, 839-855.

Peakall, R., and Beattie, A. J. (1996). Ecological and genetic consequences of pollination by sexual deception in the orchid Caladenia tentaculata. Evolution 50, 2207-2220.

Peakall, R. (1990) Responses of male Zaspilothynnus trilobatus wasps to females and the sexually deceptive orchid it pollinates. Functional Ecology 4, 159-167.

Peakall, R., Angus, C. J. and Beattie, A. J. (1990) The significance of ant and plant traits for ant pollination in Leporella fimbriata. Oecologia 84, 457-460.

Peakall, R. (1989a) The unique pollination of Leporella fimbriata (Orchidaceae) by pseudocopulating winged male ants Myrmecia urens (Formicidae). Plant Systematics and Evolution 167, 137-148.

Peakall, R., Beattie, A. J., and James, S. H. (1987) Pseudocopulation of an orchid by male ants: A test of two hypotheses accounting for the rarity of ant pollination. Oecologia 73, 522-524.

Pouyanne, A., 1917. La fecondation des Ophrys par les insectes. Bull. Soc. d’Hist. Natl. Afr. Nord., 6–7. Ruiz Zapata, T., Arroyo, M.T.K., 1978. Plant reproductive ecology of a secondary deciduous tropical forest in Venezuela. Biotropica 10, 221–230.

Schiestl, F.P., Ayasse, M., Paulus, H.F., Lofsted, C., Hansson, B.S., Ibarra, F., Francke, W., 1999. Orchis pollination by sexual swindle. Nature 399, 421–422.

Schiestl, F.P. and Peakall R. (2002) Floral odour and insect olfaction. New tricks to unlock secrets of orchid pollination. Orchids. 71, 906-915.

Schiestl, F.P., and Peakall, R. (2005) Identical sex pheromones but differing behavioral responses in pollinators of two sexually deceptive orchids. Functional Ecology, 19, 674-680.

Schiestl, F.P., Peakall, R. and Mant, J. (2004) Chemical communication in the sexually deceptive orchid genus Cryptostylis . Botanical Journal of the Linnean Society 144, 199-205.

Schiestl, F.P., Peakall, R., Mant, J.M., Ibarra, F., Schulz, C., Franke, S and Francke, W. (2003) The chemistry of sexual deception in an orchid-wasp pollination system. Science 302, 437-438. Published October 17.

Singer, R.B., 2002. The pollination mechanism in Trigonidium obtusum Lindl. (Orchidaceae: Maxillariinae): sexual mimicry and trap-flowers. Ann. Bot. 89, 157–163.

Singer, R.B., Flach, A., Koehler, S., Marsaioli, A.J., Amaral, M.C., 2004. Sexual mimicry in Mormolyca ringens (Lindl. ) Schltr. (Orchidaceae: Maxillariinae). Ann. Bot. 93, 755–762.

Steiner, K.E., Whitehead, V.B., Johnson, S.D., 1994. Floral and pollinator divergence in two sexually deceptive South African orchids. Am. J. Bot. 81, 185–194.

Van der Pijl, L., Dodson, C.H., 1966. Orchid Flowers: Their Pollination and Evolution. University of Miami Press, Coral Gables, Florida.