Orchid Pollination - University of Miami

ORCHID 

POLLINATION 

ECOLOGY 

Orchid Pollination 

Exploring a Fascinating World 

by ron mchatton 

[1] The first example of sexual deception 

in Australian orchids involved the genus 

Cryptostylis. These orchids are pollinated 

by male parasitic wasps of the genus 

Lissopimpla. Here four male Lissopimpla 

excelsa wasps compete for the favors of 

a Cryptostylis erecta flower. 

mark clements

james petranka 

[2] Silver-spotted skipper (Epargyreus 

clarus) pollinating a small purple fringed 

orchid (Platanthera psycodes) at Mt. 

Mitchell, North Carolina. The long spurs 

of the orchid have nectar at their ends 

and the skipper must push its proboscis 

deep into the spur to get to the nectar. 

Note the pollinia attached to the base of 

the skipper’s proboscis.

No one is really sure how long 

ago orchids evolved but recent fossil evidence 

suggests that the development of pollinia 

was well established at least 75 million 

years ago and perhaps earlier (Ramierez 

2007). The development of fused reproductive 

structures, the so-called column, and 

pollinia are rare outside the Orchidaceae 

and set the stage for the development of 

an extremely varied and fascinating reproductive 

ecology. Pollinia, more-or-less 

fused, relatively large masses of compact 

pollen grains, because of their mass are 

not effectively distributed by water or 

wind and require the active intervention 

of an animal, typically an insect, to effect 

pollination. In addition to the dependence 

on a pollen vector, or pollinator, orchids 

have also evolved myriad pollination syndromes 

or mechanisms that run the gamut 

from advertisement and reward to sexual 

mimicry (Dressler 1981). Humans tend 

to think of the world in anthropomorphic 

terms and forget that orchid flowers have 

the shapes and fragrances they do not for 

us, but to ensure one thing and one thing 

only — successful reproduction. 

THE POLLINATORS It has been estimated 

that about 60 percent of the orchid 

family is pollinated by wasps and bees 

(essentially pollen-collecting wasps) (van 

der Pijl and Dodson 1966). The form and 

symmetry of orchid flowers suggests that 

one of the first steps that separated orchids 

from their close relatives may have been 

the adaptation to pollination by this group 

of pollinators. Bee-pollinated flowers are 

often gullet flowers or modifications of this 

type of flower. In so-called gullet flowers 

the floral segments create a sort of chamber 

in which the pollinating insect must 

enter in the process of gathering pollen. In 

orchids, especially Sobralia and Cattleya, 

this chamber is often formed from the lip 

eric hunt 

3 4 

[3–4] Sobralia [3] and Cattleya [4], two genera 

from very different parts of the orchid 

family, share parallel evolution of the 

gullet-flower structure characteristic of bee 

pollination. In these genera, the chamber 

is formed from the lip and column alone; 

the remainder of the flower serves as 

advertisement. Here they are illustrated 

by Sobralia veitchii and Cattleya labiata. 

Growers: [3] Bruce Rogers, [4] Fred Shull. 

and column alone and the remainder of the 

flower functions only as advertisement. 

While perhaps harder to see, the flowers 

of Stanhopea species can be interpreted 

as gullet flowers since it’s the “chamber” 

formed by the complex lip and column 

that controls pollination in these orchids. 

The remainder of the flower has evolved to 

tissue-thin texture and reflexes completely 

to effectively “get out of the way” of the 

action. Because the pollinating bee must 

first enter this chamber the bee does not 

receive or deposit pollen when it enters the 

flower but only as it leaves, thereby making 

self-pollination highly unlikely. In addition, 

many orchids prevent self-pollination by 

their pollinating vector by requiring a reorientation 

of the deposited pollinaria; such 

reorganization only occurring over some 

minutes as the soft tissues of the stipe dry 

and curl. Specificity of pollinator and pollinated 

species is achieved through adaptation 

of chamber size and column placement 

to insect size. Compaction of orchid pollen 

into pollinia with sticky structures makes 

pollen placement much more precise than 

that of most other flowers and a single bee 

species may serve as the pollinator of multiple 

orchid species without interference. As 

an example, Dendrobium infundibulum and 

Cymbidium insigne appear to use the same 

eric hunt 

bumblebee species. In this case, pollinia of 

Den. infundibulum are placed on the head 

of the bee while that from the Cymbidium, 

with much longer column, becomes attached 

to the bee’s central thorax. Because 

the column of the former species is much 

shorter, only pollinia placed near the front 

of the bee will be in a position to contact 

the stigmatic surface of the Dendrobium 

column as the bee exits the flower (Du Puy 

and Cribb 2007). 

Orchids with large gullet flowers such 

as Cymbidium are typically pollinated by 

large carpenter bees and bumblebees are 

known to pollinate Spiranthes and are 

implicated in the pollination of northern 

and some high-elevation species where 

other large bees are less common or active. 

Some bees gather oils from flowers rather 

than nectar or pollen and many orchids 

have evolved to attract these pollinators. A 

couple of examples include Ornithocephalus 

and some species of Oncidium that 

have elaiophores (open, nectar-producing 

glands) on the lateral lobes of the lip (Buchmann 

1987). Orchids pollinated by bees and 

wasps also share a suite of characteristics 

in addition to the presence of some form of 

gulletlike or bowl-shaped floral structure. 

Bee- and wasp-pollinated orchids exhibit a 

wide color range with the exception of true 

red because bees and wasps cannot see red. 

Fragrance, if present, will be sweet or spicy 

and noticeable during the daylight hours 

when these pollinators are active. Bees and 

wasps land to gather pollen or nectar so the 

orchids they pollinate have well-developed 

landing platforms, nectar guides, either 

structural or visual and sturdy connection 

to the column to allow the bees to grasp 

the flowers. Rewards, offered nectar or 

food, may be real or perceived and may or 

may not be concealed in a nectary (what 

we orchidists typically think of as a spur); 

the latter is a common feature of orchids 

pollinated by nectar-feeding bees and the 

length of the nectary is directly related to 

proboscis (tongue) length in the pollinating 

species (Dressler 1987). 

In addition to wasps and bees, orchids 

are pollinated by a whole range of creatures 

including, but not limited to euglossine bees 

(also called orchid bees), flies, butterflies 

and moths, as well as many species of 

birds; each pollinator group is attracted by 

a specific combination of attractants and 

forms. Euglossine bees, closely allied to 

bumblebees, are limited to tropical America. 

They range in size from about the size 

of a typical housefly to some large species. 

This group of bees is so closely involved 

in the pollination of many orchids that they 

are often called orchid bees. However, they 

340 Orchids JUNE 2011 www.AOS.org

david du puy 

are also involved in the pollination of many 

tropical American plant families. Females 

of some species pollinate nectar-producing 

orchids such as Sobralia and in that mode 

resemble other bee pollinators. However, 

it’s the males of the species that are much 

more important in orchid pollination. Male 

euglossine bees collect fragrance oils from 

the surface of the flowers they visit and 

store them in specialized hollow pockets in 

their hind legs. These oils are then used to 

attract mates. Because fragrance is the key 

to attracting these bees, orchids pollinated 

by male euglossine bees have powerful, often 

resinous fragrances and will be fragrant 

during daylight hours. In addition, because 

nectar is unnecessary, these species do not 

produce nectar nor are nectar guides present 

in the floral structures. But wait you 

say, what about Coryanthes species. These 

plants are pollinated by male euglossine 

bees and they have unique liquid-producing 

glands that fill the characteristic bucket with 

fluid. These glands aren’t nectar producing. 

Their sole presence appears to be to create 

a liquid reservoir that, once the pollinating 

bee enters by accident, leaves the bee unable 

to fly and the only escape route is out 

a narrow, dry passage past the column with 

its anther and stigmatic cavity. 

While male euglossine bees land to 

collect fragrance oils, their mode of flight 

allows them to maneuver like helicopters 

and even land on vertical surfaces. As a 

result, orchids pollinated by such bees 

may or may not have well-developed 

landing platforms. Examples of euglossine 

bee pollinated orchids include all of the 

Catasetinae and Stanhopeinae as well as 

most members of the Zygopetalinae and 

some Oncidiinae. Pollinator specificity is 

maintained by either a specific euglossine 

bee–orchid interaction based on fragrance 

composition or, in the case of promiscuous 

bees, a specific size relationship. Such is 

the case with many species of Stanhopea 

which may be visited by many euglossine 

bee species. One species will fit correctly 

within the cavity formed by the lip epichile, 

mesochile horns and the ventral surface of 

the column, thereby properly aligning the 

5 

robert pemberton michael and patricia fogden 

pollinia with the opening of the stigmatic 

cavity. The specificity of chamber size to 

pollinating bee is so strong in the genus 

Stanhopea that two examples of the same 

species may have overall flower size that 

differ by as much as 50 percent while the 

column and lip cavity opening will be identical. 

Remember that the sepals and petals 

in this genus serve only to provide a protective 

covering during flower development 

and advertisement during anthesis. 

The complex relationship between 

male and female euglossine bees and the 

plants they pollinate can be illustrated by 

the Brazil nut tree (Bertholletia excelsa). 

Plantation farming of this nut tree has 

proven difficult to do with reasonable yield 

because of this complex interplay. The tree 

6 

7 

[5] Pollinia of Dendrobium infundibulum on 

the head of a bee (left) while that from 

Cymbidium insigne, with much longer 

column, is attached to the bee’s central 

thorax (right). 

[6] Male euglossine bees collecting chemicals 

from rotting wood. Note the enlarged 

hind legs used to store these chemicals 

and the orchid pollinia attached to the 

backs of two of the bees. 

[7] Male Euglossa viridissima visiting Gongora 

powellii in cultivation. These bees 

are known to visit other Gongora species 

in the wild. 

www.AOS.org JUNE 2011 Orchids 341

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dennis m. hansen 

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11 

eric hunt johan hermans 

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12 

johan hermans 

lynn o’shaughnessy 

[8–9] Orchids adapted to butterfly and 

moth pollination have noticeable spurs. 

The length of the spur is tightly tied to the 

proboscis length of the pollinating moth 

or butterfly. Long-tongued hawk moths 

pollinate species such as Angraecum 

dollii [8] characterized by a very long spur 

while Aerangis citrata [9] is pollinated by a 

moth with a much shorter tongue. 

[10] A sarcophagid (carrion) fly carrying pollinia 

from Satyrium pumilum, an African 

species that attracts its pollinator by 

mimicking the smell of carrion. 

[11–12] Fly pollinated orchids often are 

outfitted with hairs and other moveable 

parts that attract the attention of the 

pollinating fly. The tassellike ornamentation 

on Bulbophyllum phalaenopsis [11] 

and Stelis villosa 1 [12] appear to mimic 

maggots feeding on the flower. In the 

case of Bulb. phalaenopsis this ruse is 

further strengthened by the fragrance of 

the flower — carrion. Growers: [11] Bill 

Weaver, [12] Lynn O’Shaughnessy. 

1 

Formerly Pleurothallis schiedei. 

is pollinated by female long-tongued euglossine 

bees of the genus Eulaema (Mota 

Maues 2002). These bees are strong enough 

to lift the coiled hood on the nut tree flowers 

and possess a tongue long enough to snake 

its way through the complex, coiled flower 

to reach the nectar reserve. The presence of 

the female bees is dependent on the flowering 

of an orchid species that does not grow 

on the nut tree. The orchid flowering causes 

a swarming of the males of this bee species 

who are attracted to the orchid’s resinous 

fragrance, collecting it to be used in attracting 

female bees. The swarming of the 

male bees causes the swarming of female 

bees that feed on, and pollinate the nut tree. 

Think of it as sort of the insect equivalent 

of a singles bar. 

Butterflies and moths are nectar feeders 

and are almost as common pollinators as 

bees. Because of their long, coiled tongues, 

orchids adapted to butterfly and moth pollination 

have a narrow spur or nectary and 

belong to a floral classification called keyhole 

flowers. In many species, such as those 

of Angraecum and Aerangis, the spur may 

be pronounced. In many others, such as 

many species of Epidendrum, the nectary is 

created from the fusion of the lip to the underside 

of the column and the only feature 

that may be visible is the keyhole formed 

between the underside of the column and 

the nectar guides or lip crests. Butterflies 

and moths are normally associated with 

pollination of flowers that have few ovules. 

This is because, in nonorchids, pollen is 

usually transported adhering to the insects’ 

tongues and their long narrow tongues are 

not suited for carrying large quantities of 

pollen. Orchids avoid this problem because 

the pollinia become affixed to the insects 

head, or in the case of Tipularia discolor, to 

the eye of the pollinating miller (Whigham 

and McWhety 1980). Butterflies and some 

moths (mostly those that are day-active), 

unlike bees, see red and many butterfly pollinated 

orchids are bright pink, red, yellow 

or even vivid violet. 

Because visual clues appear to be the 

most important attractant for butterfly pollinated 

orchids, fragrance is usually lacking. 

Moths on the other hand, especially 

those that are nocturnal, can really only 

distinguish light from dark and a common 

characteristic of moth pollinated orchids is 

that they are white, cream-colored or green 


michael and patricia fogden 

and are typically fragrant, the aroma acting 

as an additional attractant in the darkness. 

Since nature wastes nothing, moth pollinated 

orchids will exhibit fragrance only 

during the period of day or night in which 

the particular pollinator is active. As examples, 

Tipularia discolor is pollinated by 

a small Miller active only just before and 

after sunset and as a result this orchid species 

is fragrant for only a period of a couple 

of hours beginning about one hour before 

sunset. By contrast, Angraecum sesquipedale 

reaches its peak fragrance only during 

the wee hours of the night. Examples of 

orchids pollinated by butterflies and moths 

include Brassavola and many Epidendrum 

species as well as most of the Angraecinae 

and Aerangidinae. There are examples of 

specificity similar to those between male 

euglossine bee and orchid as well. For 

instance, Epidendrum anceps is visited 

by males of a single species of Ctenuchid 

moth and the species which comprise the 

Epidendrum paniculatum complex attract 

male Ithomiid butterflies (Adams and Goss 

1976). Flies, including mosquitoes, represent 

a diverse group of insects and pollinate 

a wide range of orchids, including most 

pleurothallids and many Bulbophyllinae. 

The floral features involved in attracting 

flies depend on the group of flies involved. 

In the case of mosquitoes, the insects function 

essentially as tiny moths and may take 

the place of moths at high latitudes. At least 

one orchid, Platanthera obtusata is known 

to be pollinated by mosquitoes (Thien and 

Utech 1976). A second group of prominent 

orchid pollinating flies are those that collect 

oils. Orchids pollinated by oil-gathering 

flies are characterized by open-or shallowly-cupped 

flowers with disagreeable 

odors during the daylight hours. Orchids 

with fecal odors are pollinated by oil-collecting 

flies. In addition, because the fly is 

attracted to the appearance of excrement or 

something decomposing, most oil gathering 

fly-pollinated orchids are either brown or 

red-purple. Nectar, in the form of the oil 

the flies collect, will always be present and 

many orchids so pollinated will have shiny 

wet structures visible on the flower as is 

the case with many Bulbophyllum species. 

Many flies, especially carrion flies, are 

attracted to fringes and it is thought that 

the movement associated with the fringes 

is sensed as either maggots as in the case 

13 

[13] A green violetear (Colibri thalassinus) 

feeding at the flowers of an Elleanthus 

species in the Monteverde Cloud Forest 

of Costa Rica. 

of Bulbophyllum phalaenopsis and Stelis 

villosa or the buzzing activity of other flies 

around host material. Orchids pollinated by 

carrion flies have odors that mimic rotten 

flesh and often are dark red, dark brown 

or red-purple. A great many species of 

Bulbophyllum and Masdevallia appear to 

be pollinated by carrion flies. 

Even termites and crickets may get 

into the act. The majority of Angraecum 

species endemic to the Mauritius and 

Reunion islands are clearly pollinated by 

something other than long-tongued Hawk 

moths as evidenced by their development 

of short-spurred, mostly green flowers. We 

now know that at least Angraecum cadetii 

is pollinated by a species of raspy cricket 

(Micheneau et al. 2010). The cricket, also 

new to science, represents the first clearly supported 

evidence of pollination by herbivorous 

orthopterans (grasshoppers, crickets and katydids) 

in the orchid family and in flowering 

plants in general. Rhizanthella gardneri, one 


[14–15] Bird-pollinated orchids often share 

similar urn- or tubular-shaped flowers. 

Dendrobium secundum [14] is known 

to be pollinated by birds. Although the 

pollinator of Cryptochilus sanguineus 

[15] is unknown, the urn-shape, bright 

red flowers and arrangement on the 

inflorescence all strongly suggest the 

pollinator to be a bird species. 

Growers: [14] Smithsonian Institution, 

[15] Marni Turkel. 

[16] Not all bird-pollinated orchids are 

tubular or urn-shaped. This Cattleya 2 

coccinea is dramatically reduced in 

size from its larger, bee-pollinated 

relatives. The bright red color and 

narrow constricted lip are also both 

characteristic of the changes that result 

from adaptation to bird pollination. 

Grower: Anna S. Chai. 

2 

Formerly Sophronitis. 

14 15 

16 

eric hunt james osen 

of Australia’s curious underground orchids, 

appears to be regularly pollinated by termites, 

the only known example of termite pollination 

in the Orchidaceae (van der Cingel 2001). 

This rare and unusual orchid spends its entire 

life underground in association with the roots 

of a member of the genus Melaleuca. At flowering 

time, a head of small, spirally arranged 

flowers emerges from the ground but remains 

concealed under the leaf-litter. While pollination 

by a fungus gnat has been demonstrated, 

termites have also been observed to regularly 

and systematically visit newly opened flowers 

and to transport pollen masses making them 

likely pollinators. 

While birds may not seem like likely 

orchid pollinators they are however, fairly 

regularly encountered. The syndrome 

for bird pollination is similar to that of 

butterfly pollination, with tubular, vividly 

colored red, yellow, violet or white 

keyhole-flowers except that the flowers 

are typically much stiffer and fragrance is 

usually absent. Bird pollination becomes 

more and more important with increasing 

altitude and may represent a natural 

progression as insects become less and 

less active at colder, higher elevations. In 

the Old World tropics, Sunbirds (Nectariniidae) 

are efficient pollinators and many 

species of Dendrobium are probably pollinated 

by these birds. In the New World 

tropics the role is filled by hummingbirds 

(Trochilidae). Examples of hummingbird-pollinated 

orchids include species of 

Comparettia, Cochlioda, and Elleanthus 

and Epidendrum pseudepidendrum (van 

der Cingel 2001). Lastly, not all birdpollinated 

orchids are vividly colored. 

Angraecum bracteosum and Angraecum 

striatum, both endemic to the Island of 

Reunion off the coast of Madagascar, 

are pollinated by birds (Micheneau et al. 

2008). The latter species is pollinated by 

ron parsons 

a variety of Zosterops borbonicus also 

endemic to Reunion Island and represents 

a great example of the evolutionary interdependence 

of orchid and pollinator. In 

bird pollination, the pollinia are usually 

attached to the beak and, at least in hummingbird 

pollinated orchids, a significant 

percentage of species have dark pollinia 

suggesting that natural selection has favored 

those with more difficult to see dark 

pollinia over those with typically more 

visible yellow pollinia. 

Structural changes brought on by 

adaptation to pollinators can have such 

dramatic effect on flower form that closely 

related species may appear to belong to 

different genera and conversely, orchids 

not really closely related may produce very 

similar flowers — the result of convergent 

evolution. Such seems to be the case in 

the Oncidium alliance where a wide range 

of pollinators is operative. Historically, 

the generic limitations in this group of 

orchids have often been dominated by floral 

structural differences and we are now 

finding, based on DNA sequences, that 

this may not necessarily be correct. One 

interpretation of the data places a large 

number of closely related species from 

multiple genera into the genus Oncidium 

and conversely moves a group of species 

with otherwise Oncidium-like flowers into 

Gomesa (Chase et al. 2009). Basically 

flat, bright yellow-and-brown oncidiums 

appear to mimic the flowers of a group of 

plants in the Malpighiaceae. The flowers 

of these flowering trees and shrubs are 

bright yellow and one petal is modified 

into a structure called the banner petal. 

On the banner petal are specialized glands 

that produce thick brown oil. Pollination in 

this group of flowering plants is effected 

by oil-collecting bees of the genus Centris. 

These bees land on the banner petal and, 

while grasping this petal tightly, scrape 

the oil-producing glands. Oncidiums pollinated 

by Centris bees are bright yellow 

and often have brown markings that mimic 

oil-producing glands (in some oncidiums, 

actual oil glands are present on the side 

lobes or crest of the lip). When the pollinating 

bee lands to harvest the oil deposit, 

it firmly grasps the oncidium’s lip with 

its mandibles while attempting to harvest 

oil from the flower. Such a pollination 

strategy requires a solid, strong connection 

between the orchid lip and the column or 

oil-harvesting runs the risk of destroying 

the flower. As such, the lip of these 

Oncidium species is fused to the column 

across the entire lip base. In contrast, bees 

of the genus Bombus (bumblebees), like 

honey bees, feed on nectar and collect 


mark whitten 

17 

eric hunt 

18 

mark whitten 

19A 

pollen to feed their larvae. These bees are 

attracted to floral structures that resemble 

or suggest the presence of nectar and pollen 

and in orchids this can take the form 

of yellow crests and yellow dusting of 

color against light flower color resembling 

spilt pollen and shallow cavities formed 

by the narrow attachment of the lip to 

column in some species. Nectar-feeding 

bees do not grasp the flower in the same 

way as oil-collecting bees while harvesting 

and in these cases the orchid lip may be 

attached to the column by just a narrow 

claw. Finally, adaptation to bird pollination 

results in the most dramatic changes. The 

current DNA data suggests that Cochlioda 

and Symphyglossum, whose bright red, 

pink and orange flowers look nothing like 

those of an Oncidium, are in fact just that 

— oncidiums pollinated by hummingbirds 

and even Sigmatostalix, a genus that at 

first glance seems distinctive, is nothing 

more than a scaled-down version that has 

specialized in the small end of the oil-collecting 

bee spectrum. 

FLORAL ATTRACTIONS Any discussion 

of orchid pollination wouldn’t 

be complete without a discussion of the 

mechanisms by which the pollinators 

are attracted to the flower. We’ve briefly 

touched on orchids that use fragrance to 

mimic rotting flesh but that’s just the tip of 

the iceberg. No other plant family exhibits 

the range found in orchids: from real or 

perceived rewards (food or sex) to those 

with moveable parts or floral traps to those 

that attract pollinators based on mimicry of 

other flowers or insects. Food rewards, real 

or perceived, may represent the simplest of 

these attraction mechanisms. While orchid 

pollen isn’t consumed by the insects that 

pollinate orchids, most Polystachya and 

some Maxillaria and Eria species have 

evolved mealy, pollenlike substitute that 

forms on their lips. The pollinating insects, 

usually bees, eat this pseudopollen, 

thereby assuring that the bees will return 

to additional flowers (Dressler 1981). Alternatively, 

some Maxillaria species form 

waxes on their lip calluses that are collected 

by the pollinators for some purposes not 

clearly understood. The labellum of freshly 

opened Dendrobium unicum flowers is covered 

with fine trichome hairs that produce 

starch or sugar. These hairs are connected 

to the lip surface by a section of specialized 

cells that decompose as the flower ages. 

When the flowers are too old to be good 

candidates for pollination or are actually 

pollinated, these cells die and are sloughed 

off leaving a lip surface devoid of reward 

(Kjellson and Rasmussen 1987, Davies and 

Turner 2004). In some species, the promise 

of reward is hollow. The yellow crest of 

hairs in Calypso, Arethusa and Calopogon 

evidently resemble a cluster of pollenbearing 

anthers and pollination depends on 

deceiving inexperienced bees. 

Often the promise of rewards is also 

coupled with the presence of moveable 

parts. Such moving parts can be passive as 

in the fringes present on many Bulbophyllum 

flowers to those that spring shut (with 

or without the creation of a trap) when 

triggered by a pollinating insect. Fringes, 

hairs, tassels and other appendages may do 

nothing more than give the appearance of 

constant motion around the flower or they 

may provide more interest. In the case of 

Bulbophyllum phalaenopsis, the white 

structures on the flower resemble maggots 

and help to strengthen the impression of 

something dead. It has been suggested 

that the tassels present on the flowers of 

Stelis villosa are actually perceived by 

the pollinating insect as larval maggots on 

which their own larvae feed. Fringes and 

appendages may be coupled with moveable 

mark whitten 

19B 

[17–19] Nowhere are the changes induced 

by pollinator more dramatic than those 

in the genus Oncidium sensu latu. 

Historically the taxonomy of this group of 

orchids has largely been based on floral 

morphology and modern DNA sequencing 

suggests that a number of genera, 

including Oncidium, Odontoglossum and 

Cochlioda, may indeed all be oncidiums. 

The yellow-and-brown Trichocentrum 

stipitatum [17] represents the group of 

Malpighiaceae-mimics pollinated by 

Centris bees that collect oil. Odontoglossum 

crispum [18] represents the changes 

that occur when the pollinators are pollen 

and nectar-feeding Bombus species and 

Odontoglossum sanguineum [19] is hummingbird 

pollinated. As in many epidendrums, 

the nectary in Odm. sanguineum 

is formed by fusion of the lip to the 

underside of the column. The entrance to 

this short chamber is visible in the face-on 

view [19A]. The up-turned aspect of the 

flower, common in bird-pollinated orchids, 

is seen in the side-on view [19B]. Grower 

[17]: Hawk Hill Nursery. 


parts. A good example of this combination 

is the yellow fringe on the hinged labellum 

of Calopogon species that appears to 

the pollinating insect as edible pollen on 

a non-orchid flower. When the pollinating 

bee lands on the lip, the weight of the 

bee causes the lip to fall forward onto the 

column, transferring pollen. 

Trap flowers run the gamut from passive 

traps such as Cypripedium acaule 

and Phragmipedium besseae to those 

with active traps such as Pterostylis and 

Porroglossum. Passive traps function by 

creating a single point of entry and a single 

escape route that brings the pollinating 

insect out past the flower’s anther. The 

escape route can be created by a lining 

of aligned hairs that allow only one way 

out, surface texture that allows the pollinating 

insect to establish footing only 

along the appropriate route or stair like 

structures that force the insect past the 

anther. The elliptical “windows” around 

the rim of the pouch of Phrag. besseae 

are actually windows composed of colorless, 

transparent cells that are designed to 

allow sufficient light to enter the pouch 

to keep the trapped, diurnal insect active 

long enough to emerge from the pouch. 

Without them, the trapped insect would 

simply go to sleep and perhaps even die 

in the pouch. The most incredible example 

of passive traps in orchids is Coryanthes 

where the visiting euglossine bee, attracted 

by the powerful resinous fragrance, is maneuvered 

into a liquid-filled bucket from 

which the only escape for the now wet bee 

is out through a narrow channel at the back 

of the bucket. Bees too small simply leave 

the bucket without contacting the anther 

and bees too large cannot get out. When 

the bee exits the passageway, the pollinia 

attach pointed toward the back of the bee. 

Contact with the stigmatic cavity requires 

20 

[20] Sebastián Vieira observed this wasp 

inspecting every flower on the inflorescence 

of this Pleurothallis species in 

a forest in the mountains of Colombia. 

Evidently one of the “honest” species 

in this genus, the wasp was observed 

to lick the nectar from each flower for 

some seconds before proceeding to the 

next flower. In the process, the orchid’s 

pollinaria were attached to the wasp’s 

head. 

that the pollinia be upright or pointed 

forward toward the bee’s head. The orchid 

avoids this complication by the fact that 

the bee is wet when it leaves the escape 

route and cannot fly. As the bee dries, the 

pollinarium stipe dries as well and curls 

forward aligning the pollinia. 

In several orchid species, the hinged 

lip movement is not passive in nature. The 

genus Porroglossum is a good example. In 

these orchids, the hinged lip is connected 

to the column by a long column foot and 

the lip is sensitive to touch at the callus 

near the base of the lip. When touched, the 

sensitive lip quickly snaps shut, creating 

a cavity between the lip and the ventral 

surface of the column trapping the visiting 

insect. The only exit is up along the 

lip surface past the stigma and rostellum. 

Insects too small to effect pollination either 

do not trigger the lip or simply leave 

through what for them is a wide open pathway. 

Insects that are too large may force 

their way out the side of the trap. A similar 

mechanism occurs in Acostaea, Pterostylis 

and some species of Drakaea. 

FLORAL MIMICRY Orchids are 

masters of mimicry in the plant world. 

Floral mimicry in orchids ranges from 

mimicking other flowers to compete for 

sebastiÁn vieira 

pollinators to territorial insect species to 

sexual mimicry involving flowers that resemble 

females of the pollinating species. 

Some species of Tolumnia are believed to 

be mimics of Malpighiacae. Female oilgathering 

bees collect oil from the glands 

of the flowers of these vines and the same 

bees have been seen to seize Tolumnia species 

and then attempt to fly away (Nierenberg 

1972). Such mimicry may be a fairly 

common occurrence. Phragmipedium 

kovachii is believed to mimic a species 

of Tibochina, Phrag. besseae appears to 

mimic a similarly-colored gesneriad and 

Epidendrum ibaguense appears to mimic 

both lantana and a milkweed species with 

orange-and-yellow flowers (Dressler 

1981). In the last case, the mimicry may 

actually accomplish two things. Inexperienced 

insects may be fooled into visiting 

the Epidendrum flowers at a frequency 

sufficient to ensure reproductive success 

and herbivores may mistake them for the 

unpalatable species they resemble. In fact, 

there is an entire group of Epidendrum 

species that superficially resemble Epi. 

ibaguense, varying in color and details of 

the lip. Each of these species appears to 

mimic a different related shrub based on 

flower color. Recently, Du Puy and Cribb 

(2007) have suggested that Cymbidium 

insigne subsp. seidenfadenii is a floral 

mimic of Rhododendron lyi, a species with 

which it grows and flowers sympatrically. 

This subspecies is found in two forms, one 

with the characteristic red-marked white 

lip and a form devoid of red markings in 

the lip. The Rhododendron bears a white 

flower with a touch of yellow in the throat 

and freshly opening flowers of the three 

species appear remarkably similar. This 

Cymbidium, Rhododendron and Dendrobium 

infundibulum (a species which is 

also common in the same habitat) were 

observed to be visited by the same species 

of bumblebee, Bombus eximius, and 

pollinia from both orchid species was observed 

on the visiting bees. They suggest 

that the presence of the red-free color form 

in large numbers might be an excellent 

example of natural selection in process. 

This color form more closely resembles 

the Rhododendron flower when freshly 

opening and may be being selected for in 

the competition for pollinating bees. 

Floral mimicry that depends on resemblance 

of an orchid’s flowers to invading 

insects has been characterized by van der 

Pijl and Dodson as pseudoantagonism but 

it might be better called pseudotrespassing. 

This mimicry is observed in several 

species of Oncidium as well as possibly 

Tolumnia henekenii. The pollinator of 


Oncidium species, such as Oncidium 

sphacelatum and Oncidium ensatum, 

appears to be the male of a species of 

oil-collecting bee. The females collect oil 

from clusters of plants and the males tend to 

pick these clusters as bases to defend their 

territories against any flying insects that are 

not females of their species. The inflorescences 

of these orchid species wafting in 

the breeze appear sufficiently like invading 

swarms of insects that the male bees attack 

the flowers and carry pollen away on their 

faces with the “border skirmish” to be repeated 

on other inflorescences within the 

territory. Tolumnia henekenii, endemic to 

the island of Hispaniola, was long thought 

to be pollinated by pseudocopulation 

because the flowers are visited by male 

bumblebees (Dod 1976). However, it now 

appears that the mechanism may be more 

likely pseudoantagonism. Bumblebees are 

terrestrial, territorial bees and typically 

occupy fiercely defended territories. The 

bumblebeelike flowers of this orchid are 

thought to resemble male bumblebees and 

pollination may result from attack on the 

flowers by the defending male bee. 

The epitome of floral mimicry involves 

those cases where orchid flowers resemble 

females of the pollinating insect and pollination 

occurs as the result of pseudocopulation. 

Pseudocopulation in orchids has 

evolved multiple times in different parts 

of the world and with different groups of 

orchids and insects, and it’s remarkable that 

it appears to be found only in orchids. In 

some cases, the specificity is so well developed 

that natural hybrids are prevented 

while in other cases natural hybridization is 

relatively common. For instance, in European 

Ophrys, the resemblance is essentially 

visual and fairly unspecific. Recent genetic 

analyses suggest that of the 216 distinctly 

different taxa in this genus, 118 of them 

are of hybrid origin (World Checklist of 

Selected Plant Families 2011). This is in 

stark contrast to the situation observed for 

Australian terrestrial orchids, such as Chiloglottis 

and Drakeae. In these cases, the 

attraction of the male insect is pheromonal 

and reinforced visually. Natural hybrids in 

these genera are essentially nonexistent. In 

the Western Hemisphere, pseudocopulation 

has evolved in at least two orchid tribes, the 

Maxillarieae and the Epidendroideae. It is 

the main pollination syndrome in Trichoceros 

and Telipogon (both in the Maxillarieae 

tribe). Both are apparently pollinated 

by flies, the former by males of the genus 

Paragymnomma (van der Cingel 2001). It 

has recently been observed in Lepanthes 

(Blanco and Barboza 2005). In Lepanthes, 

the pollinator is a male fungus gnat 

bob sprague gary backhouse 

22 

[21] Prasophyllum is one of the few genera 

in the Diurideae known to attract its 

pollinators with the reward of nectar, 

the other genera invoking deceit or 

sexual mimicry. Pollinators, depending 

on species, are all manner of nectar- 

21 

seeking insects from bees and wasps 

to beetles and flies. Here Lissopimpla 

excelsa, known in Australia by the common 

name orchid dupe wasp, pollinates 

Prasophyllum vitetrum. The wasp is 

known as the orchid dupe because this 

species is duped into pollinating a Cryptostylis 

species by sexual mimicry. Like 

all members of its genus, Lissopimpla 

excelsa is parasitic. 

[22] Some orchids combine pollinator 

syndromes. The pronounced crest on 

the lip of this Calopogon flower looks 

strikingly like a cluster of pollen-bearing 

anthers. In addition, the lip is hinged 

near its base so that when a feeding 

bee lands on the crest, the weight of 

the bee will cause the lip to collapse forward 

dropping the bee onto the column 

in the flower’s center. 


[23–24] Sexual mimicry may take more 

than one route. Australian terrestrials 

such as this Chiloglottis species [23] 

mimic both sight and smell, producing 

fragrance compounds that mimic the 

sexual pheromones of the pollinating 

insect species. In these cases the 

combination of sight and smell make 

attraction exceptionally specific and 

natural hybrids in these systems are 

effectively unknown. On the other 

hand, in European terrestrials such as 

this Ophrys species [24] the mimicry 

appears to be strictly visual, the orchid 

appearing to the insect to be a female 

of its species. The deceit is not nearly 

perfect and as a result natural hybrids 

abound in European terrestrials. 

23 

24 

mark whitten paul piko 

and we now know that at least in one group 

of Australian terrestrials, Cryptostylis, the 

encounters lead to ejaculation by the male 

insect (Gaskett et al. 2008). The cost of 

sperm wastage is fascinating and begs the 

question as to why these insects continue 

to visit these flowers. The pollinators of 

these Australian sexually-deceptive orchids 

are almost always solitary and haplodiploid 

species. Therefore, female insects 

deprived of matings by orchid deception 

can still produce male offspring, and as a 

result increase the percentage of males in 

the population and concurrently enhance 

orchid pollination. 

In addition to these simple examples of 

mimicry in which an orchid species produces 

a fragrance suggestive of the female of 

the pollinating species or evolves a flower 

that looks like another species in order 

to compete for pollinators, some orchids 

express what might be called second-order 

mimicry. Mimicry of the second order is 

especially intriguing. In these second-order 

situations, the flower produces chemical or 

visual signals that suggest another insect, 

the presence of which attracts the pollinator 

to the flowers. As an example, Epipactis 

veratrifolia has dark structures on the labellum 

that superficially resemble a colony 

of black aphids. When aphid colonies are 

under attack by predatory insects, the aphids 

emit a chemical pheromone that alerts the 

remainder of the colony to the attack. This 

pheromone however, also attracts other 

predatory insects to the fray. In the case of 

this Epipactis species, the flower’s fragrance 

is a chemical copy of the aphid-under-attack 

pheromone. The pollinating wasp sees the 

dark lumps on the labellum and smells aphid 

colony in distress and lands on the flower to 

feed on the aphid colony. As a result, orchid 

pollinia are transferred from flower to flower 

(Stökl et al. 2010). 

The more we study the world of orchid 

pollination, the more intricate and 

convoluted it becomes. Eventually, we 

may figure out the answer to that age old 

question, “Which came first, the chicken 

or the egg?” but in the meantime one thing 

is clear: form and function are intimately 

linked when it comes to orchid pollination. 

If you can imagine a pollination mechanism 

we will probably someday find an orchid 

that utilizes it. 

References 

Adams, R.M. and G.J. Goss. 1976. The Reproductive 

Biology of the Epiphytic Orchids of Florida III. 

— Epidendrum anceps Jacquin. Amer. Orchid Soc. 

Bull. 45(6):488–492. 

Blanco, M. and G. Barboza. 2005. Pseudocopulatory Pollination 

in Lepanthes (Orchidaceae: Pleurothallidinae) 

by Fungus Gnats. Ann. Bot. 95(5):763–772. 

Buchmann, S.L. 1987. The Ecology of Oil Flowers and 


alex smart 

25 

Their Bees? Annual Review of Ecology and Systematics. 

Annual Reviews, Palo Alto. 18:343–96. 

Chase, M. and A. Pridgeon. 2009. Subtribe Oncidiinae, 

pp. 211–394. In Genera Orchidacearum Vol. 5 Epidendroideae 

(Part Two), A.M. Pridgeon, P.J. Cribb, M.W. 

Chase and F.N. Rasmussen (eds.). Oxford University 

Press, Oxford. 

Coleman, E. 1928. Pollination of an Australian Orchid 

by the Male Ichneumonid Lissopimpla semipunctata 

Kirby. Transactions of the Entomological Society of 

London 76:533–539. 

__. 1932. Pollination of Diuris pedunculata R.Br.? Victorian 

Naturalist 49:179–186. 

Davies, K.L. and M.P. Turner. 2004. Pseudopollen in 

Dendrobium unicum Seidenf. (Orchidaceae): Reward 

or Deception? Ann. Bot. 94:129–132. 

Dod, D.D. 1976. Oncidium henekenii — Bee Orchid 

Pollinated by Bee. Amer. Orchid Soc. Bull. 

45(9):792–794. 

Dressler, R.L. 1981. The Orchids, Natural History and 

Classification. Harvard University Press, Cambridge 

and London. 

Du Puy, D. and P. Cribb. 2007. The Genus Cymbidium, 

2nd Edition. Kew Publishing, Royal Botanic Gardens, 

Kew. 

Gaskett, A.C., C.G. Winnick and M.E. Herberstein. 

2008. Orchid Sexual Deceit Provokes Ejaculation. The 

American Naturalist 171:E206–E212. 

Kjellson, G. and F.N. Rasmussen. 1987. Does the 

Pollination of Dendrobium unicum Seidenf. Involve 

Pseudopollen? Die Orchidee 38:183–187. 

Micheneau, C., J. Fournel, L. Humeau and T. Pailler. 

2008. Orchid–Bird Interactions: A Case Study from 

Angraecum (Vandeae, Angraecinae) and Zosterops 

(White-Eyes, Zosteropidae) on Reunion Island. Botany 

86:1143–1151. 

Micheneau, C., J. Fournel, B.H. Warren, S. Hugel, A. 

Gauvin-Bialecki, T. Pailler, D. Strasberg and M.W. 

Chase. 2010. Orthoptera, A New Order of Pollinator. 

Ann. Bot. 105(3):355–364 doi: 10.1093/aob/mcp299. 

Mota Maues, M. 1998. Reproductive Phenology and 

Pollination of the Brazil Nut Tree (Bertholettia excelsa 

Humb. & Bompl. Lecythidaceae) in Eastern Amazonia, 

p. 208. In Pollinating Bees — The Conservation Link 

Between Agriculture and Nature, eds. P.G. Kevan and 

V.L. Imperatriz-Fonseca. Ministério do Meio Ambiente, 

Brasília. 

Nash, R.C. 1979. Observation of Native Bees on Diuris 

pedunculata. Journal of the Native Orchid Society of 

South Australia 3:7–9. 

Nierenberg, L. 1972. The Mechanism for the Maintenance 

of Species Integrity in Sympatrically Occurring Equitant 

Oncidiums in the Caribbean. Amer. Orchid Soc. 

Bull. 41(10):873–881. 

Stökl, J., J. Brodmann, A. Dafni, M. Ayasse and B. Hansson. 

2010. Smells Like Aphids: Orchid Flowers Mimic 

nathan mattinson 

Aphid Alarm Pheromones to Attract Hoverflies for 

Pollination. Proceedings of the Royal Society B, Online, 

DOI: 10.1098/rspb.2010.1770. 

Thien, L.B. and F. Utech. 1970. Thien, L.B., and F. Utech. 

The Mode of Pollination in Habenaria obtusata (Orchidaceae). 

Amer. J. Bot. 57:1031–1035. 

van der Cingel, N.A. 2001. An Atlas of Orchid Pollination, 

America Africa Asia and Australia. A.A. Balkerma, 

Rotterdam. 

van der Pijl, L. and Dodson, C.H. 1966. Orchid Flowers: 

Their Pollination and Evolution. University of Miami 

Press, Coral Gables. 

Whigham, D.F. and McWhety, M. 1980. Studies on the 

Pollination Ecology of Tipularia discolor (Orchidaceae). 

Amer. J. Bot. 67(4):550–555. 

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Published on the Internet; http://apps.kew.org/wcsp/accessed 

2011. 

Ron McHatton, PhD, has been growing orchids 

for more than 47 years. A chemist by 

training, he is the Society’s chief operating 

officer, director of education and editor of 

its monthly e-newsletter. (e-mail rmchatton@aos.org). 

26 

[25] Pollination of Diuris behrii is not well 

understood. Visitation by only males of 

one bee species while other similarly 

colored species are ignored are aspects 

of sexual mimicry while the flower’s 

bright yellow color and floral structures 

are indicative of nectar mimicry (Coleman 

1932, Nash 1979). 

[26] Cryptostylis leptochila, the smalltongued 

orchid, is pollinated by male 

Lissopimpla semipunctata wasps. The 

wasps are common throughout Australia 

but the orchid is endemic to New 

South Wales and Victoria States. If the 

orchid is introduced to an area where it 

is new the male wasps detect the flowers 

almost immediately and are drawn 

to them (Coleman 1928).

Orchid Pollination - University of Miami

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