The pollination of orchids represents a complex aspect of the biology of this plant family, characterized by intricate flower structures and diverse ecological interactions with pollinator. Notably, the topic has garnered significant scientific interest over time, including the attention of Charles Darwin, who is recognized for his contributions to the theory of evolution by natural selection. In 1862, Darwin published his observations on the essential role of insects in orchid pollination in his work The Fertilization of Orchids.[1][note 1] He noted that the various strategies employed by orchids to attract their pollinators are complex.
The phenomenon of zoophily in orchids requires that pollinating animals frequently visit the flowers and remain long enough to contact both the anthers and stigma. For successful pollen transfer, it is crucial that the pollen adheres effectively to the pollinators, enabling it to reach the stigmas of other flowers. The effectiveness of zoophily depends on the ability of these animals to recognize flowers from a distance and their attraction to flowers of the same species. Consequently, zoophilous flowers typically possess "attractive products" such as pollen and nectar, "means of attraction" like scents and colors, and pollen that is viscous or adhesive.[4]
Throughout the evolution of angiosperms, there has been significant differentiation in the means of attraction and flower morphology, allowing a broader range of animals to participate in pollination. This evolutionary process has led to the establishment of close relationships between pollinating animals and zoophilous flowers, benefiting both groups. For plants, this relationship has resulted in more precise attraction of specific pollinators, facilitating the transfer of pollen to the stigmas of other plants and reducing the overall production of pollen. In contrast to anemophilous plants, which may produce around one million pollen grains per ovule, orchids typically produce a one-to-one ratio. For specialized pollinators, this mutualism has reduced competition from other anthophilous animals, making targeted pollination advantageous.[3][4]
The evolutionary development of zoophilous angiosperms and their adapting animal partners is best understood as a process of coevolution characterized by reciprocal relationships. In some cases, orchids and their pollinators have become so interdependent that their existence is mutually exclusive.[4] Pollination mechanisms resulting from this coevolution generally benefit both parties: pollinators obtain nectar from the flowers, while orchids gain pollen transfer. However, in numerous instances, the attraction of pollinators to orchids may rely on deceptive strategies that do not offer any rewards.[3]
Orchid flowers are predominantly hermaphroditic, with unisexual forms being rare, and they typically exhibit zygomorphic (bilateral) symmetry. Most genera feature three outer elements known as sepals—two lateral and one dorsal—and three inner elements called petals, with the lower petal often modified into a lip or labellum. This labellum is usually larger and more vividly colored than the other petals, often exhibiting a trilobed or uniquely shaped structure, sometimes adorned with fleshy bumps or ridges, and may feature a basal spur with distinct color patterns.[6]
The androecium of orchids generally comprises one or two stamens, occasionally three, and is fused with the style and stigma to form a structure known as the column (or gynostema/gynostegium). The pollen is organized into masses called pollinia, which can vary in number from one to twelve, though two or four are most common. These pollinia, combined with a sticky stalk derived from the anther or stigma, form the transport unit during pollination. The gynoecium consists of three fused carpels and is situated below the calyx. It features a highly modified style that is solitary and terminal, forming a key component of the column. Near the apex, the stigma has an elongated lobe known as the rostellum, which is typically non-receptive and positioned above the stigmatic region. A part of the rostellum may develop into a sticky platform called the viscidium, which attaches to the pollinium stem.[6]
Orchids generally produce nectar as a reward for pollinators, with nectaries varying in their location and type. They may be situated on the lip spur, at the tips of the sepals, or on the septa of the gynoecium. Additionally, some orchid species are capable of self-pollinatings or are apomictics, meaning they can produce seeds without the need for pollinators.[7]
Attraction of pollinators by means of rewards
Many orchid species provide various rewards to pollinators, including nectar, food hairs, oils, and other compounds such as waxes, resins, and fragrances. These rewards serve to reinforce pollinator behavior, enhancing the likelihood of effective pollen transfer. Over time, this specialization on a single type of pollinator has led to increased morphological and structural adaptations in orchid flowers, aimed at attracting specific insect species.[3][8][9][10]
This evolutionary aspect of the interspecific relationships between orchids and their pollinators was notably examined by Charles Darwin in his studies of both British and exotic orchid species. A prominent example is the Madagascar species Angraecum sesquipedale, which features a spur exceeding 30 cm in length that contains nectar. Darwin's attempts to extract the pollinia from this flower using needles were unsuccessful; he was only able to do so by inserting a cylinder with a diameter of 2.5 mm into the spur and pulling it out, causing the viscidium to adhere to the cylinder. Darwin proposed that when a butterfly reached the bottom of the spur to access the nectar, the pollinia would attach to its head as it withdrew its proboscis. Upon visiting another flower, the butterfly would then transfer the pollinia to its stigma. Based on this reasoning, Darwin suggested that the pollinator of Angraecum sesquipedale would need to have a proboscis longer than 30 cm, an idea that seemed implausible to contemporary biologists. However, this prediction was confirmed in 1903 with the discovery of the moth Xanthopan morganii praedicta in Madagascar, which possesses a proboscis of the predicted length. The subspecific epithet "praedicta" reflects the foretelling of its existence by Darwin.[3]
The butterfly is attracted to Angraecum sesquipedale by its fragrance, particularly during the night. Upon approaching the flower, the butterfly unrolls its proboscis and inserts it into a crevice of the rostellum that leads to the spur. After accessing the nectar at the base of the spur, the butterfly lifts its head while withdrawing its proboscis, causing the viscidium to adhere to its head or another part of its body. The viscidium contains a small pedicel, known as the caudicle, which carries the pollinia. When the butterfly finishes feeding and moves to another flower, the caudicle dehydrates, altering its angle relative to the insect’s body. This positioning ensures that when the butterfly inserts its proboscis into the next flower, the pollinia are properly aligned to attach to the stigma. After successful pollination, the flowers stop producing fragrance, and their tepals wither shortly thereafter. Nonetheless, the processes of pollen transfer, fertilization, and the formation of numerous new individuals have already been secured.[3]
Many orchids, including Angraecum sesquipedale, are pollinated by nocturnal butterflies and, as a result, tend to have light-colored or nearly white flowers that emit fragrance in the evening or night. Other examples of such orchids include Bonatea speciosa, Habenaria epipactidea, species in the genus Satyrium, Disa cooperi and D. ophrydea.[11] In contrast, some orchid genera have evolved to be pollinated by diurnal butterflies, exhibiting bright colors and providing nectar as a reward.[11]
Melitophilous orchids, which are pollinated by bees, typically produce a strong fragrance during the day and are robustly colored. Examples include Satyrium erectum and Disa versicolor. These bee-pollinated orchids often offer not only nectar but also oils—a relatively rare reward in the plant kingdom—used by various bee species to nourish their larvae. Notable genera that provide oil rewards include Disperis, Pterygodium, Corycium, Ceratandra, Evotella, Satyrium, and Pachites.[11]
Pollination by flies, known as myophily, is the second most prevalent method of pollination among orchids, involving pollinators from twenty different dipteran families.[12] These flowers typically emit scents reminiscent of decaying organic materials, excrement, or carrion, which attract flies seeking food or suitable sites for egg deposition.[8] Various floral parts produce putrescent or carrion-like odors and often incorporate traps to retain the pollinators, alongside appendages and colors that may mimic flesh or other aspects of rotting matter.[13][14][15][16] For example, species in the genus Bulbophyllum, which are daciniphilous, attract true fruit flies from the Dacini tribe (Tephritidae) using pleasant or spicy scents as floral synomones, establishing mutualistic relationships that confer reproductive benefits to both the orchids and the male flies. Notably, B. hortorum has coevolved with male fruit flies to develop a unique pollination mechanism that selects for optimal-sized individuals as potential pollinators.[17]
Stelis hymenantha produces a strong aroma of sweet menthol. It secretes a sticky substance at the base of the labellum that resembles nectar. Another species, S. immersa, also emits a fragrance, described as melon-like, with its sticky substance located on both the petals and the labellum’s base. The primary visitors to these species are dipterans from various families. Typically, flies remain outside the flowers, investigating the viscous liquid on the petal surfaces. In the case of Stelis immersa, female flies from the genus Megaselia (Phoridae) are specifically adapted for effective pollen transfer. After inspecting the nectar-like substance, these insects enter the flower laterally and land on the downward-facing labellum. When this occurs, the labellum rises, pressing the fly against the viscid pollinium and trapping it. To exit, the fly backs up, causing the viscidium to adhere to its thorax. The labellum then returns to its original position, releasing the fly, which has now facilitated pollination.[18]
Attraction and reward by means of perfumes
The flowers of the subtribes Stanhopeinae and Catasetinae exhibit specialized pollination mechanisms. These species are exclusively pollinated by male euglossine bees, which collect perfumes from the flowers. The reasons behind this behavior are not entirely understood, as the perfumes do not provide nutrition or protection but may play a role in the mating rituals of these bees.
The process of scent collection is quite similar across species. The male bee approaches the osmophore, the scent-producing part of the flower, and perches on the labellum. Using the long, dense hairs on its front legs, the bee gathers the aromatic substances, which are typically liquid but can also be found in crystalline form. If the scent compounds are solid, the bee dissolves them with secretions from its salivary glands. Once saturated with the aroma, the bee transfers the scents using its middle legs to storage cavities in its hind legs, where they can be preserved for extended periods. Different orchid genera attach pollinia to various parts of the bee’s body.[19]
Orchids, like many plants, attract specific groups of male euglossine bees by producing species-specific scent mixtures, which likely act as reproductive isolation mechanisms. Some orchids exhibit morphological adaptations that ensure pollinia are only released upon visitation by particular bee species, depending on their size and behavior. Consequently, not all euglossine bees that visit a given orchid species are effective pollinators.[20]
Additionally, certain species of Bulbophyllum attract specific male Dacini fruit flies through particular male attractants such as methyl eugenol (ME), raspberry ketone (RK), or zingerone (ZN). Methyl eugenol serves as a sex pheromone precursor for several quarantine pest species of Bactrocera, including the oriental fruit fly (B. dorsalis), B. carambolae, B. occipitalis, and B. umbrosa. Similarly, raspberry ketone and zingerone function as sex pheromone components for male fruit flies like Zeugodacus caudatus, Z. cucurbitae, and Z. tau. [21][22][23][24][25][26]
Many orchids employ various deceptive tactics to attract pollinators by mimicking scents, shapes, colors, or movements associated with resources of interest to the pollinators, without providing any actual rewards. The mechanisms of deception are diverse and can be categorized as follows:[27][28]
Generalized Feeding Deception: Flowers mimic the shape and coloration of species that typically offer rewards to pollinators.
Feeding Deception Mediated through Floral Mimicry:[note 2] In this case, flowers closely resemble a specific species that rewards pollinators, cohabiting in the same environment.[29]
Mimicry of Nesting Sites: Flowers imitate the egg-laying sites of certain pollinators.
Mimicking Shelter Sites: Flowers provide potential shelter for pollinators, a strategy that may not be deceptive but can be mutually beneficial for both the insect and the orchid.
Pseudo-Antagonism: This mechanism attracts pollinators by invoking their innate defense responses. Flowers mimic the appearance of another insect species that the pollinator may perceive as a threat, prompting the pollinator to attack the flower. In doing so, the insect inadvertently transfers pollen to other flowers.[30]
Rendezvous Attraction: Flowers imitate other flowers that are attractive to female pollinators.
Sexual Deception: In this scenario, flowers mimic the visual and olfactory mating signals of female pollinators.
Among these mechanisms, generalized feeding deception is the most commonly observed in orchids, reported in 38 genera, followed by sexual deception, which has been identified in 18 genera.[27][28]
Attraction of pollinators by luring them for food
The ability to attract pollinators without offering rewards has evolved independently across several angiosperm lineages, although it typically occurs in only a few species within each family.[31] In contrast, it is estimated that approximately one-third of orchid species employ a food-deceptive mechanism.[32][33] This strategy involves signaling the presence of food, such as nectar or pollen, to attract pollinators without providing any actual reward. Orchids often achieve this by resembling species that do offer rewards and that cohabit in the same environment.[34] Feeding deception commonly manifests as a general resemblance to rewarding species, with orchids featuring large, brightly colored flowers that exploit pollinators' innate preferences for such floral characteristics.[28][33][35]
Imitation of other plants
Another method employed by orchids to attract pollinators involves mimicking the flowers of other plant species. A notable example is Epidendrum ibaguense, a terrestrial or lithophilous orchid found from Mexico to Bolivia and Brazil. This orchid features orange flowers with an intense yellow labellum, closely resembling the flowers of Asclepias curassavica, a member of the Asclepiadaceae family. The butterfly Agraulis vanillae, which typically visits Asclepias curassavica to collect nectar in exchange for pollen transport, is frequently drawn to the mimetic flowers of Epidendrum ibaguense. When the butterfly approaches the orchid, it inserts its proboscis into the narrow duct (gynostemium) of the flower. The small diameter of this duct can cause the butterfly's spiracles to become temporarily trapped. In its struggle to escape, the insect inadvertently picks up the orchid's pollinia. After being released, the butterfly may visit another Epidendrum flower, transferring the pollinia and facilitating pollination without receiving nectar for its efforts.[36]
Certain orchids have evolved deceptive flowers that mimic the shape, hairiness, and scent of female wasps or bees to attract male pollinators. A well-known example is Ophrys insectifera, found in southern Europe, which is exclusively visited by two species of wasps from the genus Argogorytes. Male wasps emerge in spring, weeks before females, and are attracted to the fragrance of Ophrys flowers, which resembles the pheromones secreted by female wasps. Additionally, the labellum of Ophrys insectifera closely resembles the shape, color, and texture of female wasps. This interaction is referred to as pseudocopulation, as male wasps attempt to mate with the flower, during which they come into contact with the anther and transfer pollinia between flowers.[37]
The phenomenon of pollination by pseudocopulation was first documented by A. Pouyanne and H. Correvon in 1916 and 1917 while studying the relationship between the orchid Ophrys speculum and the scoliidae wasp Campsoscolia ciliata in Algeria.[38][39] Their findings initially went unnoticed until Robert Godfrey validated their observations in 1925, prompting increased interest in this area of study.[note 3] Following this, Australian biologist Edith Coleman published numerous papers on the pollination of orchids from the genus Cryptostylis by males of the ichneumonoidea wasp Lissopimpla excelsa.[40][41][42]
A similar mechanism is found in Tolumnia henekenii, whose flowers mimic the female of the bee species Centris insularis. The resemblance is so convincing that male bees attempt to copulate with the flower, thereby facilitating its pollination.
In the genus Caleana, commonly known as the duck orchid due to its labellum resembling the head of a duck and the overall flower resembling a flying duck, a distinct mechanism of pseudocopulation has been observed. This species utilizes a spring mechanism to trap pollinating insects in a pouch, with their only means of escape being through the pollinium and stigma. When male insects land on the labellum, they activate a two-hinged relaxation mechanism involving the labellum-lamellum and the lamina-perianth, which flips the insect into the pouch containing the stigma and pollinia.[50][51]
Pseudocopulation is not limited to pollinators from the Hymenoptera order (such as bees and wasps); it has also been documented in certain Diptera. For instance, male mosquitoes of the genus Bradysia have been observed to pseudocopulate with species of Lepanthes, a large genus of orchids found in neotropical rainforests.[52]
In Ophrys orchids, the flowers not only mimic the shape, size, and color of female pollinators but also emit fragrances that include compounds found in female sex pheromones. This reinforces the sexual attraction of males to the flowers. Chemical and electrophysiological comparisons have been made between the volatile compounds emitted by Ophrys iricolor and the female pheromones of its pollinator, Andrena morio. More than 40 compounds have been identified, including alkanes and alkenes with 20 to 29 carbon atoms, aldehydes with 9 to 24 carbons, and two esters. Most of these compounds are present in similar proportions in both floral extracts of O. iricolor and extracts from the cuticular surface of A. morio females.[53]
The biologically active volatile compounds in Ophrys species are largely similar to those utilized by other Ophrys species that engage in pseudocopulation with males of the genera Andrena and Colletes.[54][55][56][57][58]
↑ The book was edited under the title On the various contrivances by which British and foreign orchids are fertilized by insects, and on the good effects of intercrossing. This book was the first detailed demonstration of the power of natural selection, which could explain how the complex interrelationships between orchids and their pollinators had arisen. Darwin described detailed observations of the pollination mechanism of various orchids, his experiments and dissections of flowers, and explained a number of previously ignored aspects of the pollination biology of this family, such as the Catasetumpuzzle, which was thought to consist of flowers of three different species on the same plant, and also made predictions that were later confirmed in nature.[2]
↑ There are two general types of mimicry relevant to pollination ecology: Müllerian and Batesian mimicry. In the first case (which many authors take as convergence rather than mimicry) several species with similar abundance in nature develop to their mutual advantage the same propaganda style as a warning to predators, for example. In contrast, an organism sparsely represented in nature, which mimics a more abundant one to deceive a third, develops a unidirectional advantage that is called Batesian mimicry. (Proctor & Yeo, p. 375)
↑ Subject that came to be called "Pouyanian mimicry" in honor of its discoverer.
Related Research Articles
Orchids are plants that belong to the family Orchidaceae, a diverse and widespread group of flowering plants with blooms that are often colourful and fragrant. Orchids are cosmopolitan plants that are found in almost every habitat on Earth except glaciers. The world's richest diversity of orchid genera and species is found in the tropics.
Ophrys apifera, known in Europe as the bee orchid, is a perennial herbaceous plant of the genus Ophrys, in the family of Orchidaceae. It serves as an example of sexually deceptive pollination and floral mimicry, a highly selective and highly evolved plant–pollinator relationship.
A pollinator is an animal that moves pollen from the male anther of a flower to the female stigma of a flower. This helps to bring about fertilization of the ovules in the flower by the male gametes from the pollen grains.
The genus Ophrys is a large group of orchids from the alliance Orchis in the subtribe Orchidinae. They are widespread across much of Europe, North Africa, Caucasus, the Canary Islands, and the Middle East as far east as Turkmenistan.
A pollinium is a coherent mass of pollen grains in a plant that are the product of only one anther, but are transferred, during pollination, as a single unit. This is regularly seen in plants such as orchids and many species of milkweeds (Asclepiadoideae). Usage of the term differs: in some orchids two masses of pollen are well attached to one another, but in other orchids there are two halves each of which is sometimes referred to as a pollinium.
Pseudocopulation is a behavior similar to copulation that serves a reproductive function for one or both participants but does not involve actual sexual union between the individuals. It is most generally applied to a pollinator attempting to copulate with a flower adapted to mimic a potential female mate. The resemblance may be visual, but the key stimuli are often chemical and tactile. The form of mimicry in plants that deceives an insect into pseudocopulation is called Pouyannian mimicry after the French lawyer and amateur botanist Maurice-Alexandre Pouyanne.
Entomophily or insect pollination is a form of pollination whereby pollen of plants, especially but not only of flowering plants, is distributed by insects. Flowers pollinated by insects typically advertise themselves with bright colours, sometimes with conspicuous patterns leading to rewards of pollen and nectar; they may also have an attractive scent which in some cases mimics insect pheromones. Insect pollinators such as bees have adaptations for their role, such as lapping or sucking mouthparts to take in nectar, and in some species also pollen baskets on their hind legs. This required the coevolution of insects and flowering plants in the development of pollination behaviour by the insects and pollination mechanisms by the flowers, benefiting both groups. Both the size and the density of a population are known to affect pollination and subsequent reproductive performance.
Zoophily, or zoogamy, is a form of pollination whereby pollen is transferred by animals, usually by invertebrates but in some cases vertebrates, particularly birds and bats, but also by other animals. Zoophilous species frequently have evolved mechanisms to make themselves more appealing to the particular type of pollinator, e.g. brightly colored or scented flowers, nectar, and appealing shapes and patterns. These plant-animal relationships are often mutually beneficial because of the food source provided in exchange for pollination.
Microtis, commonly known as onion orchids or mignonette orchids is a genus of about 20 species of plants in the orchid family, Orchidaceae. Onion orchids are terrestrial herbs with a single leaf at the base of the plant. They are similar to orchids in the genus Prasophyllum in that they have an onion-like leaf. The flowers are small but often scented and attractive to their insect pollinators. They are widespread in Asia, Australia and some Pacific islands.
Ophrys insectifera, the fly orchid, is a species of orchid and the type species of the genus Ophrys. It is remarkable as an example of the use of sexually deceptive pollination and floral mimicry, as well as a highly selective and highly evolved plant–pollinator relationship.
Carrion flowers, also known as corpse flowers or stinking flowers, are mimetic flowers that emit an odor that smells like rotting flesh. Apart from the scent, carrion flowers often display additional characteristics that contribute to the mimesis of a decaying corpse. These include their specific coloration, the presence of setae and orifice-like flower architecture. Carrion flowers attract mostly scavenging flies and beetles as pollinators. Some species may trap the insects temporarily to ensure the gathering and transfer of pollen.
Chiloglottis, commonly known as wasp orchids, ant orchids or bird orchids, is a genus of about 25 species of flowering plants in the orchid family, Orchidaceae and is found in eastern Australia and New Zealand. Wasp orchids are terrestrial herbs which grow in colonies of genetically identical plants. They usually have two leaves at the base of the plant and a single resupinate ("upside-down") flower. The labellum is more or less diamond-shaped and has calli resembling the body of a wingless female wasp.
Drakaea is a genus of 10 species in the plant family Orchidaceae commonly known as hammer orchids. All ten species occur only in the south-west of Western Australia. Hammer orchids are characterised by an insectoid labellum that is attached to a narrow, hinged stem, which holds it aloft. The stem can hinge only backwards, where the broadly winged column carries the pollen and stigma. Each species of hammer orchid is pollinated by a specific species of thynnid wasp. Thynnid wasps are unusual in that the female is flightless and mating occurs when the male carries a female away to a source of food. The labellum of the orchid resembles a female thynnid wasp in shape, colour and scent. Insect pollination involving sexual attraction is common in orchids but the interaction between the male thynnid wasp and the hammer orchid is unique in that it involves the insect trying to fly away with a part of the flower.
Ophrys bombyliflora, the bumblebee orchid, is a species of Ophrys, native from the Mediterranean region from Portugal and the Canary Islands to Turkey and Lebanon. The genus name Ophrys is from the Greek in reference to the hairy lips of the flowers of this genus; the specific epithet bombyliflora is from the Greek bombylios (bumblebee) in reference to the appearance of the flowers of this species.
Pouyannian mimicry is a form of mimicry in plants that deceives an insect into attempting to copulate with a flower. The flower mimics a potential female mate of a male insect, which then serves the plant as a pollinator. The mechanism is named after the French lawyer and amateur botanist Maurice-Alexandre Pouyanne. The resemblance that he noted is visual, but the key stimuli that deceive the pollinator are often chemical and tactile.
Bulbophyllum baileyi, commonly known as the fruit fly orchid, is a species of epiphytic or lithophytic orchid that is native to Queensland and New Guinea. It has coarse, creeping rhizomes, curved, yellowish pseudobulbs with a single thick, fleshy leaf, and a single cream-coloured flower with yellow, red or purple spots. It grows on trees and rocks in open forest, often in exposed places.
Cryptostylis, commonly known as tongue orchids, is a genus of flowering plants from the orchid family. Tongue orchids are terrestrial herbs with one to a few stalked leaves at the base of the flowering stem, or leafless. One to a few dull coloured flowers are borne on an erect flowering stem. The most conspicuous part of the flower is the labellum, compared to the much reduced sepals and petals. At least some species are pollinated by wasps when they attempt to mate with the flower. There are about twenty five species found in South Asia, Southeast Asia and the South Pacific.
Chemical mimicry is a type of biological mimicry involving the use of chemicals to dupe an operator.
In evolutionary biology, mimicry in plants is where a plant evolves to resemble another organism physically or chemically. Mimicry in plants has been studied far less than mimicry in animals. It may provide protection against herbivory, or may deceptively encourage mutualists, like pollinators, to provide a service without offering a reward in return.
Pollination traps or trap-flowers are plant flower structures that aid the trapping of insects, mainly flies, so as to enhance their effectiveness in pollination. The structures of pollination traps can include deep tubular corollas with downward pointing hairs, slippery surfaces, adhesive liquid, attractants, flower closing and other mechanisms.
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