Hawk-moth scale analysis and pollination specialization in the epilithic Malagasy endemic Aerangis ellisii (Reichenb. fil.) Schltr. (Orchidaceae)

The pollination biology of the epilithic endemic Aerangis ellisii (Reichenb. fil.) Schltr. (Orchidaceae) was studied on an inselberg in the Antananarivo region, central Madagascar. The flowers exhibit long nectariferous spurs indicative of hawk-moth pollination, the mechanism evidently involving pollinia transfer via the frons and palpi of the vectors. Analysis of hawk-moth scales on naturally pollinated stigmas showed that the principal pollinators were Agrius conuolvuli (Linnaeus) and Panogena lingens (Butler). Proboscis morphology of long-tongued Malagasy Sphingidae in relation to nectar position and spur morphology in A. ellisii also indicated that A. conuolvuli and P. lingens were best suited to interact with the plant's floral adaptation. Aerangis ellisii seems to display a relatively moderate specialization versus the Malagasy hawk-moth guild since its nectar is accessible even to those long-proboscis hawk-moth species which are not able to act as pollinators.     

Hawkmoth pollination of the African epiphytic orchid Mystacidium venosum, with special reference to flower and pollen longevity

 The pollination biology of Mystacidium venosum Harv. ex Rolfe, an epiphytic orchid endemic to southern Africa, was investigated. The white flowers of this orchid have long spurs (c 4.5 cm) and contain dilute sucrose-dominated nectar which is secreted during the afternoon and early evening. Scent, dominated by Jasmine lactone and (E, E)-Farnesol, is emitted in the evening. A breeding system experiment established that self pollination results in inferior quality fruits. Field observations at three sites in Kwa-Zulu Natal, South Africa, in 1998 and 1999 showed that hawkmoths were frequent visitors to the orchid shortly after dusk, and carry pollinaria along the length of their tongues. No evidence for directional selection on spur length was found at any of the three sites. Results showed that flower longevity is strongly reduced by pollination, while pollinaria removal had only a small effect. Pollinia removed from flowers remained viable for up to 20 days under outdoor conditions. Received August 15, 2000 Accepted April 17, 2001     

Observations of hawkmoth pollination in the South African orchid Disa cooperi

Johnson, S. D. 1995. Observations of hawkmoth pollination in the South African orchid Disa cooperi . - Nord. J. Bot. 15: 121–125. Copenhagen. ISSN 0107–055X.
Disa cooperi is a robust grassland orchid with long-spud white flowers which are strongly scented in the evening. Observations at a site in Natal province, South Africa showed that hawkmoths are frequent visitors to the orchid at dusk. The hawkmoth Basiothia schenki was an effective pollinator of D. cooperi; this hawk-moth has a medium length proboscis (x = 4.3 cm) which can be fully inserted into the nectar-containing spur of the orchid. Pollinaria are attached ventrally to the basal portion of the proboscis where it joins the head. Another hawkmoth, Agrius convolvuli , commonly foraged on nectar from D. cooperi , but did not carry pollinaria, probably because its proboscis is too long (x = 10 cm) to allow contact between the thick basal portion of the proboscis and the orchid column. Lips in Disa are typically linear and do not produce scent, thus the autapomorphic spathulate and scent-producing lip of D. cooperi indicates that hawkmoth pollination is derived in this species, probably from a long-proboscid fly pollinated ancestor.     

Diverse pollination systems of the twin-spurred orchid genus Satyrium in African grasslands

The large terrestrial orchid genus Satyrium underwent evolutionary radiations in the Cape floral region and the grasslands of southern and eastern Africa. These radiations were accompanied by tremendous diversification of the unusual twin-spurred flowers that characterize the genus, but pollination data required to interpret these patterns of floral evolution have been lacking for grassland species in the genus. Here we document pollinators, nectar properties, and levels of pollination success for 11 grassland Satyrium species in southern and south-central Africa. Pollinators of these species include bees, beetles, butterflies, hawkmoths, noctuid moths, long-proboscid flies, and sunbirds. Most species appear to be specialized for pollination by one functional pollinator group. Long-proboscid fly pollination systems are reported for the first time in Satyrium (in S. macrophyllum and a high-altitude form of S. neglectum). Floral morphology, especially spur length and rostellum structure, differs markedly among plants with different pollinators, while nectar volume, concentration, and sugar composition are fairly uniform across species. Most taxa exhibited high levels of pollination success (>50% of flowers pollinated), a trend that can be attributed to the presence of nectar in the twin spurs.     

Effective pollinia transfer by settling moths’ legs in an orchid Habenaria aitchisonii

A great diversity of flower morphology in orchids has long been thought to be selected by diverse pollinators. Habenaria Willd. (Orchidaceae) species are generally characterized by long nectar spurs and pollinated by long‐tongued insects (Lepidoptera), the mechanical fit between the spur and pollinator proboscis length being supposedly caused by “arms race” reciprocal selection. Here, we report that flowers of Habenaria aitchisonii Rchb. f. with nectar spurs (approximately 9 mm) were pollinated by three species of settling noctuid moths whose proboscises varied in length from 10 to 16 mm. When a settling moth crawled on the spikes and probed the flowers for nectar, pollinia were placed on the moths’ legs rather than on other body parts. Our 5‐year survey of pollinia movement and 3‐year supplemental pollination experiments indicated that fruit and seed production in this orchid were not often pollen‐limited at flower level. In a natural population in Shangri‐La, Southwest China, the proportions of pollinia removal and deposition on stigmas by moth legs were 93.8% and 83.5%, respectively. This finding of efficient pollen transfer by the pollinators’ legs in H. aitchisonii adds a new example of diverse pollinia placement on pollinators (here settling moths) in the Orchidaceae.     

Specialization for pollination by beetles and wasps: the role of lollipop hairs and fragrance in Satyrium microrrhynchum (Orchidaceae)

Exposed nectar presentation is a key trait in flowers specialized for pollination by short-tongued insects. We investigated the pollination of Satyrium microrrhynchum, a rare South African orchid in which nectar is secreted as droplets on long floral hairs ("lollipop hairs") at the mouth of a shallow labellum. Our observations indicate that this orchid is pollinated specifically by two insect species: a cetoniid beetle (Atrichelaphinus tigrina) and a pompilid wasp (Hemipepsis hilaris). Both insects have short mouthparts and remove nectar from the hairs with sweeping motions of their mouthparts. Pollinaria become attached to the upper surface of their heads while they feed on the nectar. Beetles damage the hairs while feeding, which may explain the positive relationship between hair damage and pollination success in plants of S. microrrhynchum from populations where beetles were common. The orchid has cryptic green-yellow flowers with spectral reflectance similar to that of its leaves. The fragrance from plants in three populations, analyzed using gas chromatography coupled to mass spectrometry, was dominated by various terpenoids; linalool was the most abundant. Plants in different populations emitted similar compounds, but eugenol and derivatives of this compound were found in only one of the three populations. In an electrophysiological study (gas chromatography coupled to electroantennography), using antennae of A. tigrina, clear signals were elicited by some of the floral scent compounds.     

Pollination ecotypes of Satyrium hallackii (Orchidaceae) in South Africa

Intraspecific variation in floral traits may reflect adaptive shifts in the pollination system of a plant. This idea was tested by examining the pollination biology of Satyrium hallackii H. Bolus., an orchid which has spurs varying from 8 to 36 mm among populations in southern Africa. Field observations showed that the short-spurred form ( S. hallackii subsp. hallackii ) in coastal fynbos habitats is pollinated by bees, while the long-spurred form ( S. hallackii subsp. ocellatum ) in grassland habitats is pollinated primarily by hawkmoths and, secondarily, by long-tongued flies. The shift between hawkmoths and bees as pollinators may have been promoted by an ecological gradient in South Africa: carpenter bees are common in coastal fynbos habitats, but rare in grassland habitats where there are few available nesting sites. On the other hand, hawkmoths are common in grassland habitats, but rare in fynbos where there are few palatable host-plants. The formation of pollination ecotypes across pollinator gradients has probably been a major factor in the diversification of South African plants with specialized pollination systems.     

Pollination in Lilium sargentiae (Liliaceae) and the first confirmation of long‐tongued hawkmoths as a pollinator niche in Asia

The geographical distribution of pollinators is an important factor determining the biogeographical pattern of floral evolution and diversification. In Africa, a guild of plant species has converged in a floral syndrome for pollination by long‐tongued hawkmoths (predominantly Agrius convolvuli). It is hypothesized that such floral convergence could track the geographical distribution of long‐tongued hawkmoths, so it may not be confined to Africa. We investigated the pollination biology of Lilium sargentiae E. H. Wilson, which is endemic to China and exhibits traits suggestive of long‐tongued hawkmoth pollination. Lilium sargentiae was visited by A. convolvuli as well as small beetles (Nitidulidae) but pollination was mostly or totally effected by the moth. It was consistent with other hawkmoth‐pollinated plants in terms of floral tube length, nectar traits, tepal reflectance, and scent composition. We present the first experimental evidence for the hypothesis proposed above and for hawkmoth pollination in the widespread and ornamentally and economically important genus Lilium L. in a natural habitat. Our findings imply that long‐tongued hawkmoths (especially the extremely widespread A. convolvuli) as a distinctive pollinator niche may have underlain the evolution of long‐tubed (>8 cm) flowers across different continents.     

Pollination of Habenaria tridactylites on the Canary Islands

We investigated the pollination of Habenaria tridactylites, an endemic orchid of the Canary Islands. The entirely green, widely open flowers have a long spur containing nectar. We carried out fieldwork, a molecular clock analysis, herbarium surveys, identified pollinators by both morphology and DNA barcoding, and measured the length of floral spurs and insect tongues using a combination of traditional and innovative micro‐CT scanning methods to 1) determine the pollinator of this orchid and 2) investigate correlations between local mean spur length and age, altitude and longitude of the island. Habenaria tridactylites was found to be pollinated on Tenerife by both small and intermediate sized moth species with variable tongue lengths and mostly belonging to Geometridae and to a lesser extent Crambidae, Erebidae, Noctuidae and Tortricidae. Of the sixteen moth species identified, nine are endemic to the Canary Islands or Macaronesia. The different local populations of H. tridactylites on the islands of Gran Canaria, El Hierro, La Gomera, La Palma and Tenerife with different ages and distances from mainland Africa, did not show a significant correlation of mean spur length and altitude, but did show a significant and positive linear correlation with longitude and the geological age of the island. The latter is congruent with the evolutionary arms race theory first proposed by Darwin, suggesting that flowers gradually evolve longer spurs and pollinators longer tongues.     

The pollination of Habenaria rhodocheila (Orchidaceae) in South China: When butterflies take sides

Habenaria is one of the largest terrestrial genera in the family Orchidaceae. Most field studies on Habenaria species with greenish–white and nocturnal scented flowers are pollinated by nocturnal hawkmoths and settling moths. However, H. rhodocheila presents reddish flowers lacking a detectable scent and fails to fit the moth pollination syndrome. We investigated the pollinators, breeding system, and functional traits of H. rhodocheila in South China and found that two diurnal swallowtail butterflies Papilio helenus and Papilio nephelus (Papilionidae) were the effective pollinators. When butterflies foraged for nectar in the spur, the pollinia became attached between the palpi. A triangular projected median rostellar lobe was found at the entrance (sinus) of the spur of H. rhodocheila. This lobe divided the spur opening into two entrances forcing butterflies to enter their proboscides through the left or right side. When the projection of median rostellar lobe was removed, the site of pollinium attachment changed to the eyes of the butterflies, leading to a higher rate of pollinium removal but lower rate of pollinium deposition. Our quartz glass cylinder choice experiment suggested that visual rather than olfactory cues provided the major stimuli for butterflies to locate these flowers. Hand pollination experiments suggested this species was self‐compatible but pollinator‐dependent. However, the proportion of seeds with large embryos produced in self‐pollinated fruits was significantly lower than in cross‐pollinated fruits, indicating a significant inbreeding depression. Unlike many other orchid species, fruit set was higher than rates of pollinium removal, indicating a high level of pollination efficiency in a species with friable pollinia. Shifts from moth to butterfly pollination in the genus Habenaria parallel other orchid lineages providing insights into the potential for pollinator‐mediated floral trait selection.     

Armament imbalances: match and mismatch in plant-pollinator traits of highly specialized long-spurred orchids

Background

Some species of long-spurred orchids achieve pollination by a close association with long-tongued hawkmoths. Among them, several Habenaria species present specialized mechanisms, where pollination success depends on the attachment of pollinaria onto the heads of hawkmoths with very long proboscises. However, in the Neotropical region such moths are less abundant than their shorter-tongued relatives and are also prone to population fluctuations. Both factors may give rise to differences in pollinator-mediated selection on floral traits through time and space.

Methodology/Principal Findings

We characterized hawkmoth assemblages and estimated phenotypic selection gradients on orchid spur lengths in populations of three South American Habenaria species. We examined the match between hawkmoth proboscis and flower spur lengths to determine whether pollinators may act as selective agents on flower morphology. We found significant directional selection on spur length only in Habenaria gourlieana, where most pollinators had proboscises longer than the mean of orchid spur length.

Conclusions/Significance

Phenotypic selection is dependent on the mutual match between pollinator and flower morphologies. However, our findings indicate that pollinator-mediated selection may vary through time and space according to local variations in pollinator assemblages.     

The Pollinators of the Malagasy Star Orchids Angraecum sesquipedale,A. sororium and A. compactum and the Evolution of Extremely Long Spurs by Pollinator Shift

The pollination process of the extremely long-spurred orchids Angraecum sesquipedale and A. sororium is described and documented here for the first time. The pollinaria and viscidia load was examined in moths captured in central and south Madagascar. Visits to orchids by hawkmoths were rarely observed in the field and were therefore systematically recorded in large flight tents using a night-vision video technique and flashlight photography. Angraecum sesquipedale in Fort Dauphin is pollinated by Xanthopan morgani praedicta and A. sororium on Mt. Angavokely by Coelonia solani. By combining a deep nectar spur of extraordinary length with a protruding labellum functioning as a landing platform, these orchids overcome the moth's stereotypic swing-hovering flight thus enabling full insertion of the long tongue. Angraecum compactum in Forêt d'Ambohitantely is pollinated by both the shorter and longer-tongued forms of Panogena lingens which never swing-hover but is also exploited by X. morgani and C. solani with wastage of pollinaria. The duration of tongue insertion, nectar exploitation and tongue withdrawal were analyzed: legitimate and illegitimate visitors differ in their time budget and approach to the flower. Nectar volume, nectar level and sugar concentration of A. sesquipedale and A. sororium were compared with the nectar requirements of the pollinating hawkmoths. The evolution of very long spurs in these orchids is likely to have involved a series of pollinator shifts. The orchids adapted to different hawkmoth species with increasingly long tongues which primarily evolved to avoid predator attacks during visits to less specialized flowers. This “pollinator shift” model modifies the classical “coevolutionary race” model. The relevance of the taxon Angraecum bosseri Senghas is questioned.     

Mast fruiting in a hawkmoth-pollinated orchid Habenaria glaucifolia: an 8-year survey

Aims The hypothesis of predator satiation has been proposed to explain mast fruiting in various flowering plants. It considers that the simultaneous production of large numbers of seeds by a plant population reduces the risk of seed predation for each individual. Orchids produce huge numbers of seeds per fruit and rarely experience seed predation. It remains unclear which factors may affect fluctuating fruit production in orchids, which generally suffer a widespread pollen limitation. To explore the temporal pattern of fruiting and potential factors related to fluctuation in fruit production, we investigated reproductive success of a long-spurred orchid (Habenaria glaucifolia) in an alpine meadow with thousands of individuals over 8 years.Methods To estimate reproductive success, pollinator observation was conducted by day and at night, and pollinia removal and receipt were recorded in the field population for 8 years. To examine whether fruit set and seed set are pollen limited, we conducted supplementary pollination experiments and compared fruit set, seed set and pollinia movement of open-pollinated flowers from 2011 to 2013. We measured lengths of spurs and pollinator proboscises, and nectar volume and concentration, to identify potential pollinators.Important findings Hawkmoths were seen to be effective pollinators for H. glaucifolia in 3 years, whereas in the remaining 5 years no pollinators were observed, and consequently pollinia were rarely transferred. Numerous pollinia movements were observed in 2012, 2013 and 2014 (pollinia removal: 48, 59 and 85%; pollinia receipt 51, 70 and 80%), and correspondingly fruit set was significantly higher in 2012 and 2013 (59 and 46%) than in 2011 (25%). It was fruit set, rather than seed set, that was pollen limited in this orchid in the 3 years, in that supplementary pollination increased fruit set but did not increase seed set per fruit compared to natural. Three species of hawkmoths had proboscis lengths that matched the spur length of H. glaucifolia. Fruit set in this long spurred orchid depends on the activity of long-tongued hawkmoths, resulting in significant temporal variation in fruit production. Mast fruiting in this alpine orchid could be attributed to a 'sit and wait' strategy, awaiting an abundance of effective pollinators.     

An examination of nectar production in 34 species of Dendrobium indicates that deceptive pollination in the orchids is not popular

Nectar, the most common floral reward, is generally used to determine whether an orchid species involves deceptive pollination. Estimates of the deceptive pollination systems with nectarless flowers have ranged from one quarter to one third of the nearly 30 000 species of orchids. These estimates, however, are biased towards temperate-zone, usually terrestrial, orchids. Here we investigated nectar production and property in 34 epiphytic orchid species of the Southeast Asian genus Dendrobium. Twenty-one species were observed producing nectar. The amount and sugar concentration (in bagged flowers) of 12 species varied from 0.45 to 2.78 μL and from 8.1% to 31.1%. The nectar was sucrose-dominant, typical of bee-pollinated flowers. Reconstruction of phylogenetic relationship indicated that transition of nectar secretion occurred in the genus. Spur length was positively correlated with flower size but species with relatively long spurs tended to produce small volume of nectar. Nectar production was strikingly variable among and within individuals in some species, suggesting that a vital measurement of bagged and fresh flowers is needed. Given that the quantitative measurement of nectar or floral reward in orchid species remains scarce, an estimate of deceptive pollination systems awaits further survey in diverse genera.     

Flower reshaping in the transition to hummingbird pollination in Loasaceae subfam. Loasoideae despite absence of corolla tubes or spurs

Many angiosperm lineages present transitions from bee to hummingbird pollination. The flower design in most of these lineages includes either corolla tubes or nectar spurs, structures that commonly experienced an elongation with the acquisition of hummingbird pollination. It is proposed that this increases the fit between the bird head and flower structures, and isolates or partially blocks bees from the interaction. But can this transition occur if the ancestral flower design lacks tubes or spurs? Here we focus on the transition from bee to hummingbird pollination in the Loasaceae subfamily Loasoideae. Loasoideae flowers have radial corollas with separated petals; therefore, they do not display corolla tubes nor nectar spurs. These flowers also present a whorl of nectar scales and staminodes, unique to the subfamily, which is involved in flower–pollinator fit and in nectar harvesting. To explore flower shape adaptation to hummingbird pollination, we tested for correspondence between pollinators and flower shape in Loasoideae. In order to achieve this, we first compared the evolutionary history of flower phenotype and pollination mode, and then used stochastic character mapping and geometric-morphometric variables in a comparison of alternative evolutionary models. The results of our study suggest that the transition from bee to bird pollination was accompanied by changes in the shape of the staminodial complex, along with the evolution of relatively closed corollas. Moreover, while bird pollination seems to be the end point in the evolution of pollination syndromes in many angiosperm lineages, rodent pollinated flowers probably evolved from ancestral bird pollinated flowers in Loasoideae. Our findings suggest that the evolution of bird pollinated flowers from ancestral bee pollinated flowers does not require the presence of corolla tubes or spurs, and can take place as long as the flower design includes structures participating in flower–pollinator fit.     

Bird pollination in South African species ofSatyrium (Orchidaceae)

Field observations showed that three South African orchid species,Satyrium carneum, S. coriifolium andS. princeps, are pollinated by sunbirds. Foraging sunbirds insert their bills into the labellum chamber of the flowers and suck nectar from the labellum spurs with their tongues. The column overarches the entrance to the labellum and pollinaria become affixed to the upper mandible of the bill. Birds often attempt to remove pollinaria by rubbing their bills against a branch, but are mostly unsuccessful due to the large plate-like viscidia which attach the pollinaria very firmly to the bill. Other modifications for bird pollination in theseSatyrium species may include the red, carmine or orange colour of the flowers, sturdy inflorescence stems used for perching and large amounts of dilute nectar in the spurs. EachSatyrium species was pollinated by several species of sunbird, and despite some differences in flowering time, occasional hybrids occur at a site whereS. carneum andS. coriifolium share the same sunbird pollinators.     

Postpollination nectar reabsorption in the African epiphyte Aerangis verdickii (Orchidaceae)

Nectar reabsorption in flowering plants seems to be a rare phenomenon and to our knowledge it has not been reported previously in the Orchidaceae. In this study we present data that show statistically significant differences in nectar sugar concentrations before and after pollination. Virgin flowers of the African epiphytic orchid Aerangis verdickii showed mean sugar concentrations in the nectar of 14.4 and of 18.3% in 1994 and 1995, respectively (sucrose mass by mass equivalent). Sugar concentration in nectar from naturally pollinated flowers had a mean of 3.2 and of 8.7% for the same period. In an experiment in the wild 60 flowers were protected against nectar theft by the ants. Half of those flowers were hand-pollinated. Control (virgin) flowers had nectar with a mean value of 13.8%, while flowers measured 48 h after pollination had a mean sugar concentration of 4.3%. The mean volume of nectar based on measurements of 23 virgin flowers from nine different plants was 19 uL. We estimated an average energy value for the nectar produced of 684 J per plant per season. At the study site, over 60% of the unmanipulated flowers were robbed of all their nectar by arboreal ants, Polyrachis spp. (Formicinae). The adaptive significance of nectar reabsorption for A. verdickii is probably a function of the environmental stresses to which it has been exposed and the relative costs of nectar production.     

Floral specialization for different pollinators and divergent use of the same pollinator among co‐occurring Impatiens species (Balsaminaceae) from Southeast Asia

Floral variation among closely related species is thought to often reflect differences in pollination systems. Flowers of the large genus Impatiens are characterized by extensive variation in colour, shape and size and in anther and stigma positioning, but studies of their pollination ecology are scarce and most lack a comparative context. Consequently, the function of floral diversity in Impatiens remains enigmatic. This study documents floral variation and pollination of seven co‐occurring Impatiens spp. in the Southeast Asian diversity hotspot. To assess whether floral trait variation reflects specialization for different pollination systems, we tested whether species depend on pollinators for reproduction, identified animals that visit flowers, determined whether these visitors play a role in pollination and quantified and compared key floral traits, including floral dimensions and nectar characteristics. Experimental exclusion of insects decreased fruit and seed set significantly for all species except I. muscicola, which also received almost no visits from animals. Most species received visits from several animals, including bees, birds, butterflies and hawkmoths, only a subset of which were effective pollinators. Impatiens psittacina, I. kerriae, I. racemosa and I. daraneenae were pollinated by bees, primarily Bombus haemorrhoidalis. Impatiens chiangdaoensis and I. santisukii had bimodal pollination systems which combined bee and lepidopteran pollination. Floral traits differed significantly among species with different pollination systems. Autogamous flowers were small and spurless, and did not produce nectar; bee‐pollinated flowers had short spurs and large floral chambers with a wide entrance; and bimodally bee‐ and lepidopteran‐pollinated species had long spurs and a small floral chamber with a narrow entrance. Nectar‐producing species with different pollination systems did not differ in nectar volume and sugar concentration. Despite the high frequency of bee pollination in co‐occurring species, individuals with a morphology suggestive of hybrid origin were rare. Variation in floral architecture, including various forms of corolla asymmetry, facilitates distinct, species‐specific pollen‐placement on visiting bees. Our results show that floral morphological diversity among Impatiens spp. is associated with both differences in functional pollinator groups and divergent use of the same pollinator. Non‐homologous mechanisms of floral asymmetry are consistent with repeated independent evolution, suggesting that competitive interactions among species with the same pollination system have been an important driver of floral variation among Impatiens spp.     

Long Tongues and Loose Niches: Evolution of Euglossine Bees and Their Nectar Flowers1

I examined relationships between tongue length of orchid bees (Apidae: Euglossini) and nectar spur length of their flowers in the genera Calathea, Costus, and Dimerocostus using phylogenetically independent contrasts. Long‐tubed flowers have specialized on one or several species of long‐tongued euglossine bees, but long‐tongued bees have not specialized on long‐tubed flowers. Whereas long tongues may have evolved to provide access to a wider variety of nectar resources, long nectar spurs may be a mechanism for flowers to conserve nectar resources while remaining attractive to traplining bee visitors.     

Pollinators and nectar robbers cause directional selection for large spur circle in Impatiens oxyanthera (Balsaminaceae)

Nectar spurs have an important role in floral evolution and plant–pollinator coadaptation. The flowers of some species possess spurs curving into a circle. However, it is unclear whether spur circle diameter is under direct selection pressure from different sources, such as pollinators and nectar robbers. In this study, we quantified selection on some floral traits, such as spur circle diameter in Impatiens oxyanthera (Balsaminaceae) using phenotypic selection analysis and compared the relative importance of pollinators and nectar robbers as selective agents using mediation analysis. The study showed that pollinators caused significant selection on corolla length, spur curvature and spur circle diameter while nectar robbers only imposed strong selection on spur circle diameter. Pollinators favored flowers with large corolla, curly spurs and large spur circle while nectar robbers preferred flowers with small spur circle. More pollinator visits resulted in higher female reproductive success, while robbery reduced female fitness. Conflicting selection on spur traits from pollinators and nectar robbers was not found. Mediation analysis showed that selection on floral traits through nectar robbing was stronger than selection through pollination. The results suggested that pollinators and nectar robbers jointly mediated the directional selection for large spur circle, and nectar robbers caused stronger selection than pollinators on floral traits.