Pollination partners of Mucuna macrocarpa (Fabaceae) at the northern limit of its range

Mucuna macrocarpa is a plant found in tropical and subtropical regions that requires an “explosive opening.” Explosive opening is the process that exposes the stamen and pistil from the opening of the carina. This process is needed for cross pollination; however, the plant cannot open itself and opening by an animal is needed. The most common opener of Mucuna flowers is several nectar‐eating bats (e.g., Syconycteris), but the flying fox, Pteropus dasymallus, is the only opener of M. macrocarpa on the subtropical island of Okinawajima. Here, we present the explosive openers and possible pollinators in the northernmost and temperate Kamae region, Kyushu, Japan, where nectar‐eating bats are absent. The Japanese macaque, Macaca fuscata, and the Japanese marten, Martes melampus, were the explosive openers observed during our survey in Kamae. Martens opened flowers using their snout in a manner similar to that of the flying fox, whereas macaques opened flowers using their hands. This is the first time that an animal has been observed opening these flowers with its hands rather than snout. In total, 97% (n = 283) of explosively opened flowers were opened by macaques, and the macaque largely contributed to the overall flower opening. Because many pollen grains become attached to the explosive openers, they are considered to be primary pollinators. Furthermore, two bee species, Apis cerana japonica and Bombus ardens ardens, also visited opened flowers and collected pollen, and they were possibly secondary pollinators.     

Effects of trophic morphology and behavior on foraging rates of three Hawaiian honeycreepers

Summary The effects of trophic morphology and behavior of three Hawaiian honeycreepers (Fringillidae: Drepanidinae) upon their foraging rates on the flowers of Vaccinium calycinum are examined. The Maui Creeper (Paroreomyza montana), the Amakihi (Hemianathus virens), and the I'iwi (Vestiaria coccinea) show shifts from a short straight bill to a long decurved bill, from a tongue adapted for insect feeding to one specialized for nectar, and diets ranging from primarily probing for insects to primarily nectarivorous. This diversity is examined feeding on the simple straight tubular corollas of the Vaccinium. Significant differences (P<0.001, t-tests) exist among the birds with respect to foraging rates on these flowers. The I'iwi, with its greatly decurved (64°) bill feeds the quickest (2.09 s/flower) while the Maui Creeper with its rather straight (18°) bill feeds the slowest (3.87 s/flower). These differences are seen to be the result of differing abilities of tongues to extract nectar as well as differing behavioral tactics of floral exploration and nectar extraction. This suggests that predictions of foraging efficiency based solely on bill morphology are not necessarily valid, and that other factors (tongue morphology, foraging maneuvers, and typical food spectrum) must also be considered.     

Nectar secretion pattern of the dish-shaped flower,Cayratia japonica (Vitaceae), and nectar utilization patterns by insect visitors

We studied the relationship between the diurnal nectar secretion pattern of flowers of Cayratia japonica and insect visiting patterns to these flowers. Flower morphology of C. japonica changed greatly for about 12 hours after flower-opening and the maximum duration of nectar secretion was 2 days. The nectar volume peaked at 11∶00 and 15∶00, and declined at night and at 13∶00 regardless of time elapsed after flower-opening. The nectar volume at the two peaks was, on average, 0.25 μl on bagged inflorescences and 0.1μl on unbagged inflorescences (both, sugar concentration=60%). The flower secreted nectar compensatory when the nectar was removed. This means that insects consume more nectar than the difference of nectar volume between bagged and unbagged flowers. Apis cerana is a primary visitor of this flower, and was the only species for which we confirmed pollen on the body, among many species of flower visiting insects to this flower. Apis cerana visited intensively at the two peaks of nectar secretion. Visits of the other insects were rather constant or intensive only when there was no nectar secretion. Thus flowers of C. japonica with morphologically unprotected nectaries may increase likelihood that their nectar is used by certain pollinators, by controlling the nectar secretion time in day. In this study the pattern of nectar secretion allowed A. cerana maximum harvest of nectar.     

Choice of flowers by foraging honey bees (Apis mellifera): possible morphological cues

Abstract.

• 1 Honey bees foraging for nectar on lavender (Lavandula stoechas) chose inflorescences with more of their flowers open. The number of open flowers predicted whether an inflorescence was visited by bees, inspected but rejected, or ignored. Inflorescences chosen arbitrarily by observers had numbers of open flowers intermediate between those of visited and ignored inflorescences.
• 2 Differences in morphological characters between types of inflorescence correlated with nectar volume and sugar weight per flower so that visited inflorescences had a disproportionately greater volume of nectar and weight of sugar per flower and greater variance in nectar volume.
• 3 Although there were significant associations between nectar content and the morphological characters of inflorescences, discriminant function analysis revealed discrimination on the basis of morphology rather than nectar content.
• 4 Visited inflorescences tended to have smaller than average flowers but bees tended to probe the largest flowers on visited inflorescences.
• 5 Choice of flowers within inflorescences is explicable in terms of the relationship between flower size and nectar content.

     

Rat- and bat-pollination of Mucuna championii (Fabaceae) in Hong Kong

Mucuna (Fabaceae) species possess gullet-type flowers that open explosively and which are thought to be specifically adapted for bat- or bird-pollination. However, recent studies have shown that non-flying mammals are also important pollinators of this genus in Asia. Here we report on the pollination system of Mucuna championii (endemic in southeast China) in Hong Kong. As is typical for the genus, explosive opening is essential for fruit set, but flowers are unable to open in the absence of manipulation by an effective pollinator. Camera trap surveys of three individuals revealed both chestnut spiny rats (Niviventer fulvescens) and short-nosed fruit bats (Cynopterus sphinx) to be capable of triggering explosive opening. The number of flowers opened by each species did not differ significantly, and both removed most pollen grains from the flowers they visited, but either species visited different individuals. Sucrose-rich nectar was secreted by flowers throughout the day. Our results reveal that M. championii can be pollinated by both rats and bats, with this representing only the second report of rat-pollination in tropical Asia. The sympatric M. birdwoodiana often occurs in close proximity to M. championii and has an overlapping flowering season, suggesting that pollinator segregation may have played a role in shaping the evolutionary ecology of these two species.     

Why do nectar-foraging bees and wasps work upwards on inflorescences?

Summary Wasps (Dolichovespula and Vespula spp.) worked predominantly upwards when foraging for nectar on inflorescences of the protogynous Scrophularia aquatica, in which the standing crop of nectar sugar per flower showed no clear pattern of vertical distribution up an inflorescence. Bumblebees taking nectar (Bombus hortorum visiting legally, and certain individuals of B. terrestris which positioned themselves head-upwards while taking nectar through holes bitten in the corolla) worked predominantly upwards on the racemose inflorescences of Linaria vulgaris, although the standing crop of nectar sugar per open flower increased up the raceme. Individuals of B. terrestris which robbed Linaria flowers in a head-down position worked predominantly downwards on inflorescences. The upward or downward directionality of intra-inflorescence movements by foraging insects may depend in part on the position these adopt during their flower visits.     

Flowering patterns of long-lived Heliconia inflorescences: implications for visiting and resident nectarivores

Summary Flowering patterns of four Heliconia (Heliconiaceae) species in Trinidad, West Indies were examined for their predictability and availability to the nectarivores that rely on Heliconia floral nectar. Principal flower visitors are trapling hermit hummingbirds; inflorescences are inhabited by nectarivorous hummingbird flower mites that move between inflorescences by riding in the hummingbirds' nares. Heliconia inflorescences flower for 40–200 days, providing long-term sources of copious nectar (30–60 l per flower), but each Heliconia flower lasts only a single day. As an inflorescence ages the interval increases between open flowers within a bract; wet-season inflorescences produce open flowers more slowly than dry-season conspecifics.Estimated daily energy expenditures for hermit hummingbirds demonstrate that slow production of short-lived open flowers plus low inflorescence density preclude territorial defense of Heliconia by the hermits. Heliconia flowering patterns are viewed as a means of (i) regulating reproductive investment by the plants through staggered flower production over long periods of time, and (ii) maintaining outcrossing by necessitating a traplining visitation pattern by its hummingbird pollinators. I suggest that Heliconia exhibit a two-tiered pollination system by using hermit hummingbirds primarily for outcrossing and using hummingbird flower mites primarily for self-pollination.     

The exploitation of floral nectar in Eucalyptus incrassata by honeyeaters and honeybees

Summary During October and November, 1977, a study of nectar production and nectarivore foraging in Eucalyptus incrassata was conducted at Wyperfeld National Park in south-eastern Australia in order to evaluate the extent to which introduced honeybees (Apis mellifera) compete with native honeyeaters for floral nectar. Data on nectar production, nectar availability, ambient air temperature and the numbers of visiting honeyeaters and honeybees were collected. Most of the daily nectar production in E. incrassata occurs early in the morning when temperatures are too low for insects to forage. In addition, insects, particularly honeybees, are unable to exploit nectar in the youngest flowers because the stamens are clustered tightly around the style. As a result of these temporal and structural characteristics of the flowers, honeyeaters are able to harvest most of the nectar. Honeybees potentially have access to 35–47% of the average daily production of floral nectar in E. incrassata and actually harvest considerably less. These data show that E. incrassata flowers are adapted to restrict insect foragers despite their superficially unspecialized appearance. Eight forest and woodland eucalypts do not have a flower stage which excludes insects and the significance of this difference is discussed.     

Nectar secretion dynamic links pollinator behavior to consequences for plant reproductive success in the ornithophilous mistletoe Psittacanthus robustus

The mistletoe Psittacanthus robustus was studied as a model to link flower phenology and nectar secretion strategy to pollinator behaviour and the reproductive consequences for the plant. The bright‐coloured flowers presented diurnal anthesis, opened asynchronously throughout the rainy season and produced copious dilute nectar as the main reward for pollinators. Most nectar was secreted just after flower opening, with little sugar replenishment after experimental removals. During the second day of anthesis in bagged flowers, the flowers quickly reabsorbed the offered nectar. Low values of nectar standing crop recorded in open flowers can be linked with high visitation rates by bird pollinators. Eight hummingbirds and two passerines were observed as potential pollinators. The most frequent flower visitors were the hummingbirds Eupetomena macroura and Colibri serrirostris, which actively defended flowering mistletoes. The spatial separation between anthers, stigma and nectar chamber promotes pollen deposition on flapping wings of hovering hummingbirds that usually probe many flowers per visit. Seed set did not differ between hand‐, self‐ and cross‐pollinated flowers, but these treatments set significantly more seeds than flowers naturally exposed to flower visitors. We suggest that the limitation observed in the reproductive success of this plant is not related to pollinator scarcity, but probably to the extreme frequency of visitation by territorial hummingbirds. We conclude that the costs and benefits of plant reproduction depend on the interaction strength between flowers and pollinators, and the assessment of nectar secretion dynamics, pollinator behaviour and plant breeding system allows clarification of the complexity of such associations.     

Ineffectiveness of nectar scent in generating bumblebee visits to flowers of Impatiens textori

To clarify if bumblebees can recognize nectar through its scent in Impatiens textori flowers, we examined the behavior of Bombus diversus on nectarless flowers in which the spurs had been artificially removed. Bumblebee visits to both natural flowers and spur‐cut flowers were captured using a long‐term video recording system. Visiting behavior and frequency were compared between the two flower types. Many bumblebees visited both types of flower, and their visit frequencies were not significantly different. However, the length of stay on each flower type did differ, with the bumblebees remaining on the spur‐cut flowers for a significantly shorter time than on the natural flowers. Our results suggest that bumblebees cannot detect the absence of nectar in I. textori flowers before probing them. Therefore, the nectar scent of I. textori does not serve to attract bumblebees although the presence of nectar will detain bumblebees on flowers for longer periods.     

The sweet jelly of Combretum lanceolatum flowers (Combretaceae): a cornucopia resource for bird pollinators in the Pantanal, western Brazil

 The pollination biology of the neotropical scandent shrub Combretum lanceolatum was studied in the seasonally-flooded Pantanal region in western Brazil. This plant bears horizontally oriented inflorescences, whose yellowish green flowers begin to expand at dusk and are fully open at dawn. Instead of fluid nectar the flowers produce sweet gelatinous secretion in form of pellets. The glandular complex of the flower is composed of the inner wall of the receptacle and its tubular extension, being equivalent to the nectariferous disk of the nectar-producing species within the genus. The jelly is produced at night, contains mannan and is imbibed by free hexoses. It originates by swelling and disintegration of the inner wall, after contact with the nectar generated concomitantly in the mesophyll. Combretum lanceolatum is unique within the genus in its production of jelly pellets instead of liquid nectar. A new term, the jelly-flower, is proposed for flowers with this kind of reward. The pellet is not replaced once removed by a bird, and thus resembles a fruit in its availability to consumers, another unique feature that distinguishes this species within the genus. The jelly pellets offered by the many flowered branches attract a great diversity of bird visitors (28 species from eight families), which feed on this copious food resource and pollinate the flowers. The most effective pollinators probably are thrushes, tanagers, and orioles. Flocking parakeets and macaws sometimes feed on the petals, thus acting as flower plunderers. Combretum lanceolatum presents a high fruit set under natural conditions, which likely favours its spreading and becoming a weed species. Received July 11, 2000 Accepted November 18, 2000     

Intra‐floral resource partitioning between endemic and invasive flower visitors: consequences for pollinator effectiveness

1. Sympatric flower visitor species often partition nectar and pollen and thus affect each other's foraging pattern. Consequently, their pollination service may also be influenced by the presence of other flower visiting species. Ants are solely interested in nectar and frequent flower visitors of some plant species but usually provide no pollination service. Obligate flower visitors such as bees depend on both nectar and pollen and are often more effective pollinators. 2. In Hawaii, we studied the complex interactions between flowers of the endemic tree Metrosideros polymorpha (Myrtaceae) and both, endemic and introduced flower‐visiting insects. The former main‐pollinators of M. polymorpha were birds, which, however, became rare. We evaluated the pollinator effectiveness of endemic and invasive bees and whether it is affected by the type of resource collected and the presence of ants on flowers. 3. Ants were dominant nectar‐consumers that mostly depleted the nectar of visited inflorescences. Accordingly, the visitation frequency, duration, and consequently the pollinator effectiveness of nectar‐foraging honeybees (Apis mellifera) strongly decreased on ant‐visited flowers, whereas pollen‐collecting bees remained largely unaffected by ants. Overall, endemic bees (Hylaeus spp.) were ineffective pollinators. 4. The average net effect of ants on pollination of M. polymorpha was neutral, corresponding to a similar fruit set of ant‐visited and ant‐free inflorescences. 5. Our results suggest that invasive social hymenopterans that often have negative impacts on the Hawaiian flora and fauna may occasionally provide neutral (ants) or even beneficial net effects (honeybees), especially in the absence of native birds.     

Departure rules used by bees foraging for nectar: A field test

Summary Departure rules used by solitary long-tongued bees (Anthophora spp. andEucera spp.) collecting nectar from flowers ofAnchusa strigosa (Boraginaceae) were studied. The amount of nectar a bee receives from an individual flower was estimated by measuring the time elapsed since the previous bee visit to that flower. Measurements of nectar accumulation in experimentally emptied flowers indicated that this time interval is an accurate predictor of nectar volumes in flowers. We found that nectar rewards influence the probability of departure from individual plants, as well as distances of movements within plants. The probability of departure from individual plants was negatively related to the amount of reward received at the two lastvisited flowers. This result indicates that the bees used a probabllistic departure rule, rather than a simple threshold departure rule, and that rewards from both the current and the previously visited flower were important in determining departure points. Distances of inter-flower movements within plants were negatively related to the amount of reward received at the current flower. The overall results suggest that the pollinators ofA. strigosa make two types of departure decisions-departures from the whole plant and departures from the neighbourhood of individual flowers-and that they use different departure rules for each scale. Factors influencing the decision-making processes of the observed foraging behaviour are discussed.     

A preliminary study of nectar production of the field cress,Rorippa indica,in relation to the age of its flowers

Floral nectar production quantity and changes in sugar concentration of the nectar ofRorippa indica were investigated. Flowers were sorted by age. The small flower size was a major factor for the use of BCG test paper in nectar sampling. In flowers of the youngest age category, the nectar volume increased toward the night and decreased during the day. The amount of sugar in the nectar decreased with flower age. The sugar concentration was lower at night than daytime. The variability and changes in nectar concentration enabled various species of insects to forage for nectar.     

REPRODUCTIVE BIOLOGY,POLLEN AND SEED DISPERSAL,AND NEIGHBORHOOD SIZE IN THE HUMMINGBIRD-POLLINATED ECHEVERIA GIBBIFLORA (CRASSULACEAE)

Little is known about the reproductive biology of the Crassulaceae. We studied a population of Echeveria gibbiflora in the Pedregal de San Angel ecological preserve in Mexico City, Mexico. Each flower is open and producing nectar 7 to 8 days. On the days of maximum nectar production (flowers 4–6 days old) an average of 14.5 μl accumulates in a flower per day. The maximum rate of nectar production is between 0700 and 0900 hours. The average sugar concentration in the nectar is 43.7%. In a given flower, pollen is exposed and the stigmas are receptive at the same time. The average natural fruit-set and seed-set are 56.6% and 35.5%, respectively. The pollen-ovule ratio is 124, and the plants are fully self-compatible. The flowers are pollinated by only one species of hummingbird (Cynanthus latirostris) and are never visited by insects. Pollen movement is very limited (mean of pollinator flight distances = 0.72 m, mean distance fluorescent dyes = 0.92 m). Seed dispersion is by gravity and wind, and also is very limited (an average of 1.07 m). The total genetic neighborhood area is 15 m² to 17 m², producing a neighborhood effective population size (N_b) of 5.01 to 39.7 individuals. This is a very small N_b, indicating that genetic drift may be a dominant force in the evolution of this species.     

Nectar robbers deter legitimate pollinators by mutilating flowers

Nectar robbing – harvesting nectar illegitimately – can have a variety of outcomes for plant sexual reproduction and for the pollinator community. Nectar robbers can damage flowers while robbing nectar, which could affect the behavior of subsequent flower visitors and, consequently, plant reproduction. However, only nectar manipulation by nectar robbers has so far received attention. We found a short-tongued bee, Hoplonomia sp. (Halictidae), mutilating the conspicuous lower petal of the zygomorphic flowers of Leucas aspera (Lamiaceae) while robbing nectar. We hypothesized that the mutilation of the conspicuous lower petal deters legitimate pollinators on L. aspera flowers, which, in turn, might affect plant reproduction. We first assessed the proportion of naturally-robbed flowers in plant populations for three years to confirm that it was not a purely local phenomenon due to a few individual bees. We then studied diversity, community and visitation characteristics of pollinators, nectar dynamics and fruit set in unrobbed and robbed open flowers in naturally-robbed populations. The proportion of robbed flowers varied significantly across sites and years. Robbing did not affect nectar dynamics in flowers, but it did alter flower morphology, so much so that it reduced pollinator visitation and altered the pollinator community on robbed flowers. However, the maternal function of plant reproduction was not affected by nectar robbing. This study for the first time shows that a nectar robber can have an ecologically significant impact on floral morphology.     

Spatial distribution of nectar production in a natural Echium vulgare population: Implications for pollinator behaviour

The distribution of trait values in many populations is not homogenous but creates a mosaic of patches. This may lead to differences in selection on the patch level compared to selection on the population level. As an example we investigated the spatial distribution of nectar production and its effects on pollinator behaviour in a natural population of Echium vulgare. Nectar production per flower, number of flowers and total nectar production showed a hierarchy and spatial aggregation as expressed by Gini coefficients and significant Moran's I values. Plants in patches of high nectar production received significantly more pollinator visits and had a significant emanating effect on pollinator visits of neighbouring plants. The same was true for plants in patches with high number of flowers. To disentangle these effects a path analysis was applied, which suggested that the direct effect of nectar production per flower although present, seems to be small compared to the effect of the number of flowers. Nectar production per flower affected pollinator visits mainly indirectly by way of total nectar production, which includes the effect of number of flowers. Assuming a minor pollinator-mediated selection for number of flowers, pollinator-mediated selection for total nectar production equals that for nectar production per flower. If so, the observed spatial structure of nectar production and its emanating effect on pollinator behaviour is of importance for natural selection. Plants of low nectar production occurring close to patches of plants with high nectar production benefited from the enhanced pollinator service of their neighbours while saving costs of increased nectar production. Consequently, plants with low nectar production may have a selective advantage at patch level while plants with high nectar production may have a selective advantage at population level. Results presented stress the importance of small-scale patterns for ecological relationships and evolutionary change.     

Visitation rate of pollinators and nectar robbers to the flowers and inflorescences of Tabebuia aurea (Bignoniaceae): effects of floral display size and habitat fragmentation

Large floral displays favour pollinator attraction and the import and export of pollen. However, large floral displays also have negative effects, such as increased geitonogamy, pollen discounting and nectar/pollen robber attraction. The size of the floral display can be measured at different scales (e.g. the flower, inflorescence or entire plant) and variations in one of these scales may affect the behaviour of flower visitors in different ways. Moreover, the fragmentation of natural forests may affect flower visitation rates and flower visitor behaviour. In the present study, video recordings of the inflorescences of a tree species (Tabebuia aurea) from the tropical savannah of central Brazil were used to examine the effect of floral display size at the inflorescence and tree scales on the visitation rate of pollinators and nectar robbers to the inflorescence, the number of flowers approached per visit, the number of visits per flower of potential pollinators and nectar robbers, and the interaction of these variables with the degree of landscape disturbance. Nectar production was quantified with respect to flower age. Although large bees are responsible for most of the pollination, a great diversity of flower insects visit the inflorescences of T. aurea. Other bee and hummingbird species are highly active nectar robbers. Increases in inflorescence size increase the visitation rate of pollinators to inflorescences, whereas increases in the number of inflorescences on the tree decrease visitation rates to inflorescences and flowers. This effect has been strongly correlated with urban environments in which trees with the largest floral displays are observed. Pollinating bees (and nectar robbers) visit few flowers per inflorescence and concentrate visits to a fraction of available flowers, generating an overdispersed distribution of the number of visits per inflorescence and per flower. This behaviour reflects preferential visits to young flowers (including flower buds) with a greater nectar supply.     

How specialized is the plant–pollinator association between Eucalyptus globulus ssp. globulus and the swift parrot Lathamus discolor?

Abstract The swift parrot Lathamus discolor (Shaw) (Psittacidae) evolved from granivorous ancestors to become a specialized flower‐feeder in a monotypic genus. Its reproduction is dependent largely on flowers of Eucalyptus globulus Labill. ssp. globulus (Myrtaceae), the birds migrating to breed within the natural distribution of this tree. This paper investigates the extent to which this dependence of L. discolor on E. globulus is mirrored by dependence of the tree on the bird. It was found that L. discolor carried significantly more eucalypt pollen within 22 mm of its bill tip than did the New Holland honeyeater, Phylidonyris novaehollandiae (Latham) (Meliphagidae), and that pollen was concentrated on the regions of the head of L. discolor that consistently contact stigmas. Larger pollen loads on L. discolor can be attributed to it consuming both pollen and nectar, while honeyeaters take nectar only. The short thick bill of L. discolor necessitates regular stigmatic contact while the long slender bills of honeyeaters are unlikely to contact stigmas as often in these bowl‐shaped flowers. These factors suggest that L. discolor has a greater capacity to deposit pollen on stigmas of E. globulus than do honeyeaters. However, the characteristics of L. discolor that make it such an effective pollinator of E. globulus are also exhibited by a lorikeet (Psittacidae) that feeds on flowers of E. globulus. The association between E. globulus and L. discolor is therefore only moderately specialized because the flowers are also adapted to the more recently associated lorikeet and are almost certainly also pollinated by honeyeaters.     

Pollination and breeding system of Eritrichium nanum (Boraginaceae)

 Flower development, pollination and breeding system of the high alpine cushion plant, Eritrichium nanum (Boraginaceae), were investigated in nine populations from the European Alps at altitudes of 2700 m–3200 m. Peak flowering period lasts longer than a month, from mid-June to the end of July. In contrast to statements in the literature that flowers are protogynous and nutlets remain in their calyx until spring we found a distinct protandry and nutlets being dispersed before mid-September. Various insects from 12 families, but mostly Diptera, frequently visited E. nanum flowers, with flies from the families Anthomyiidae and Muscidae being the predominant visitors. Under optimal conditions (max. solar radiation, min. wind force), visitation rates of 200 simultaneously observed flowers reached 32.5–46.7 insects per hour, i.e. 0.16–0.24 insects per flower per hour. However, the commonly observed Anthomyiidae and Muscidae clearly preferred the white-yellowish flowers of Saxifraga exarata and Saxifraga bryoides which are abundant at E. nanum sites and which are certainly also pollinated by species of these two fly families. The flowers of these Saxifraga species offer plenty of nectar and may compete for pollinators with E. nanum, when they are flowering in its proximity. However, various other insects like Pontia callidice and Psodos sp. (Lepidoptera) as well as Andrena sp. (Hymenoptera) and especially Eristalis tenax and closely related hoverflies showed a higher degree of flower constancy to E. nanum, often flying from a blue Eritrichium cushion to the next and hence causing outcrossing. The five fornices of E. nanum flowers which obstruct the tube containing stamens and nectar, are a feature which differs distinctly from the syndrome of fly-pollinated flowers with easily accessible nectar. Consequently pollination by flies in E. nanum seems to be caused mainly by the unfavorable ecological conditions at high altitudes, where flies are the most frequent insects. Bagging experiments showed that outcrossing and geitonogamy are the prevailing pollination modes, and autogamy, although possible, plays only a minor role. Received February 13, 2001 Accepted November 23, 2001