Cuticular Hydrocarbon Profiles of Himalayan Bumble Bees (Hymenoptera: Bombus Latreille) are Species-Specific and Show Elevational Variation.

Bumble bees are important pollinators in natural environments and agricultural farmlands, and they are in particular adapted to harsh environments like high mountain habitats. In these environments, animals are exposed to low temperature and face the risk of desiccation. The Eastern Himalayas are one of the recognized biodiversity hotspots worldwide. The area covers subtropical rainforest with warm temperature and high precipitation as well as high mountain ranges with peaks reaching up to 7,000 m, shaping a diverse floral and faunal community at the different elevational zones. To identify possible adaptation strategies, we investigated the cuticular hydrocarbon profiles of four bumble bee species occurring at different elevational ranges in Arunachal Pradesh, the northeastern most state in India. At 17 locations along an elevational gradient, we collected workers of two species from lower elevations (B. albopleuralis and B. breviceps; ~ 100 m - 3,000 m asl) and two species from higher elevations (B. prshewalskyi and B. mirus; ~ 2,800 m - 4,500 m asl). The CHC profiles of all four species showed a significant degree of variation in the composition of hydrocarbons, indicating species specificity. We also found clear correlation with elevation. The weighted mean chain length of the hydrocarbons significantly differed between the low and high elevation species, and the proportion of saturated hydrocarbons in CHC profiles significantly increased with the elevational range of the bumble bee species. Our results indicate that bumble bees living at high elevations reduce the risk of water loss by adapting their CHC composition on their cuticle, a phenomenon that has also been found in other insects like ants and fruit flies.

Color preference and spatial distribution of glaphyrid beetles suggest a key role in the maintenance of the color polymorphism in the peacock anemone (Anemone pavonina, Ranunculaceae) in Northern Greece.

In the Mediterranean region, a group of unrelated plant species share an unusual deep-red flower color and are pollinated by glaphyrid beetles. Some of these species possess different color morphs, but the mechanisms maintaining this color polymorphism are unknown. The peacock anemone, Anemone pavonina, is a color polymorphic species with red or purple flowers. We investigated the spatial distribution of its color morphs and its potential glaphyrid pollinators, Pygopleurus spp., along an elevational gradient on the southern slopes of Mount Olympus, Greece. We found a correlation between relative proportions of the two color morphs with both elevation and beetle abundance. At low elevations (< 1000 m a.s.l.), beetles were abundant and anemone populations comprised only red flowers. Above a steep transition zone with mixed-colored populations (c. 1000-1300 m) most flowers were purple and beetles were rare. Color-trapping experiments revealed a strong preference for red over other colors in beetles and colorimetric modeling suggests that a simple chromatic mechanism is sufficient to explain their color choices. We thus hypothesize that beetles select for red flowers and that with increasing elevation and decreasing beetle density, other flower visitors (e.g., bees) gain importance as pollinators and select for a different color.

Species composition and elevational distribution of bumble bees (Hymenoptera, Apidae, Bombus Latreille) in the East Himalaya, Arunachal Pradesh, India.

The East Himalaya is one of the world's most biodiverse ecosystems. However, very little is known about the abundance and distribution of many plant and animal taxa in this region. Bumble bees are a group of cold-adapted and high elevation insects that fulfil an important ecological and economical function as pollinators of wild and agricultural flowering plants and crops. The Himalayan mountain range provides ample suitable habitats for bumble bees. Systematic study of Himalayan bumble bees began a few decades ago and the main focus has centred on the western region, while the eastern part of the mountain range has received little attention and only a few species have been verified. During a three-year survey, more than 700 bumble bee specimens of 21 species were collected in Arunachal Pradesh, the largest of the north-eastern states of India. The material included a range of species that were previously known from a limited number of collected specimens, which highlights the unique character of the East Himalayan ecosystem. Our results are an important first step towards a future assessment of species distribution, threat, and conservation. Clear elevation patterns of species diversity were observed, which raise important questions about the functional adaptations that allow bumble bees to thrive in this particularly moist region in the East Himalaya.

Body size limits dim-light foraging activity in stingless bees (Apidae: Meliponini).

Stingless bees constitute a species-rich tribe of tropical and subtropical eusocial Apidae that act as important pollinators for flowering plants. Many foraging tasks rely on vision, e.g. spatial orientation and detection of food sources and nest entrances. Meliponini workers are usually small, which sets limits on eye morphology and thus quality of vision. Limitations are expected both on acuity, and thus on the ability to detect objects from a distance, as well as on sensitivity, and thus on the foraging time window at dusk and dawn. In this study, we determined light intensity thresholds for flight under dim light conditions in eight stingless bee species in relation to body size in a Neotropical lowland rainforest. Species varied in body size (0.8-1.7 mm thorax-width), and we found a strong negative correlation with light intensity thresholds (0.1-79 lx). Further, we measured eye size, ocelli diameter, ommatidia number, and facet diameter. All parameters significantly correlated with body size. A disproportionately low light intensity threshold in the minute Trigonisca pipioli, together with a large eye parameter P eye suggests specific adaptations to circumvent the optical constraints imposed by the small body size. We discuss the implications of body size in bees on foraging behavior.

Flower detection and acuity of the Australian native stingless bee Tetragonula carbonaria Sm.

We tested the endemic Australian Tetragonula carbonaria bee as a model of how colour vision may allow these small bees to find flowers. In a Y-Maze apparatus, we presented stimuli that contained both chromatic- and green-receptor contrasts, or only had chromatic contrast to free flying bees. Stimuli were detected at visual angles of 9.5° and 9.3°, respectively. We next made morphological measurements of the compound eye under high magnification using a digital microscope, and despite a relatively small eye size with a surface area of 0.64 ± 0.02 mm(2), the compound eye contained 3010 ± 10 ommatidia. Measurements of diverging rays of light using antidromic illumination revealed a mean interommatidial angle in the frontal visual field measures 1.56° ± 0.10°. Finally, we calculate that the minimum number of ommatidia that need to be excited for object detection is 33, which is much higher than for object detection in bumblebees and for the detection of objects providing both colour and green contrasts by honeybees, but lower for the detection of an object lacking green contrast in honeybees. We discuss reasons that may explain potential tradeoff for foraging bees.

Functional Significance of Labellum Pattern Variation in a Sexually Deceptive Orchid (Ophrys heldreichii): Evidence of Individual Signature Learning Effects.

Mimicking female insects to attract male pollinators is an important strategy in sexually deceptive orchids of the genus Ophrys, and some species possess flowers with conspicuous labellum patterns. The function of the variation of the patterns remains unresolved, with suggestions that these enhance pollinator communication. We investigated the possible function of the labellum pattern in Ophrys heldreichii, an orchid species in which the conspicuous and complex labellum pattern contrasts with a dark background. The orchid is pollinated exclusively by males of the solitary bee, Eucera berlandi. Comparisons of labellum patterns revealed that patterns within inflorescences are more similar than those of other conspecific plants. Field observations showed that the males approach at a great speed and directly land on flowers, but after an unsuccessful copulation attempt, bees hover close and visually scan the labellum pattern for up to a minute. Learning experiments conducted with honeybees as an accessible model of bee vision demonstrated that labellum patterns of different plants can be reliably learnt; in contrast, patterns of flowers from the same inflorescence could not be discriminated. These results support the hypothesis that variable labellum patterns in O. heldreichii are involved in flower-pollinator communication which would likely help these plants to avoid geitonogamy.

Sex and caste-specific variation in compound eye morphology of five honeybee species.

Ranging from dwarfs to giants, the species of honeybees show remarkable differences in body size that have placed evolutionary constrains on the size of sensory organs and the brain. Colonies comprise three adult phenotypes, drones and two female castes, the reproductive queen and sterile workers. The phenotypes differ with respect to tasks and thus selection pressures which additionally constrain the shape of sensory systems. In a first step to explore the variability and interaction between species size-limitations and sex and caste-specific selection pressures in sensory and neural structures in honeybees, we compared eye size, ommatidia number and distribution of facet lens diameters in drones, queens and workers of five species (Apis andreniformis, A. florea, A. dorsata, A. mellifera, A. cerana). In these species, male and female eyes show a consistent sex-specific organization with respect to eye size and regional specialization of facet diameters. Drones possess distinctly enlarged eyes with large dorsal facets. Aside from these general patterns, we found signs of unique adaptations in eyes of A. florea and A. dorsata drones. In both species, drone eyes are disproportionately enlarged. In A. dorsata the increased eye size results from enlarged facets, a likely adaptation to crepuscular mating flights. In contrast, the relative enlargement of A. florea drone eyes results from an increase in ommatidia number, suggesting strong selection for high spatial resolution. Comparison of eye morphology and published mating flight times indicates a correlation between overall light sensitivity and species-specific mating flight times. The correlation suggests an important role of ambient light intensities in the regulation of species-specific mating flight times and the evolution of the visual system. Our study further deepens insights into visual adaptations within the genus Apis and opens up future perspectives for research to better understand the timing mechanisms and sensory physiology of mating related signals.

Sexual dimorphism in the olfactory system of a solitary and a eusocial bee species.

Sexually dimorphic sensory systems are common in Hymenoptera and are considered to result from sex-specific selection pressures. An extreme example of sensory dimorphism is found in the solitary bee tribe Eucerini. Males of long-horned bees bear antennae that exceed body length. This study investigated the pronounced sexual dimorphism of the peripheral olfactory system and its representation in higher brain centers of the species Eucera berlandi. Eucera males have elongated antennae, with 10 times more pore plates and three times more olfactory receptor neurons than females. The male antennal lobe (AL) comprises fewer glomeruli than the female AL (∼100 vs. ∼130), of which four are male-specific macroglomeruli. No sex differences were found in the relative volume of the mushroom bodies, a higher order neuropil essential for learning and memory in Hymenoptera. Compared with the Western honeybee, the degree of sexual dimorphism in Eucera is more pronounced at the periphery. In contrast, sex differences in glomerular numbers are higher in the eusocial honeybee and a sexual dimorphism of the relative investment in mushroom body tissue is observed only in Apis. The observed differences between the eusocial and the solitary bee species may reflect differences in male-specific behavioral traits and associated selection pressures, which are discussed in brief.

Floral visual signal increases reproductive success in a sexually deceptive orchid.

Sexually deceptive orchids mimic signals emitted by female insects in order to attract mate-searching males. Specific attraction of the targeted pollinator is achieved by sex pheromone mimicry, which constitutes the major attraction channel. In close vicinity of the flower, visual signals may enhance attraction, as was shown recently in the sexually deceptive orchid Ophrys heldreichii. Here, we conducted an in situ manipulation experiment in two populations of O. heldreichii on Crete to investigate whether the presence/absence of the conspicuous pink perianth affects reproductive success in two natural orchid populations. We estimated reproductive success of three treatment groups (with intact, removed and artificial perianth) throughout the flowering period as pollinaria removal (male reproductive success) and massulae deposition (female reproductive success). Reproductive success was significantly increased by the presence of a strong visual signal-the conspicuous perianth-in one study population, however, not in the second, most likely due to the low pollinator abundance in the latter population. This study provides further evidence that the coloured perianth in O. heldreichii is adaptive and thus adds to the olfactory signal to maximise pollinator attraction and reproductive success.

Why sexually deceptive orchids have colored flowers.

Sexually deceptive orchids provide no reward to their pollinators. Instead, they mimic the sex pheromone of receptive insect females to attract males which pollinate the flowers in mating attempts. Nearly all species of the Mediterranean orchid genus Ophrys are sexually deceptive and pollinated by solitary bees and wasps. Due to the use of a highly specific olfactory communication channel most Ophrys species have, in contrast to food deceptive or rewarding orchids, an inconspicuous greenish perianth and a dark brownish labellum. However, some species possess a bright pink or white perianth, and the functional significant of such color signals in the orchid-pollinator communication system is unknown. We recently showed that the pink perianth of Ophrys heldreichii increases the performance of its bee pollinator, males of the long-horned bee Eucera (Tetralonia) berlandi, to detect the flower at short-range. At great distances (>30 cm) from the flower, male search behavior was found to be olfactory guided and unaffected by the spectral property of the perianth, i.e., chromatic and green receptor-specific contrast. However, in the near vicinity of the flower (<30 cm), where spatial vision is sufficient to detect the flower, search time only correlated with the green receptor-specific contrast between the perianth and the background.

Floral colour signal increases short-range detectability of a sexually deceptive orchid to its bee pollinator.

Orchids of the genus Ophrys are pollinated by males of solitary bees and wasps through sexual deception. The flowers mimic the behaviourally active compounds of the sex pheromone of receptive females and thus attract males that seek to copulate. Odour is the main attractant while visual stimuli have been assumed so far to play only a minor role. In contrast to most species of the genus, Heldreich's orchid Ophrys heldreichii, which is pollinated by males of the long-horned bee Tetralonia berlandi, possesses a bright pink perianth that appears conspicuous to a human observer. We investigated the role of this floral colour signal in pollinator attraction. We filmed approach flights of male bees to flowers in which we removed the original perianth and in which we substituted the perianth with an artificial one of a particular selected colour. At distances >30 cm, male search time correlated only with wind speed but not with the spectral parameters of the perianth, i.e. chromatic and green receptor-specific contrast. By contrast, in the close range (<30 cm), where the perianth subtends a visual angle of at least 5 deg. to the bee's eye, search time decreased with increasing green receptor contrast between perianth and background; however, no correlation with chromatic contrast or wind speed was found. Our results indicate that pollinators are first attracted by olfactory signals from a distance. Once in the vicinity of the flower where spatial vision of the males is sufficient, they are guided exclusively by vision. However, it can be expected that possession of a ;non-private' colour signal would increase the risk of pollen loss in sexually deceptive orchids by accidentally attracting non-specific flower visitors. We therefore discuss the occurrence of colour signals in the genus Ophrys in respect to the species-specific visual system of the pollinators.