Most bees are diurnal, with behaviour that is largely visually mediated, but several groups have made evolutionary shifts to nocturnality, despite having apposition compound eyes unsuited to vision in dim light. We compared the anatomy and optics of the apposition eyes and the ocelli of the nocturnal carpenter bee, Xylocopa tranquebarica, with two sympatric species, the strictly diurnal X. leucothorax and the occasionally crepuscular X. tenuiscapa. The ocelli of the nocturnal X. tranquebarica are unusually large (diameter ca. 1 mm) and poorly focussed. Moreover, their apposition eyes show specific visual adaptations for vision in dim light, including large size, large facets and very wide rhabdoms, which together make these eyes 9 times more sensitive than those of X. tenuiscapa and 27 times more sensitive than those of X. leucothorax. These differences in optical sensitivity are surprisingly small considering that X. tranquebarica can fly on moonless nights when background luminance is as low as 10(-5) cd m(-2), implying that this bee must employ additional visual strategies to forage and find its way back to the nest. These strategies may include photoreceptors with longer integration times and higher contrast gains as well as higher neural summation mechanisms for increasing visual reliability in dim light.

Bees are mostly active during the daytime, but nocturnality has been reported in some bee families. We studied temporal flight activity in three species of carpenter bees (genus Xylocopa) in relation to light intensities. X. leucothorax is diurnal, X. tenuiscapa is largely diurnal being only occasionally crepuscular, while X. tranquebarica is truly nocturnal. Occasional forays into dim light by X. tenuiscapa are likely to be due to the availability of richly rewarding Heterophragma quadriloculare (Bignoniaceae) flowers, which open at night. X. tranquebarica can fly even during the moonless parts of nights when light intensities were lower than 10(-5) cd m(-2), which makes this species the only truly nocturnal bee known so far. Other known dim-light species fly during crepuscular or moonlit periods. We compare eye and body sizes with other known diurnal and dim-light bees. We conclude that while extremely large ocellar diameters, large eye size:body size ratio, large number of ommatidia and large ommatidial diameters are all adaptations to dim-light foraging, these alone do not sufficiently explain the flights of X. tranquebarica in extremely dim light. We hypothesise that additional adaptations must confer extreme nocturnality in X. tranquebarica.

the Indian carpenter bee Xylocopa tranquebarica (Fabricius), which flies even on moonless nights [3], uses colour vision to discriminate artificial landmarks at the nest in starlight. Humans, in contrast, are colour-blind at half-moon illumination. This finding, obtained using natural nests under natural illumination, is remarkable because insensitive apposition eyes were thought unable to support nocturnal colour vision. Hitherto, nocturnal colour vision was known only in nocturnal hawkmoths [4] and geckos [5], animals with eyes well adapted to nocturnality.Outdoor experiments were conducted at natural nests within Bhimashankar Wildlife Sanctuary, Maharashtra State, in the Western Ghats of India (see [3]), between December 2007 and March 2008. Experiments were performed each night when the bees started flying, approximately half an hour after sunset (which was between 18:00 and 19:00 hours). Experiments usually continued until 03:00 the

SUMMARYBees of the genus Apis are important foragers of nectar and pollen resources. Although the European honeybee, Apis mellifera, has been well studied with respect to its sensory abilities, learning behaviour and role as pollinators, much less is known about the other Apis species. We studied the anatomical spatial resolution and absolute sensitivity of the eyes of three sympatric species of Asian honeybees, Apis cerana, Apis florea and Apis dorsata and compared them with the eyes of A. mellifera. Of these four species, the giant honeybee A. dorsata (which forages during moonlit nights) has the lowest spatial resolution and the most sensitive eyes, followed by A. mellifera, A. cerana and the dwarf honeybee, A. florea (which has the smallest acceptance angles and the least sensitive eyes). Moreover, unlike the strictly diurnal A. cerana and A. florea, A. dorsata possess large ocelli, a feature that it shares with all dim-light bees. However, the eyes of the facultatively nocturnal A. dorsata are much less sensitive than those of known obligately nocturnal bees such as Megalopta genalis in Panama and Xylocopa tranquebarica in India. The differences in sensitivity between the eyes of A. dorsata and other strictly diurnal Apis species cannot alone explain why the former is able to fly, orient and forage at half-moon light levels. We assume that additional neuronal adaptations, as has been proposed for A. mellifera, M. genalis and X. tranquebarica, might exist in A. dorsata.

Night, dawn, and dusk have abiotic features that differ from the day. Illumination, wind speeds, turbulence, and temperatures are lower while humidity may be higher at night. Nocturnal pollination occurred in 30% of angiosperm families across 68% of orders, 97% of families with C3, two-thirds of families with crassulacean acid metabolism (CAM), and 71% dicot families with C4 photosynthesis. Despite its widespread occurence, nocturnal pollination occurs in more families with xerophytic adaptations than helophytes or mesophytes, suggesting that nocturnal flowering is primarily an adaptation to water stress since flowering is a water-intensive process. We propose the arid or water stress hypothesis for nocturnal flowering suggesting that plants facing water stress in a habitat (e.g., deserts) or a habitat stratum (e.g., upper canopy for epiphytes) gain a selective advantage by nocturnal flowering by reducing water loss through evapotranspiration, leading to larger flowers that provide more nectar or other resources, to support pollinators with higher rewards. Contrary to the wide taxonomic occurrence of nocturnal flowering, few animal taxa serve as nocturnal pollinators. We discuss the sensory and physiological abilities that enable pollinator movement, navigation, and detection of flowers within the nocturnal temporal niche and present a unified framework for investigation of nocturnal flowering and pollination.

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We provide the first data on nocturnal pollination by bees. Heterophragma quadriloculare is a self–incompatible hermaphroditic tree solely pollinated at night by the carpenter bee Xylocopa (Mesotrichia) tenuiscapa Westwood, whose pollinating flights were lunar insensitive and unaffected by low nighttime temperatures (2–l4°C). The number of carpenter bees visiting a tree per minute and the number of flowers visited per foraging bout were positively related to the size of the floral display. We found that floral display size was negatively related to fruit set but positively related to absolute fruit numbers. In pollen carryover experiments, cross–pollen did not travel far in sequences of non–emasculated flowers (up to the 4th flower), but traveled farther in emasculated flower sequences (up to the 15th flower) as evidenced by fruit set in these sequences. The relatively poor fruit set on trees with many open flowers may have resulted from deposition of self–pollen on touch–sensitive stigmas, which close on first contact. Because large–girth trees tend to have large floral displays, trees may increasingly realize a higher percentage of fitness through male rather than female function as they age. The steady–state flowering phenology of H. quadriloculare may limit reduction in the female function compared to the cornucopia strategy of many other members of the Bignoniaceae and may also provide a reliable resource for X. tenuiscapa. RESUMES Investigou‐se a relação entre salinidade intersticial, esclerofilia da folha, vigor da planta e densidade de galhas provocadas pelo inseto galhador foliar, Cecidomyia avicenniae (Diptera: Cecidomyiidae), em sua planta hospedeira Avicennia germinans (Avicenniaceae). As coletas foram realizadas em seis mangues e em uma mata de várzea, no Maranhão, nordeste do Brasil. Em cada local, dez ramos foram coletados aleatoriamente em cinco árvores de A. germinans. De cada ramo foram obtidos o número total de galhas e o comprimento (cm). Médias do comprimento, largura, biomassa e área das folhas por ramo foram também registradas. A esclerofilia foliar foi quantificada atraves da biomassa por unidade de área foliar (g/cm2). Amostras da água intersticial foram obtidas através de um tubo de PVC de 1,3 cm de diàmetro á uma profundidade de 80 cm, e a salinidade (ppm) medida com um refratômetro. A esclerofilia das folhas apresentou uma forte corrclação positiva com a salinidade intersticial (R2=0,77; P < 0,05). Observou‐se também correlações entre a densidade de galhas por unidade de área foliar (cm2) e a salinidade intersticial (r=0,36; P < 0,05), e entre densidade de galhas e a esclerofilia das folhas (r=0,40; P < 0,05). Salinidade e esclerofilia juntas explicaram 22 por cento da variação na densidade de galhas de C. avicenniae. Observou‐se, ainda, uma relação negativa entre a densidade de galhas por centímetre e o comprimento do ramo (R2=0,50; P < 0,05). Portanto, ramos mais longos de A. germinans apresentaram menor densidade de galhas. Nossos resultados sugerem que a densidade de galha...

Sexual dimorphism in eye structure is attributed to sexual selection in animals that employ vision for locating mates. In many male insects, large eyes and eye regions of higher acuity are believed to facilitate the location of females. Here, we compare various features of male and female eyes in three sympatric carpenter bee species, which include two diurnal species (Xylocopa tenuiscapa and X. leucothorax) as well as a nocturnal species (X. tranquebarica). In X. tenuiscapa, males have larger eyes than females, while in the nocturnal X. tranquebarica, males have slightly smaller eyes and in X. leucothorax, the eyes are of similar size in both sexes. X. tenuiscapa males detect females by perching near nest sites (resource defence) or along fly-ways and other open areas with good visibility. Males of the other two species search for females by patrolling. We postulate that the larger eyes of male X. tenuiscapa are beneficial to their mode of mate detection since perching males may benefit from a larger visual area of high resolution detecting moving stimuli across the sky, and which may be germane to the more social and gregarious nesting behaviour of this species, compared to the other solitary bees. We tested the performance of the eyes of male X. tenuiscapa behaviourally and find that a perching male can detect a flying female at a distance of 20 m, which darkens the visual field of a single ommatidium by just 2%. This, together with the bee’s high spatial resolution permits detection of moving stimuli at least as well or even better than achieved by honey bee drones.