Abstract
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.
Highlights
Honeybee colonies comprise three adult phenotypes, males, reproductive females and sterile females
As an initial step to explore the variability of sensory and neural structures and the interaction between size limitations and sex and caste-specific selection pressures in Apis, we studied the periphery of the visual system
Specimens were collected near Bangalore, India (Doddaballapur, 13u17’32"N, 77u32’35"E) between 2003 and 2012 (A. florea, A. dorsata, A. cerana), in Chiang Rai Province, Northern Thailand (Mae Sai, 20u25’60"N, 99u52’60"E; Mae Fang Luang, 19u52’25"N, 99u43’23"E) in 2011 (A. andreniformis), in Vienna, Austria (48u13’47"N, 16u21’32"E) and Wurzburg, Germany (49u46’48"N, 9u58’25"E) between 2009 and 2011 (A. mellifera carnica) and obtained from the collection maintained at the bee research unit, Bremen, Germany (A. florea, obtained from Feyriz, Fars Province, Iran in 1991)
Summary
Honeybee colonies comprise three adult phenotypes, males (drones), reproductive females (queens) and sterile females (workers). Queens and workers differ in reproductive organs, as well as in the morphology of mouthparts, flight musculature, number of glands, sensory systems and structural organization of brains [1,2,3,4,5]. The workers perform all tasks necessary to maintain the colony, e.g. brood care, foraging, and colony defense. Corresponding to this division of labor, queens show reductions in many morphological traits, e.g. mouthparts, pollen collecting structures, olfactory system and brain size [1,4]. Drones engage neither in social nor foraging tasks; they serve predominantly for reproduction, i.e. searching for and mating with queens. Drones show enlarged and elaborated olfactory [2,7,8] and visual systems [9], as well as flight musculature that is adapted for fast pursuit flights [10]
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