Mushroom bodies (MBs) are prominent neuropils in the insect brain involved in higher order processing such as sensory integration, learning and memory, and spatial orientation. The size and general morphology of MBs are diverse across insects. In this study we comparatively investigated the microstructure of synaptic complexes (microglomeruli) in major sensory input regions of the MBs, the calyces, across various neopteran insect species. Pre- and postsynaptic compartments of microglomeruli were analyzed using anti-synapsin immunocytochemistry, f-actin-phalloidin labeling and high-resolution confocal microscopy. Our results suggest that calycal microglomeruli are present across all investigated neopteran insect species, but differences are found in the distribution of synapsin and f-actin within their pre- and postsynaptic compartments. Hymenopteran MBs contain the highest number and packing density of microglomeruli compared to all other species from the different insect orders we investigated. We conclude that the evolution of high numbers of microglomeruli in Hymenoptera may reflect an increase in synaptic microcircuits, which could enhance the computational capacities of the MBs.
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