Abstract Division of labor among workers is a key feature of social insects and frequently characterized by an age-related transition between tasks, which is accompanied by considerable structural changes in higher brain centers. Bumble bees (Bombus terrestris), in contrast, exhibit a size-related rather than an age-related task allocation, and thus workers may already start foraging at two days of age. We ask how this early behavioral maturation and distinct size variation are represented at the neuronal level and focused our analysis on the mushroom bodies (MBs), brain centers associated with sensory integration, learning and memory. To test for structural neuronal changes related to age, light exposure and body size, whole-mount brains of age-marked workers were dissected for synapsin immunolabeling. MB calyx volumes, densities and absolute numbers of olfactory and visual projection neuron (PN) boutons were determined by confocal laser scanning microscopy and three-dimensional image analyses. Dark-reared bumble bee workers showed an early age-related volume increase of olfactory and visual calyx subcompartments together with a decrease of PN-bouton density during the first three days of adult life. A 12:12 light-dark cycle did not affect structural organization of the MB calyces compared to dark-reared individuals. MB calyx volumes and bouton numbers positively correlated with body size, whereas bouton density was lower in larger workers. We conclude that, in comparison to the closely related honey bees, neuronal maturation in bumble bees is completed at a much earlier stage, suggesting a strong correlation between neuronal maturation time and life style in both species. This article is protected by copyright. All rights reserved.