HANNAFORD, S.; FOSTER, R.L.; GIBSON, J.; SATTLER, A.; Univ. of Puget Sound; Univ. of Puget Sound; Univ. of Puget Sound: Effects of age and caste on the brain and mushroom bodies of the bumblebee, Bombus huntii
Work with honeybees reports age and caste related differences in brain structures, as well as systematic correlations between brain region size and task specialization in workers (i.e., forager and nurse bees). A striking result is the difference in the mushroom bodies (MBs), which are the centers for learning and memory. Our research on bumblebees extends this work to another social insect for which behavior is well characterized but the neuroanatomy has not yet been described. An important difference between honeybees and bumblebees is that task specialization is a function of worker age in honeybees, but is independent of age in bumblebees. Here we describe brain structure in male, worker, and queen bumblebees. We show total hemisphere volume for queens (1.3 x 10 �3 mm3) was significantly larger than for males and workers (0.9 x 10�3 mm3). The brains are dominated by three structures (optic lobes, mushroom bodies, and antennal lobes). Brain organization was similar in queens, males, and workers. However, we found several differences among castes. Males and workers have similarly sized optic lobes (~40% brain volumes) and mushroom bodies (21% and 18% respectively), but males have larger antennal lobes (28% versus 22% for workers). We examined the MBs in detail. Each MB is comprised of two areas of synaptic integration (calyx neuropil), which give rise to a stalk and two lobes; the Kenyon cells (soma of MB neurons) fill the calyx cups. In male brains the calyx neuropil consistently accounted for ~ 9% of total brain volume. Worker brains showed greater variability (6 to 9% of total brain volume), which may correlate with task specialization. We found no difference in total brain volume with age. However, one profound difference was that, in all castes, the Kenyon cell volume was greater in callow (<1 day) than mature (>4 days) bees.
