Mushroom body volumes and visual interneurons in ants: comparison between sexes and castes

The mushroom bodies are brain centers involved in complex behaviors such as learning and orientation. Here we examine the organization of mushroom bodies in ants, focusing on visual input. We describe the structure of visual neurons and compare the volume of brain structures involved in visual processing, especially the optic lobes and parts of the mushroom bodies receiving visual input in males, winged females, and workers of carpenter ants (Camponotus). A relatively small number of neurons connect the medulla with the mushroom bodies, and these neurons have relatively large dendritic fields in the medulla, suggesting low spatial resolution in ants. These neurons terminate in different yet overlapping strata in the mushroom bodies' collar region. While males have larger optic lobes than workers, their collar region is smaller than in females. Male ants have an additional type of medulla-mushroom body neuron with dendrites probing the distal medulla. These neurons are absent in female and worker ants. Most mushroom body Kenyon cells that are postsynaptic to visual input neurons appear to integrate visual as well as antennal input. This is in contrast to honey bees, where visual input to the mushroom bodies is more prominent and where Kenyon cells are not known to combine visual and antennal input.