Organization of visual afferents to the mushroom bodies in the carpenter ant

Images from brain reconstructions of workers (left), unmated queens (middle) and males (right) of the carpenter ant
Camponotus ocreatus. top row: frontal view, bottom row: view from above.

There are considerable differences in behavior between male and female ants, as well as between queens and workers within the females, a difference which is apparent in their morphology and which extends to the nervous system. In the central nervous system the conspicuous differences are in the elaboration of brain structures, not the presence of brain structures unique to one sex. This difference can be attributed to the contrasting requirements for male and female ants to succeed. In general the sole purpose of a male ant is to leave the nest, fly and mate with a female ant before they die. Though pheromones play a role in mate finding visual guided behavior is very important for this task, so males have in general much larger eyes than females, especially than the mostly sterile worker caste (Gronenberg and Hölldobler 1999). Females can be either winged reproductives, which commonly fly once on the mating flight before they shed their wings and found a colony, or sterile workers which will eventually do all the tasks of the colony. Workers are never winged, and except in a few uncharacteristically visual ant species live in what must be mainly an olfactory landscape, as judged by the size of primary sensory neuropil dedicated to olfaction versus vision.
In the carpenter ant Camponotus ocreatus the optic lobes are in their relative size twice as large than that of females, but the volume of the collar (the area of the mushroom bodies where afferents from the optic lobe terminate) is smaller than that of females. The same is true for the antennal lobes, though the differences are clearly less: the relative size of the antennal lobes is only about 17% smaller in males.

Unlike in the honey bee, where visual afferents terminate in clear layers in the mushroom bodies, in ants afferents from neurons from the upper medulla are more likely to terminate in outer collar regions, whereas neurons from the lower medulla are predominantly found in the inner collar regions, though this regards more the density than absolute distribution.

a) two different dyes in the medulla and lobula results in inequal staining in the collar of the mushroom bodies (b). If a different dye is, however, inserted in each medulla, no difference in the distribution of those colors can be observerd (c). d) examples of the branching within the medulla of two cells which connect the medulla with the mushroom bodies.

Looking at the arborization pattern of those neurons which connect the mushroom bodies with the optic lobes it is interested to note that a sexual dimorphism exists: in the ventral eye region those cells arborize much more into the outer medulla than those either of workers or unmated queens.

Arborization of medulla-mushroom body cells in males (top), unmated queens (middle), and workers (bottom).