The antennal lobes of fungus-growing ants (Attini): neuroanatomical traits and evolutionary trends.

Ants of the tribe Attini are characterized by their obligate cultivation of symbiotic fungi. In addition to the complex chemical communication system of ants in general, substrate selection and fungus cultivation pose high demands on the olfactory system of the Attini. Indeed, behavioral studies have shown a rich diversity of olfactory-guided behaviors and tremendous odor sensitivity has been demonstrated. To allow fine-tuned behavioral responses, adaptations within the olfactory system of the Attini are expected. We compared the number, volumes and position of the glomeruli (functional units) of the antennal lobe of 25 different species from all three major Attini groups (lower, higher and leaf-cutting Attini). The antennal lobes of all investigated Attini comprise a high number of glomeruli (>257). The highest number (630) was found in Apterostigma cf. mayri. This species is at a basal position within the Attini phylogeny, and we suggest that a high number of glomeruli might have been advantageous in the evolution of the advanced olfactory systems of the Attini. In the leaf-cutting Attini, an extremely large glomerulus (macroglomerulus) near the antennal nerve entrance was recently described in two species. Preliminary results show that this macroglomerulus is involved in processing of trail-pheromone information. In our comparative study, we find this macroglomerulus in all investigated leaf-cutting Attini, but in none of the lower and higher Attini species. It is found only in large workers, and for all investigated species it is located close to the entrance of the antennal nerve. Our results indicate that the presence of a macroglomerulus in large workers of leaf-cutting Attini is a derived over-expression of a trait in the polymorphic leaf-cutting species. It presumably represents an olfactory adaptation to elaborate foraging and mass recruitment systems, and adds to the complexity of division of labor and social organization known for this group.