A macroglomerulus in the antennal lobe of leaf-cutting ant workers and its possible functional significance.

Ants have a well-developed olfactory system, and pheromone communication is essential for regulating social life within their colonies. We compared the organization of primary olfactory centers (antennal lobes, ALs) in the brain of two closely related species of leaf-cutting ants (Atta vollenweideri, Atta sexdens). Both species express a striking size polymorphism associated with polyethism. We discovered that the ALs of large workers contain a substantially enlarged glomerulus (macroglomerulus, MG) at the entrance of the antennal nerve. This is the first description of an MG in non-sexual individuals of an insect. The location of the MG is laterally reversed in the two species, and workers of different size express a disproportional allometry of glomerular volumes. While ALs of large workers contain an MG, glomeruli in small workers are all similar in size. We further compared electroantennogram (EAG) responses to two common trail pheromone components of leaf-cutting ants: 4-methylpyrrol-2-carboxylate and 2-ethyl-3,6-dimethylpyrazine. At high concentrations the ratio of the EAG signals to 2-ethyl-3,6-dimethylpyrazine versus 4-methylpyrrol-2-carboxylate was significantly smaller in A. vollenweideri compared with the ratio of EAG signals to the same two components in A. sexdens. The differences in EAG signals and the species specific MG location in large workers provide correlative evidence that the MG may be involved in the detection of the trail pheromone.