Leaf-cutting ants (Atta vollenweideri) express a remarkable size polymorphism across the two sexual castes (queens and males) but in particular within the worker caste. Worker size is related to behavior (alloethism), separating workers into behavioral subcastes. The neuronal mechanisms underlying differences in behavior within the worker caste are still unknown. In this study, we first compared selected neuropils, in particular, the antennal lobes (AL) in males, queens, and workers. The males' ALs contain three extremely large, sex-specific glomeruli (macroglomeruli; MGs) and in total comprise fewer glomeruli (242) than the ALs of queens (about 346 glomeruli). In contrast to males, the queen ALs contain only one large glomerulus at a lateral position. The largest number of glomeruli was found in workers (396-442). In a previous paper, we described an MG in the workers' AL, and, in the second part of this study, we show that within workers two distinct, size-related AL phenotypes exist: the MG phenotype (containing a macroglomerulus) and the RG phenotype, with all glomeruli of regular size. This neuroanatomical polyphenism is established during pupal development and separates the worker caste into two neuroanatomical subcastes. Third, we investigate the functional significance of the MG in workers. By using calcium imaging to monitor activity of AL projection neurons, we show that the releaser component of the trail pheromone is represented in the same region as the MG. We propose that phenotypic trait variation in the organization of the ALs leads to differences in odor information processing that finally result in size-related differences in trail-following behavior.