Different growth patterns of two adjacent glomeruli responsible for sex-pheromone processing during postembryonic development of the cockroach Periplaneta americana.

In many insect species, sex pheromone is processed by specific, enlarged glomeruli in the antennal lobes of males. In the male American cockroach, two closely located glomeruli (A and B) are responsible for processing the major pheromone components (periplanone-A and -B, respectively), and these collectively form the macroglomerular complex. Afferents originating from the dorsal and ventral surfaces of the antenna tend to project to the anterior and posterior regions of the macroglomerular complex via the dorsal and ventral antennal nerves, respectively. This topographic segregation of afferents is seen only in the macroglomerular complex, and not in other glomeruli that process normal environmental odors. Using differential, anterograde dye injection into the two antennal sensory nerves, we show that the macroglomerular complex is not formed by fusion of several glomeruli, as suggested in previous studies, but that the precursors of the A- and B-glomeruli already exist in the first larval instar. The volume of afferents in the macroglomerular complex precursor increases nearly exponentially with molting times, 430-fold from the first instar to the adult. The A- and B-glomeruli both undergo continuous growth during postembryonic development, but peak growth rates occur in different larval stages. The growth rate of the B-glomerulus peaked in the mid-developmental stage then declined, while growth of A-glomerulus was maintained at low level in early- to mid-developmental stages but increased greatly in later stages. These results suggest perception of sex pheromone occurs in early instars, and that PA and PB have distinct roles in different developmental stages.