Glomerular organization in the antennal lobe of the oriental armyworm Mythimna separata

In insects, the number and specificity of antennal lobe glomeruli often reflect the number and diversity of expressed chemosensory genes, which are linked to its ecological niche and specific olfactory needs. The oriental armyworm, Mythimna separata, is an important and common lepidopteran pest of cereal crops. Given its reliance on the olfactory system for crucial behaviors, understanding the evolutionary potential of this system requires a thorough characterization of the anatomical structure of the primary olfactory center. Here, we systematically identified all antennal lobe glomeruli of M. separata based on synaptic antibody immunostaining and mass staining of the olfactory sensory neurons. A total of 69 glomeruli were identified in females and 65 in males, and an intuitive nomenclature based on glomerular positions was applied. Our findings uncovered some sex-specific glomeruli in this species. There were ten female-specific glomeruli and three male-specific glomeruli, except for the macroglomerular complex(MGC) units, with a notable observation that the female labial pit organ glomerulus was larger than its male counterpart. Additionally, we identified four antennal-lobe tracts(ALTs) and retrograde labeling from the calyx revealed that all glomeruli were innervated by the medial ALT projection neurons. The comparison of the olfactory system structures between M. separata and sympatric moths supports their evolutionary convergence in noctuid moths. These results collectively lay the foundation for future studies on olfactory processing in M. separata.