A prerequisite for understanding how odor information is coded in the central nervous system is to know the morphology and spatial relationship of the principal neurons forming the olfactory pathways. The present account provides an anatomical description of the morphology of the neuronal connections between the antennal lobe and the protocerebrum in the moth Heliothis virescens, a species used for studies of olfactory processing and learning. Intracellular labeling and antennal lobe focal injections with dextran fluorescent markers were combined with neuropil immunostaining and three-dimensional reconstructions. The experiments revealed four antennocerebral tracts, the inner, middle, outer, and dorsomedial, and eight morphological types of projection neurons in addition to a neuron with an unpaired median soma in the subesophageal ganglion. Multiglomerular projection neurons, present in all but the dorsomedial antennocerebral tract, project in several olfactory foci of the protocerebral neuropil. With few exceptions, these neurons do not innervate the calyces of the mushroom body. Uniglomerular projection neurons appear most numerous in the inner antennocerebral tract but are also present in the outer and dorsomedial tracts. These neurons always ramify in the calyces of the mushroom body and in the lateral horn. The projection areas of the neurons following different tracts are largely separated in the secondary olfactory centers. This is most evident in the lateral horn, whereas, in the calyces, the axonal ramifications are more intermingled. The mushroom body architecture, revealed by neuropil immunolabeling, showed striking similarities to that of other lepidopteran species as well as insects of other taxa.

The glomeruli of the antennal lobes in insects reflect the organization of the olfactory system, which is important for species-specific behaviors in response to insect- and plant-produced odorants. We studied the antennal lobes of the polyphagous moth Helicoverpa armigera and the oligophagous H. assulta (Heliothinae; Lepidoptera; Noctuidae) in order to see whether there are any anatomical differences that might elucidate how information about odorants is analyzed. Three-dimensional models of the antennal lobes were made, based on synaptic antibody staining combined with confocal laser scanning microscopy. These showed 65 glomeruli in each sex of H. armigera and 66 glomeruli in females of H. assulta. Sixty-two of the glomeruli were identified in both sexes and species and were given the same numbers. The sex-specific glomeruli included three macroglomerular units in H. armigera males, as well as three and four female-specific glomeruli in H. armigera and H. assulta, respectively. The species specificity of H. assulta females also appeared by the particular large size of two ordinary glomeruli. The accumulating knowledge on how biologically relevant information is encoded in receptor and antennal lobe neurons in heliothines makes these moths particularly interesting for studying the functional organization of the glomeruli. The anatomical atlases of the antennal lobes, as presented here, are prerequisites for identifying glomeruli ascribed to particular functions across sexes and species.