Distribution and Organization of Descending Neurons in the Brain of Adult Helicoverpa armigera (Insecta)

SIMPLE SUMMARY: One of the fundamental aims of neuroscience is to understand how the brain coordinates motor behaviors. Insects are perfect models for such studies because they have a simple but elaborate brain and a complex but stereotypical behavioral reservoir. The descending neurons (DNs) of insects connect the brain and thoracic ganglia and play essential roles in regulating insect behavior. A complete survey of DNs in the brain facilitates identifying the candidate neurons that may correlate with specific behaviors. In the present study, we comprehensively examined the distribution and organization of the DNs in the brain of a moth species, Helicoverpa armigera. The DN clusters are conserved across insect species. However, the cluster of DNd in H. armigera was not found in other studied species, and this cluster was only observed in males. This result suggests that the novel DNd cluster may consist of species- and sex-specific descending neurons. The innervation patterns of DNs in the brain are conserved across insect species, which indicates that the ventral part of the central brain plays multiple essential roles in triggering insect behaviors. ABSTRACT: The descending neurons (DNs) of insects connect the brain and thoracic ganglia and play a key role in controlling insect behaviors. Here, a comprehensive investigation of the distribution and organization of the DNs in the brain of Helicoverpa armigera (Hübner) was made by using backfilling from the neck connective combined with immunostaining techniques. The maximum number of DN somata labeled in H. armigera was about 980 in males and 840 in females, indicating a sexual difference in DNs. All somata of DNs in H. armigera were classified into six different clusters, and the cluster of DNd was only found in males. The processes of stained neurons in H. armigera were mainly found in the ventral central brain, including in the posterior slope, ventral lateral protocerebrum, lateral accessory lobe, antennal mechanosensory and motor center, gnathal ganglion and other small periesophageal neuropils. These results indicate that the posterior ventral part of the brain is vital for regulating locomotion in insects. These findings provide a detailed description of DNs in the brain that could contribute to investigations on the neural mechanism of moth behaviors.