Reorganization of visual interneurons during metamorphosis in the swallowtail butterfly Papilio xuthus

The metamorphic fate of larval visual interneurons in the swallowtail butterfly Papilio xuthus has been examined by using reduced silver impregnation and immunocytochemistry of gamma-amino butyric acid (GABA). Visual interneurons in the second larval optic neuropile (medulla) can be followed throughout metamorphosis because of large somata located in the anterior edge of the imaginal medulla. Ten to 12 neurons in the larval medulla were immunoreactive to a GABA-antiserum. They arborize in the larval medulla and extend dendritic processes to the first larval optic neuropile (lamina). After pupation, the medulla neurons lose GABA-immunoreactive larval processes and differentiate new processes that enter tangentially into the developing imaginal medulla. Axons of the surviving larval neurons follow an optic tract between the lobula and the lobula plate and extend to the lateral area of the protocerebrum. Thus, larval visual interneurons are incorporated into the imaginal optic lobe and may play a new role in the visual processing of the butterfly.