Metamorphosis of flight motor neurons in the moth Manduca sexta

1. In Manduca sexta a nerve, IIN1b, innervating the dorsal musculature of the larva, survives the metamorphosis and innervates the dorsal longitudinal muscles of the adult. That is, the adult nerve is a modified larval nerve (Figs. 1, 3). 2. Groups of motor neurons with their axons in IIN1b can be identified with the cobalt backfilling technique in the adult, and then located in the developing adult and larva (Fig. 4). 3. In the prothoracic ganglion, the axons of the motor neurons studied travel in a single bundle and send their dendrites into the same region of the neuropile. These dendrites increase in length during the last two-thirds of metamorphosis (Figs. 5, 6). 4. During metamorphosis, the prothoracic ganglion of Manduca sexta exhibits a pattern of allometric growth typical of insect postembryonic development. The volumes of cellular cortex and neuropile both increase, but the increase in neuropile is greater (Fig. 7). 5. The basic structure of the larval neuropile defined by its fiber tracts is maintained through metamorphosis (Figs. 9, 10, 11).