Metamorphosis of the abdominal ganglia of the tobacco hornworm,Manduca sexta

Abstract

1. Motor neurons in the fourth abdominal ganglion were identified by diffusing cobalt into the ganglion through the cut ends of the peripheral nerves. 2. The ganglion from the last instar larva had a total of 74 motor neurons. These were divided into 10 groups which were characterized by the position of the cell body in the ganglion and the root through which the axon travelled to the periphery. 3. In the transition to the pupal stage, 12 larval neurons were lost and 8 new motor neurons appeared. 4. There was no change in the number of motor neurons during adult development. Most of the larval neurons persist through metamorphosis and are used by the adult at emergence. At this stage there are 70 motor neurons. 5. After adult eclosion there was a loss of 40 motor neurons. Only “larval” neurons degenerated. Those which remained included both those which were larval in origin and those which appeared in the pupa. After emergence the interneuron number dropped from approximately 380 to 200. 6. At the time of emergence the eclosion hormone “turns-off” pupal behavior and “turns-on” adult behavior. It also triggers the breakdown of various muscle groups in the abdomen. It is concluded that the neural death was the end result of this aspect of eclosion hormone action.