Sound production in the cockroach,Gromphadorhina portentosa: The sound-producing apparatus

1. The giant Madagascar cockroach,Gromphadorhina portentosa, hisses by expelling air from a pair of specialized abdominal spiracles. The anatomy and innervation of serially homologous respiratory and sound-producing spiracles were compared in order to determine the evolutionary steps by which a new behavior has developed.

2. The trachea leading to the sound-producing (fourth) spiracle shows a constriction proximally; distally it is greatly elongated with a conical bore (Fig. 2). These features, which are lacking in other spiracles, are sufficient to account for the character of the sound (Fig. 10).

3. The motoneurons innervating both types of spiracles were located by axonal diffusion of cobalt, and their morphology was determined in wholemounted ganglia. The number, ganglionic locations, and in some cases branching patterns of motoneurons serving the sound-producing and respiratory spiracles were essentially identical (Figs. 4, 5, 6).

4. Physiological activity was recorded along spiracle nerves and within spiracle muscle fibers; four units were identified for each spiracle, agreeing with the number of cells located anatomically. These included, for each abdominal spiracle, an opener exciter motoneuron, two closer exciter motoneurons, and one closer inhibitor motoneuron (Figs. 7, 8).

5. During normal respiration the output of these 4 units had similar phase relationships in all abdominal spiracles which were examined; lower firing rates in the motoneurons innervating the hissing spiracles rendered these nonfunctional during normal respiration (Fig. 9).

6. The findings are consistent with conservation of motor innervation and of central pattern generators during evolution.