On the role in stridulation of plurisegmental interneurons of the acridid grasshopperOmocestus viridulus L.

Summary

• 1. In the experimental set-up described by Hedwig (1986), which permits a grasshopper (Omocestus viridulus, Acrididae) to stridulate while various parameters of the stridulation (sound, movement, neuromuscular activity) are monitored, the activity of ascending and T-shaped plurisegmental neurons was recorded intracellulary, followed by staining of the neurons.

• 2. Interneurons ascending to the supraesophageal ganglion originate in the abdominal (Figs. 1, 2) and thoracic (Figs. 3, 5, 9) ganglia. T-shaped fibers (Figs. 7, 8) originate in the mesothoracic or prothoracic ganglion, with one branch descending to the metathoracic ganglion and another ascending to the supraesophageal ganglion. In almost all the ganglia through which they pass, the axons of the ascending and T-shaped neurons send out more or less abundant collaterals.

• 3. All the ascending and T-shaped neurons discharge in a pattern synchronized with the stridulation rhythm. The various neurons are active at different phases of the cycle.

• 4. The fibers ascending from the metathoracic ganglion (Figs. 3, 5), in particular, belong to the category of ‘auditory’ interneurons; that is, when the animal is at rest and is stimulated with the conspecific song, these neurons copy the sound pattern (Fig. 4C).

• 5. In the singing animal, however, these ‘auditory’ interneurons either fail to respond to acoustic stimuli (Fig. 4 E) or respond only in certain phases of the stridulation cycle (Fig. 6C). They are not generally inhibited during stridulation, however, for they discharge in the stridulatory rhythm.

• 6. During stridulation, there is parallel transmission of activity corresponding to the stridulation pattern and of unpatterned activity to all thoracic and cephalic ganglia, by a system of plurisegmental ascending, descending and T-shaped neurons (Fig. 10).

• 7. Although there is a thoracic pattern generator for stridulation (Hedwig 1986), the nervous system appears to operate as a unit in controlling this behavior. The physiological features of the ‘auditory’ interneurons involved here imply a close structural relationship between detection and production of the songs.