Postembryonic development in the auditory system of the locust

Summary

• 1. The postembryonic development of three identified ascending interneurones in the auditory system ofLocusta migratoria was studied using intracellular recording and staining techniques.

• 2. The terminal arborisations in the brain of G, B and C neurones were identified in the adult, 5th and 4th instars, and for the G neurone in 3rd and 2nd instars as well (Figs. 2, 3). Further, the development of the soma and dendrites of the G neurone in the mesothoracic ganglion was followed from 2nd instar to adulthood (Fig. 4).

• 3. The adult G, B and C neurones all terminate in the lower lateral lobe of the protocerebrum, and G and B neurones share a further projection area ventrally in the anterior protocerebrum (Fig. 2). The lower lateral protocerebrum may represent an auditory neuropil in the brain.

• 4. The larval G, B and C neurones possess the general adult morphology throughout postembryonic development and neural growth is allometric. Only at the second instar level may some reduction in density of arborisation in brain and mesothoracic ganglion occur (Figs. 3, 4).

• 5. The physiological responses of the identified adult and larval neurones are very similar. Expressed firstly as constant intensity curves (frequency response spectra) (Figs. 5, 6), the shapes of the adult spectra are seen reflected in larval G, B and C neurones throughout development. With each step back in development there is an accompanying loss in sensitivity which affects higher more than lower frequencies and is not always linearly related to age (Fig. 8). The thresholds obtained from second instar animals suggest that newly hatched (first instar) locusts are probably deaf to airborne sound.

• 6. The intensity response and latency of response functions (Fig. 9 A) of the adult neurones are repeated in each larval instar studied; similarly the ipsilateral-contralateral difference characteristic of the B neurone is found intact in the 4th instar, although at a higher absolute intensity (Fig. 9B). These results suggest that the dynamic properties and synaptology described elsewhere for the adult neurones are already formed early in development. A role for these interneurones in the escape behaviour of the larva is considered.