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Keywords:

  • insects;
  • sound communication;
  • neuroanatomy;
  • central nervous system

Abstract

The morphological characteristics of five types of local spiking interneurons in the metathoracic ganglion of the acridid grasshopper Omocestus viridulus L. have been revealed by intracellular injection of the fluorescent dye Lucifer Yellow. All these neurons are active during induced stridulation and discharge in the stridulation rhythm. They are structurally unlike any of the metathoracic local interneurons previously described in the Acrididae.

  • Type 1: Unilateral interneurons in the metathoracic neuromere; soma in anterior position; arborization dorsal, in anterolateral, posterolateral, and midline regions.

  • Type 2: Unilateral interneurons in the metathoracic neuromere; neurite with characteristic hairpin bend; soma in lateral position; arborization through the lateral neuropil region.

  • Type 3: Unilateral interneurons in the metathoracic and abdominal neurons; soma anterior at the base of the connective; arborization dorsal, in anterolateral, posterolateral, and midline regions, with ladderlike branches into the abdominal neuromeres.

  • Type 4: Symmetrically bilateral neurons; soma in ventral median position; arborization dorsal, in anterolateral, posterolateral, and midline regions. Ipsilateral branches mostly smooth, contralateral mostly varicose.

  • Type 5: Asymmetrically bilateral neurons, soma in ventral median position; ipsilateral branches mostly smooth, contralateral mostly varicose. The ipsilateral branches in particular penetrate all layers of the ganglion.

The main arborizations of interneurons of types 1 and 4, and to some extent those of types 3 and 5, occupy the same region, which extends very far dorsally into the vicinity of the median and lateral tracts. There are additional antero- and posterolateral branches in the metathoracic neuromere, in positions where in principle they can overlap with the arborizations of the motoneurons. The interneurons of type 2, and to some extent type 5, arborize mainly in a lateral region that also includes deeper layers of the ganglion and hence encompasses the projections of mechanoreceptive proprioceptors.

The interneurons of types 1, 3, and 4 have so-called “sister” neurons, identical to the typical neurons in their metathoracic arborizations but with additional collaterals that pass into the mesothoracic ganglion.