Projection areas and branching patterns of the tympanal receptor cells in migratory locusts, Locusta migratoria and Schistocerca gregaria.

In Locusta migratoria and Schistocerca gregaria, the projection areas and branching patterns of the tympanal receptor cells in the thoracic ganglia were revealed. Four auditory neuropiles can be distinguished on each side of the ventral cord, always located in the anterior part of the ring tract in each neuromere (two in the meta-, one in the meso-, and one in the prothoracic ganglion). Some of the receptor fibres ascend to the suboesophageal ganglion. There are distinct subdivisions within the auditory, frontal metathoracic and mesothoracic neuropiles. The arrangement of the terminal arborisations of the four types of tympanal receptor cells according to their different frequency-intensity responses is somatotopic and similar in the two ganglia. Here the receptor cells of type-1 form a restricted lateroventral arborisation. Cells of type-4 occupy the caudal part with a dorsorostral extension. Cells of type-2 and -3 arborise in a subdivision between both. Most of the stained low-frequency receptors (type-1, -2, and -3) terminate either in the metathoracic or, predominantly, in the mesothoracic ganglion. In contrast, the high-frequency cells (type-4) ascend to the prothoracic ganglion. The receptor fibres of the different types of receptor cells differ in diameter.