CENTRAL PROJECTIONS OF TIBIAL SENSORY FIBERS WITHIN THE 3 THORACIC GANGLIA OF CRICKETS (GRYLLUS-CAMPESTRIS L, GRYLLUS-BIMACULATUS DEGEER)

Cobalt sulfide (CoS) staining, via axonal filling, was used to reveal the central projections of fibers of the tympanal nerves in the cricket prothoracic ganglion, as well as those of serially homonymous nerves in the mesothoracic and metathoracic ganglia. The main results are: 1. The branch of the anterior leg nerve termed the tympanal nerve (TB in Fig. 2) comprises a few motor fibers and many sensory axons, as do serially homonymous nerves in the two other thoracic segments. The sensory axons are those of sense cells in the tympanal organ (or the serially homonymous parts of the meso and metathoracic segments), the subgenual organ, the campaniform sensilla, and the tibial sensory hairs (Eibl, 1978). 2. On the basis of their branching and projections in the three thoracic ganglia, the sensory fibers of nerve TB can be classified into five types (Figs. 3-7). With respect to function, those of Type 5 can be regarded as auditory fibers (Fig. 10). They project only to the crescent-shaped region (Fig. 8), which represents the auditory neuropile in the strict sense, and are tuned to the carrier frequency of the calling song (Fig. 10B). It is characteristic of Types 1 to 4 that the fibers branch immediately after entering the ganglion (Figs. 3-6); typically, these branches run dorsally and may overlap with the lateral projections from the motor neuropile. In addition, the fibers of Types 2 to 4 project into the rostral and caudal margins of the auditory crescent (Figs. 4-6). 3. The similarity among the three thoracic ganglia with respect to sensory fiber types and projections (Fig. 11) suggests that serially homonymous sensory structures found in each of the leg-bearing segments resemble one another in their sensory projection areas and thus in their connectivity with other neural subsystems within the three thoracic ganglia. That 'typical auditory nerve fibers' occur in the meso and metathoracic segments, projecting exclusively to the crescent (Figs. 7 and 11), is anatomical evidence that auditory function need not be restricted to the first thoracic segment. 4. The stained leg motoneurons (Fig. 12), with axons that innervate the femur muscles, are distinguished by a dendritic field that must be assigned to the ventral part of the so-called sensorimotor integration zone (Altman, 1976). These fields are of lesser extent than, for example, those of the flight and singing motoneurons of meso and metathorax. 5. All the sensory and motor elements marked with CoS are bilateral and arranged with approximately mirror-image symmetry. Their central projections are restricted entirely to the ipsilateral half of the ganglion (Fig. 13). © 1979 Springer-Verlag.