Position-specific central projections of mechanosensory neurons on the haltere of the blow fly, Calliphora vicina.

The halteres of Dipteran insects play an important role in flight control. They are complex mechanosensory devices equipped with approximately 400 campaniform sensilla, cuticular strain gauges, which are organized into five fields at the base of each haltere. Despite the important role of these mechanosensory structures in flight, the central organization of the sensory afferents originating from the different field campaniforms has not been determined. We show here that in the blow fly, Calliphora vicina, sensory afferents from the campaniform fields project to the thorax in a region-specific manner. Afferents from different fields have different projection profiles and in addition, the projection pattern of afferents from different regions of the same field may show further variation. However, central target regions of these afferents are not exclusive to particular sensory fields because cells from different fields can possess similar projection profiles. Convergence of afferent projections is not limited to axons from the haltere fields, but is also observed between afferents originating from the haltere fields and those from serially homologous fields on the radial vein of the wing. Although we have not determined the specific cellular targets of the haltere sensory cells, the afferents of a dorsal field could make potential contact with at least one identified wing steering motoneuron that is known to be important in turning maneuvers. Our results, thus, provide the anatomical basis for studying how mechanosensory information encoded by the complex fields on the base of the haltere is mapped onto different functional regions within the CNS.