DISTRIBUTION AND MORPHOLOGY OF DESCENDING BRAIN NEURONS IN THE CRICKET GRYLLUS-BIMACULATUS

The number and distribution of descending brain neurons have been inve stigated in the cricket. The results are based on retrograde labeling of these cells with either Lucifer yellow or Neurobiotin via whole or small split portions of the cervical connectives. Various groups of ce lls and single neurons have been identified, and the morphology of mor e than 40 cells is described. Nearly 200 descending brain neurons can be stained via one cervical connective. Their perikarya are concentrat ed in clusters that occur ipsi- and contralateral to the filled connec tive and that lie dorsal and ventral in the brain. Descending cells on ly arborize in the nonglomerular neuropils of the brain and never bran ch in the optic lobe. Cells descending ipsilaterally never arborize in the contralateral hemisphere, whereas contralateral descending neuron s often branch in both hemispheres. Irrespective of soma position, cel ls can arborize in the ventral and/or dorsal neuropils of the brain. N eurons with somata in the protocerebrum often have branches in the deu tocerebrum and vice versa. The main arborizations of the cells from th e prominent ventral i5 group are found in the same part of the protoce rebrum. In contrast, various cells arborize in the ventral posterior d eutocerebrum but their somata are not located in different clusters. T hus, neurons from the same cluster may, but need not necessarily, arbo rize in the same brain area.