Comparison of morphology and physiology of two plurisegmental sound-activated interneurones in a bushcricket

The unusual morphology of a sound-activated plurisegmental ascending intern eurone (AN5-AG7) in an insect (Ancistrura nigrovittata, Ensifera, Phaneropt eridae) is described. This neurone's soma is located in the penultimate abd ominal ganglion. The most prominent arborisations with smooth endings are f ound in the prothoracic ganglion. The neurone terminates with numerous bead ed endings in the brain (protocerebrum). All abdominal ganglia including th e penultimate contain only tiny side branches of beaded appearance. The neu rone's morphology is compared to the morphology of a 'typical' sound-activa ted plurisegmental neurone of bushcrickets with its soma in the prothorax. In the prothoracic ganglion and in the brain the arborisations of the two c ells are very similar. Graded potentials and action potentials are generate d in the prothoracic portion of both neurones. Both receive excitation main ly by ultrasound, and inhibition by soma-ipsilateral stimuli. Neither wind, substrate vibration nor touch of the abdomen evoke responses in AN5-AG7. I t is assumed that early in evolution this neurone had its dendrites in the ganglion which houses the cell body (like cereal interneurones of this neur omere). Profound evolutionary changes probably have taken place to bring ab out this neuron's modern morphology.