Anatomy of locust ocellar interneurons: Constancy and variability

Abstract

1. The anatomy of the ocellar interneurons in the brain of the locust,Scchstocerca vaga, was revealed by axonal iontophoresis of cobalt chloride. The normal bilaterally symmetric anatomy, as represented in 41 of 50 preparations, includes 17 large and identified ocellar interneurons (Fig. 1). Each ocellar nerve contains the axons of 7 large interneurons (Fig. 2), 4 of the interneurons having axons in two ocellar nerves. 2. The anatomy of each large interneuron is described (Fig. 5). Only one, with an axon in the median ocellar nerve, projects bilaterally; all others are confined to one side of the brain and have contralateral homologues. Two interneurons run between each lateral ocellus and the median ocellus. Their axons run through the brain without branching, except for the neurite, and they may be the “efferent” units previously described. Several pairs of interneurons are described with effectively identical gross morphology. 3. Individual variation of the same cell in different preparations is described (Fig. 6), showing constancy of general shape but with variation in fine branching patterns characteristic of most preparations. 4. Major asymmetric variations were found in 9 of 50 preparations, including cases in which axons of identified cells extended into lobes of the brain in which they normally were not found (Fig. 7), cases in which normally occuring cells sent extra axons into ocellar nerves in which they normally were not represented, and cases in which extra cells occured which were anatomical duplicates of normally occuring cells (Fig. 9).