J. comp. Physiol. 95, 185--201 (1974) 9 by Springer-Verlag 1974 Anatomy of Locust Ocellar Interneurons : Constancy and Variability Corey Goodman Department of Zoology, University of California, Berkeley, California, USA Received July 25, 1974 Summary. 1. The anatomy of the ocellar interneurons in the brain of the locust, Schisto- ce~va 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 intcrneurons (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 inter- neurons 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 character- istic 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 ceils 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 oceuring cells (Fig. 9). Introduction Most insects possess simple cup-shaped eyes, called dorsal ocelli, in addition to their large compound eyes. They consist of a common lens, a few hundred receptor cells, and a synaptic zone. The receptor cells synapse with the processes of over 30 ocellar interneurons, whose axons form the ocellar nerve which extends from the eye to the protocerebrum. Extracellular recordings have been made from the ocellar nerves (ttoyle, 1955; Ruck, 1961; Metschl, 1963; Chappell and Dowling, 1972; Rosser, 1974), from unknown ocellar units in the insect brain (Horridge et al., 1965; Mimura et al., 1969, 1970), and from unknown ocellar units in the ventral nerve cord (e.g. Goodman, 1971a, b; Cooter, 1973). Intracellular recordings have been made from receptors and unidentified post-synaptic units in the dragonfly median ocellus (Chappell and Dowling, 1972). One recent study (l~osser, 1974) has sug- gested that the lateral ocellar nerve of the dragonfly contains at least one large efferent fiber in addition to both large and small afferent fibers. Physiological studies have suffered from a lack of anatomical information. Though the ocelli themselves have been the subject of numerous anatomical studies (Cajal, 1918; Ruck and Edwards, 1964; Goodman, 1971b; Toh et al., 13 5. comp.Physiol., Vol.95