The first optic ganglion of the bee

Each visual unit (ommatidium) of the compound eye of the honey bee contains nine retinula cells, six of which end as axons in the first synaptic ganglion, the lamina, and three in the second optic ganglion, the medulla. A technique allowing light- and electron microscopy to be performed on the same silver-impregnated sections has made it possible to follow all types of retinula axons of one ommatidium to their terminals in order to study the shape of the terminal branches with their position in the cartridge. 1. The axons of retinula cells 1-6 (numbered according to Menzel and Snyder, 1974) end as three different types of short visual fibres (svf) in the lamina; the axons of retinula cells 7-9 run through the lamina to terminate in the medulla and are known as long visual fibres (lvf). Retinula cells of each type are identified by the location of their cell bodies and by the direction of their microvilli. The retinula cells 1 and 4 (group I according to Gribakin, 1967) end as svf type 1 with three tassel-like branches in stratum B of the first synaptic region. The pair of cells 3, 6 and the pair 2, 5 (group II) end in the first synaptic region in stratum A. Cells 3 and 6 have forked endings, svf type 2, whereas cells 2 and 5 have tapered endings, svf type 3. The remaining retinula cells 7, 8 and 9 have long fibres. Nos. 7 and 8 (group III) have tapered endings and are termed lvf types 1 and 2, respectively. The 9th cell is the lvf type 3 with a highly branched ending. 2. The nine axons in the bundle from one ommatidium have relative positions which do not change from the proximal retina to the monopolar cell body layer. 3. By following silver-stained retinula cells and their corresponding axons, it is possible to describe mirror-image arrangements of fibres in the axon bundles in different parts of the eye. This correlation of numbered retinula cells with specific axon types, together with the highly organized pattern in an axon bundle, allows the correlation between histological and physiological findings on polarization and colour perception.