The first optic ganglion of the bee. III. Regional comparison of the morphology of photoreceptor-cell axons.

The nine receptor cells in each ommatidium of the worker bee end as six short visual fibres in the lamina and as three long visual fibres in the medulla. Behavioural and physiological evidence for regional variation in spectral sensitivity prompted observations on the morphology of the visual units. The distribution, branching pattern, diameter and the arrangement of axonal protusions of the characteristic receptor-cell axons were studied in various regions of the lamina. The six short visual fibres and two of the long visual fibres in each laminar cartridge are uniform over the total eye surface. Only the receptor axons of the ninth cell a UV and polarised light-sensitive cell, show obvious regional variation. In view of the regional constancy in morphology of eight of the nine receptor-cell axons, the regional variations in spectral sensitivity demand either functional subdivision of morphologically indistinguishable photoreceptors (e.g., content of different visual pigments) or a highly complex connectivity pattern of their axons in the first optic ganglion.