The ultrastructural organization of the visual system of the wax moth, Galleria mellonella: the optic tract.

The ultrastructural organization of the axons of retinula cells of the eye of the wax moth Galleria mellonella are described. The axons traverse an appreciable distance between the basement membrane of the retina and the lamina ganglionaris of the optic lobe of the brain. The optic tract was reconstructed from serial thin sections. Axons emanating from a single ommatidium are closely associated together in the optic tract. Adjacent cartridges fuse together to form large clusters of axons (8 to 10 cartridges). There is further coalescence between these large clusters. Extracellular space within the optic tract is severely limited and axons are sheathed by glial lamellae. Extracellular space between the axons and glia has been measured between 50 and 120 A. Calculations are presented that suggest that the glial interstices between the axons could increase the space constants of the axons significantly. Potentials could be transmitted along the length of the axons with between 59 to 37% decremental decrease, depending upon the number of glial interstices.