Direct connections between the R7/8 and R1-6 photoreceptor subsystems in the dipteran visual system

Musca and related flies have three main photoreceptor subsystems. The R1-6 group has short axons that terminate in the cartridges of the first optic neuropile, the lamina. The cartridges are bypassed by the longer axons of R7 and R8, which run together to terminate at different levels in the underlying medulla neuropile. The present account describes a shallow, previously unidentified zone in the lamina within which R7/8 make glancing contact with R1-6. At the distal border of the cartridge over no more than 3-4 microns depth, the tangentially directed short axon of R6 squeezes between the pair from R7 and R8, forming quite large areas of mutual contact (approximately 7 microns2). Less frequently, R1 is contacted. At least some of these sites contain smaller membrane specialisations indistinguishable from the more numerous gap junctions found more proximally that interconnect the terminals of R1-6. The R7/8 junctions with R6 are of comparable size (0.15 micron 2) and likewise possess symmetrical membrane densities. They provide proposed pathways for direct electrical interaction to account for observed electrical input from R7/8 to the R1-6 subsystem. In two cases R7/8 was possibly postsynaptic to R1-6 at a multiple-contact synapse, but even if functional, these sites were so rare that they are unlikely to have much operational significance.