The long visual fibers of the dragonfly optic lobe: their cells of origin and lamina connections

Abstract

A pair of long visual fibers projects from each ommatidium of the compound eye of the dragonfly Sympetrum. They arise from retinular cell R7 and its slender partner R6. To investigate the segregation of lamina input pathways between these and the well-described short retinular terminals, we have examined the cell morphology and en passant synaptic involvements of R7 and 6 in the ventral lamina using serial-EM and combined Golgi-EM. In the retinula, R7 has a large apical rhabdomere with microvilli aligned along the animal's horizon, and a basal axon. R6 has a few microvilli at all depths, which align with those of neighboring rhabdomeres. In the lamina, R7 and 6 neighbor on MV, the fifth monopolar cell. The stout axon of R7 has many diffusely distributed spines which contact the neighboring terminals of R5 and 8, and MII. In one entire cartridge R7 was presynaptic to MII at 24 dyads and triads throughout the lamina and also to MV at dyads exclusively in the distal lamina. Another class of element which derives from the unidentified processes called alpha was also postsynaptic to R7 at dyads, and, mostly in the proximal lamina, was reciprocally presynaptic to R7, along with R5 and 8. R6 is slender with a single conspicuous spine. At a total of three zones in the proximal lamina and distal chiasma it was presynaptic at 38 dyads--also upon MII and MV and often in combination with other elements; it was not postsynaptic in the lamina. R6 and 7 thus provide the sole input to MV and contribute to the general retinular input to MII. They form the lowest ratios of triad:dyad synapses of all lamina retinular elements. Comparison is made with the long visual fibers of other arthropods but known examples are too diverse to detect functional commonalities.