The lamina monopolar cells in the optic lobe of the dragonfly sympetrum

The connectivities of five monopolar cells, M I-M V, within the ventral cartridge of the lamina of the dragonfly Sympetrum have been analysed from serial electron microscopy and their morphologies confirmed from Golgi-electron microscopy. The results of synaptic analyses are presented from a single cartridge photographed in its entirety in one series of transverse sections through the complete depth of the lamina and corroborated from shorter series of sections of additional cartridges. Each monopolar cell is defined by and identified from the location of its soma and the characteristic position of its axon in the cartridge cross section. M I and M II are two axial monopolar cells with large-calibre axons, while axons of M III-M V are slender and occupy polar positions, M III and M IV next to M I, M V next to the long visual fibres R 6 and R 7. M I and M II contribute postsynaptically at the triad synapses of all six retinular terminals, M I contributing exclusively at its dendrites, which number about 50 % more than those of M II. The distribution of M I and M II dendrites in general reflects the geometry and extent of synaptic engagement with the surrounding retinular terminals. In addition M II is postsynaptic at synapses of the long visual fibres R 6 and R 7, thus receiving a comprehensive and exclusive receptor input; it is only postsynaptic in the lamina. M I, on the other hand, forms an output back upon certain of its retinular inputs and upon M IV. M III too forms an important output upon M IV and it receives a selective retinular input from R 1 at synapses that are the focus of an unexpected asymmetry within the cartridge. M V, like M III, receives a selective retinular input (from R 7) while M IV receives its retinular input only indirectly, from both M I and M III. M IV and M V, like M II, have no output within the lamina. Finally, all monopolar cells excluding M II receive input from an unidentified cell type or types, called a, an input that for M I and M III is reciprocal. To judge from the diversity of their synaptic configurations, the numbers of their dendrites and probably the numbers of their synapses too, the monopolar cells form a sequence in ascending richness M V -M I. Definite parallels exist between, respectively, M I and M II of Sympetrum and L 2 and L 1 of Musca and Apis and between M III of Sympetrum and L 3 of Apis ,but further homologies are unclear