Synaptic organization in the lamina of the superposition eye of a skipper butterfly, Parnara guttata

The first optic neuropil of the compound eye, the lamina, of the skipper butterfly Parnara guttata , was examined by light microscopy after Golgi‐impregnation and by electron microscopy (EM) to clarify the cellular and synaptic organization. In the lamina, five different types of lamina neurons (L neurons) were characterized by using Golgi‐impregnation. By EM, each cartridge was found to contain all nine receptor axons from an ommatidium, five L neurons, and a few putative centrifugal elements. Axons from photoreceptors (retinula cells) R2, R3, R4, R6, R7, and R8 terminate as short visual fibers (svfs) in the lamina cartridge. Those from R1, R5, and R9 penetrate the lamina and terminate in the medulla as long visual fibers (lvfs). In the cartridges, the synaptic contacts were formed from svfs onto L neurons, from the lvfs of R1 and/or R5 to the lvf of R9 and L neurons, and from the lvf of R9 to L neurons. The putative centrifugal fibers also make synapses to svfs and L neurons. At the most distal level of the cartridge, one of the centrifugal fibers containing dense‐core vesicles makes presynaptic contacts to the putative long collaterals of the L neuron. A novel characteristic feature of this lamina is that svfs of R3 and R7 and the lvfs of R1 or R5 have long collaterals extending into neighboring cartridges. Presynaptic contacts were confirmed in such long collaterals from the svf. These results imply that receptor axons provide direct intercartridge connections as well as providing indirect connections to neighboring cartridges by way of their input upon L neurons. J. Comp. Neurol. 408:107–124, 1999. © 1999 Wiley‐Liss, Inc.