Nonconventional Interactions between Photoreceptor Axons in the Butterfly Lamina Ganglionaris

Butterflies have nine photoreceptors within each ommatidium which have been named from their positional orientation across the rhabdom. Within all ommatidia ex­ amined, there are four morphologically discrete receptor types: 1. Two dorsal-ventrally aligned cells — the vertical receptors; 2. two anterior-posteriorly aligned cells — the horizontal receptors; 3. four diagonally aligned cells the diagonal receptors; 4. one basally occuring bilobed cell — the basal receptor. The nine retinula cell axons pass through the basem ent membrane and enter an optic car­ tridge of the lamina ganglionaris as a single bundle. The organization of these axons within their cartridge has been investigated in the nymphalid-type butterfly Agraulis vanillae. Prior to entry into the optic cartridge, the axonal bun­ dle undergoes a 90° positional rotation. Retinula axons be­ com e grouped into three regions: a dorsal triad and a vent­ ral triad of three axons, one horizontal and two diagonal retinula cells each; a central group composed of two verti­ cal retinula cells, the basal cell, and 4 —5 lamina monopolar cells. Retinula axons of the triads appear to be coupled by bulbous cytoplasmic projections. Two types of coupling ap­ pear to occur: 1. Within each triad, the horizontal retinula cell axon is coupled to both diagonal axons. 2. The horizon­ tal retinula cell axon o f one triad is coupled to the horizon­ tal cell of the adjacent triad. Because of the 90° reposition­ ing of the ommatidial axon bundle, specific cell types from one ommatidium appear to be linked to retinula cells of ommatidia immediately above or below them. These non­ conventional receptor cell interactions are described and a possible function in receptor coupling suggested.