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Butterflies have nine photoreceptors within each ommatidium which have been named from their positional orientation across the rhabdom. Within all ommatidia ex­amined, 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 basement membrane and enter an optic car­tridge 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 bun­dle undergoes a 90° positional rotation. Retinula axons become grouped into three regions: a dorsal triad and a vent­ral triad of three axons, one horizontal and two diagonal retinula cells each; a central group composed of two verti­cal retinula cells, the basal cell, and 4 -5 lamina monopolar cells. Retinula axons of the triads appear to be coupled by bulbous cytoplasm ic projections. Two types of coupling ap­pear to occur: 1. Within each triad, the horizontal retinula cell axon is coupled to both diagonal axons. 2. The horizon­tal retinula cell axon of one triad is coupled to the horizon­tal cell of the adjacent triad. Because of the 90° reposition­ing 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 non­conventional receptor cell interactions are described and a possible function in receptor coupling suggested.

Nonconventional Interactions between Photoreceptor Axons in the Butterfly Lamina Ganglionaris