Microvillar Orientation in the Retina of the Nymphalid Butterfly

Abstract

The photoreceptor cells of the nymphalid butterfly, Agraulis vanillae, have been structurally characterized. Four distinct types of retinulae can be found in each ommatidium.Two vertically oriented cells contribute microvilli to the to rhabdom only in the distal retina. These microvilli are or-ha]dered into discrete packets. Although the distal-most micro­villi enter the rhabdom in a dorsal-ventral axis, these rhabdomeral packets begin altering their direction of orientation becoming directed at 45° to the vertical alignment of the cells. Abrupt alternations in orientation produce microvillar packets oriented to each other at approximately 90° along the distal portion of each vertical cell. The vertical reti­nulae lose their microvilli at mid-retina and become axonal.Four retinular cells, oriented diagonally across the ommati­dium, contribute microvilli to the rhabdom aligned at 45° to the vertical axis. These diagonal cells produce microvilli throughout the depth of the retina. Two horizontally ordered photoreceptors produce microvilli aligned along a horizontal axis, these cells contributing rhabdomeres along their entire length. A ninth bilobed eccentric cell is arranged in a vertical plane in the basal region of the ommatidium. A few short microvilli are added to the rhabdom from each lobe.These are oriented in a vertical direction.