Microvillar orientation in the photoreceptors of the ant Cataglyphis bicolor

Polarization sensitivity in arthropod photoreceptors is crucially dependent on the arrangement of the microvilli within the rhabdom. Here, we present an electronmicroscopical study in which the degree of microvillar alignment and changes in the cross-sectional areas of the rhabdoms along their lengt h were studied in the compound eye of the desert ant, Cataglyphis bicolor. Serial cross-sections through the retina were taken and the orientation of the microvilli was determined in the photoreceptors of individually identif ied ommatidia. The reconstructions of microvillar alignment were made in th e three anatomically and functionally distinct regions of the Cataglyphis c ompound eye: the dorsal rim area (DRA), the dorsal area (DA), and the ventr al area (VA). The following morphological findings are consistent with pola rization sensitivities measured previously by intracellular recordings. (1) The microvilli of the DRA photoreceptors are aligned in parallel along the entire length of the cell from the distal tip of the rhabdom down to its p roximal end, near the basement membrane. The microvilli of the retinular ce lls R1 and R5 are always parallel to each other and perfectly perpendicular , with only minor deviation, to the microvillar orientation of the remainin g receptor cells. (2) In the DA and VA regions of the eye, the microvillar tufts of the small receptors R1, R3, R5, R7, and R9 change their direction repetitively every 1-4 mu m for up to 90 degrees. In contrast, the large re ceptor cells R2, R4, R6, and R8 maintain their microvillar orientation rigi dly. (3) In the DRA ommatidia, the cross-sectional areas of the rhabdomeres do not change along the length of the rhabdom, but substantial changes occ ur in the DA and VA ommatidia.