Regional distribution of three ultrastructural retinula types in the retina of Cataglyphis bicolor Fabr. (Formicidae, Hymenoptera).

The retina of Cataglyphis bicolor was investigated by electron microscopy. Three types of structurally distinct retinulae were found and mapped throughout the compound eye: Type I is composed of four unpigmented thin cells, four larger pigmented cells as well as a basal ninth cell. Its rhabdom possesses a round cross section and four microvilli directions. This type occupies most of the dorsal two-thirds of the retina. Type II consists of two thin cells, two intermediate cells and four large cells. A basal ninth cell is also present; the rhabdom is as in type I. Type II retinulae are located in the ventral third of the retina. Type III ommatidia are unique within the Hymenoptera: there are four large pigmented cells, four thinner unpigmented cells and a basal ninth cell. The rhabdom, however, has a dumb-bell shaped cross section; two small cells lie at its opposed extremities and the remaining six cells have mutually perpendicular microvilli orientations. This type of retinula is found at the dorso-medial eye margin. Serial sectioning in this region revealed a conical shaped rhabdom without any torsion along the longitudinal axis. The rhabdomere cross section was calculated from distal and proximal thin sections. Angular statistics were applied to the microvilli directions of all three ommatidial types to determine the degree of order. A possible functional significance of the structural specializations of the different eye regions is discussed.