The structure of the eye ofPieris brassicae L. (Lepidoptera)

Abstract

The apposition eye of the large Cabbage Butterfly,Pieris brassicae L., was investigated electronmicroscopically (TEM). Diagrams based on micrographs show details of the structure of an ommatidium. The dioptric apparatus is made up of cornea, corneal process, and crystalline cone. The sensory part consists of nine visual cells. The retina is tiered. Four distal visual cells (type I) participate in forming the distal rhabdome, the proximal part being formed by four proximal visual cells (type II) with proximal main sections. There are ultrastructural differences between these two types of receptors. The basal ninth visual cell forms, if any, only stubby microvilli that have no part in the formation of the ‘banded rhabdome’. There is no gap between the distal and proximal parts of the rhabdome, but the two parts differ morphologically. In the distal part the rhabdomeres do not interlock at right angles and are, for the most part, not exactly perpendicular to the optical axis of the ommatidium; their microvilli may be bent and sometimes extend more than halfway across the rhabdome. In the proximal rhabdome the microvilli are straight and parallel, though they too vary in extent across the rhabdome. The microvilli plates of neighbouring rhabdomeres interlock here at right angles. Basally the rhabdome ends at a large tracheal fork that acts as a tapetum. There are five different types of pigment granules in the ommatidium each belonging to a particular cell type. Distal and proximal visual cells each have one type of pigment; the other three types of pigment granules belong to two principal, six secondary, and six basal pigment cells, respectively. Each distal process of the secondary pigment cells contains microtubule bundles and is joined horizontally to the processes of neighbouring secondary pigment cells. Bundles of microtubules running parallel to the optical axis are found in the primary pigment cells, too. The ultrastructure is described, and its relevance for the eyes function, especially in the ultraviolet wavelength-range, is discussed.