PHOTORECEPTOR STRUCTURES. THE RETINAL CELLS OF THE COCKROACH EYE

The retinal structures of the compound eyes of insects are being investigated as part of a comparative study of the structure and biochemistry of visual organs (1 5). The compound eye of the insect is composed of numerous ommatidia, each of which consists of from four to eight retinula cells. Each retinula cell contains a differentiated structure , the rhabdomere, a photoreceptor. The rhab-domeres are analogous in function to the retinal rods of the vertebrate eyes. One of the more primitive of the unspecialized insects is the cockroach. It has been suggested from electrophysiological studies that the cock) roach eyes contain two types of visual photore-ceptor systems (6, 7). It was, therefore, of interest to us to determine the fine structure of their rhabdo-meres and to isolate their photosensitive pigments in order to make a comparison with the photore-ceptors of other insects and invertebrate compound eyes. Two common, large species of cockroaches, Periplaneta americana and Blaberus giganteus, made available to us from the Gulf Research Laboratory, were used in this study. 1 MATERIALS AND METHODS The cockroaches, Periplaneta americana and Blaberus giganteus, were dark-adapted and their eyes excised in dim red light. The eyes were immediately dropped into a solution of I per cent osmium tetroxide, buffered with veronal-acetate pH 7.0 (8). Under a binocular microscope each eye was dissected into a lower half and an upper half and returned to the fixative for 4 hours at 5°C. After fixation, the eyes were washed with buffer and distilled water, and then dehydrated by a graded water-ethanol series. While in 75 per cent alcohol, muscle, tracheae, fat bodies and other extraneous eye tissues were cut away. The completely dehydrated eye parts were then carried through ethanol-n-butyl methacrylate mixtures (9 parts n-butyl methacrylate to 1 part methyl metha-crylate) to a pure methacrylate monomer, embedded 1 Thanks to Dr. A. Mallis, Gulf Research Laboratories , for the cockroaches used in these studies. in gelatin capsules, and polymerized by heat. Using a Servall (Porter-Blum) microtome with a glass knife, sections less than 0.05 # thick were cut from the polymerized blocks. The sections were mounted on formvar or carbon-coated grids and examined with a Philips EM-100A electron microscope. Transverse and longitudinal sections, 2 to 5 # in thickness, were also cut from the tissue blocks for light micros-copy. OBSERVATIONS The compound eye of the cockroach is composed of approximately 2,000 ommatidia that are separated …