Graded illumination potentials from retinula cell axons in the bug Lethocerus

Glial membrane specializations and the compartmentalization of the lamina ganglionaris of the housefly compound eye

Membrane specializations in the lamina ganglionaris of the housefly are investigated using conventional thin-section EM, freeze-fracture replication and the diffusion of colloidal lanthanum. All glial cells in the lamina are coupled by gap junctions. Desmosomes also link all glia except the epithelial glia. Extensive glia-glial and glia-neuronal septate junctions are present in the pseudocartridge zone and nuclear layer. Septate junctions in the nuclear layer intermingle with bands of interglial and glia-neuronal tight junctions. Tight junctions are also found between satellite and epithelial glia at the border of the nuclear and plexiform layers, between adjacent epithelial glial cells in the plexiform layer, between epithelial and marginal glia at the proximal boundary of the optic neuropil, between marginal glial cells, and between marginal glia and axons. Colloidal lanthanum, introduced through an incision in the cornea, penetrates the retina but is occluded from the neuropil by septate junctions in the pseudocartridge zone. The disposition of tight and septate junctions is described in relation to the compartmentalization of the lamina. Two major compartments are delineated. The first represents the nuclear layer and contains the cell bodies of second-order visual neurons (monopolar neurons). The second compartment constitutes the plexiform layer of the lamina. Within the plexiform layer, each optic cartridge is partitioned into a separate subcompartment. Also, tracheoles and axons of long visual fibres are isolated from the optic cartridges by glial tight junctions. Morphological evidence for compartmentalization is correlated with previously established electrical properties of the insect lamina ganglionaris.

The organization of visual fields in the hemipteran acone eye

The angle between optical axes of adjacent photoreceptors in a single ommatidium ofLethocerushas been measured by optical methods and confirmed by electrophysiological recording and the optical system by which this comes about has also been demonstrated by tracing rays through a model eye in which the relevant optical constants have been determined. In the dark-adapted anterior part of the eye, in air, the inclination between rhabdomere axes within a single ommatidium is approximately 10° but the interommatidial angle is 2.5°-3.0°. Therefore each receptor sees the same field as a particular pattern of other receptors 3-4 facets away in each direction. Although adjacent rhabdomeres lie side by side, they have separate fields of view, which reach 10° wide at the 50 % contours in the dark-adapted state. In the light-adapted eye the cone extends as a crystalline tract between the corneal cone and the rhabdomeres. The fields of view of all receptors of an ommatidium now coincide. The changes upon adaptation imply an interesting problem as to how the projection of the outside world can apply to both the light-adapted and the dark-adapted eye. Apparently cells 7 and 8 form aphotopicsystem which retains acuity in both states of adaptation, and cells 1-6 are functional only as a high-sensitivity large fieldscotopicsystem.