The tiered retina of Dytiscus: a new type of compound eye

The superposition eye of skipper butterflies

1. The anatomy of the eye is described in seven representative genera of Australian Hesperioidea. Between the crystalline cones and the long rhabdom is a wide clear zone crossed by narrow extensions of the retinula cells. The distal pigment remains between the cones even in daylight. 2. No evidence of functional light guides crossing the clear zone could be found. 3. A real erect image is formed on the receptor layer. The acuity and origin of this image were investigated by several methods. 4. The angular sensitivity curve of the receptors is 6 to 8°wide at the 50% sensitivity contour. 5. Optomotor experiments with stripes of differing widths show that the angular sensitiv­ity of the receptors is fully utilized behaviourally, and that the eye functions in relatively dimlight. Adaptation changes are small. 6. A parallel beam falling on the eye reaches a single receptor via a circular patch of facets subtending about 30° at the centre of the eye. This was directly demonstrated by recording from a retinula cell and stimulating the eye by a moveable slit of parallel rays. This also demonstrates the wide acceptance angle of the exposed ends of the rhabdom columns. 7. When the eye is illuminated by a parallel beam, light is reflected back out of the eye. All the reflected light is contained in an angle of ±5° to the incident beam, although it enters and emerges via the large patch of facets mentioned above (6). Again, no effects of adaptation were observed. 8. The relation between the direction in which a ray enters the optical system of the cornea and cone and the direction it leaves was measured directly by a rotatable microscope of narrow aperture. 9. The mechanism by which the optical system forms the erect image on the receptor layer was demonstrated by tracing rays through scale drawings of the components. To do this the refractive index was measured in all components. The corneal surface acting as a lens forms the first image within the cone. The crystalline cone is non-homogeneous and acts as second lens in each ommatidium. 10. The skipper eye therefore illustrates Exner’s superposition principle, and it does so in daylight.

Movement on dark–light adaptation in beetle eyes of the neuropteran type

Beetles of several species belonging to the families Carabidae, Dytiscidae, Gyrinidae and Hydrophilidae have an eye of the neuropteran type which is characterized as follows. In the dark-adapted state a long column formed by retinula cells (in these families numbering seven) stretches from the cone tip to the rhabdom layer. In the light a crystalline tract, formed from the outer layer of the cone, extends about 100 µ m from the cone and is surrounded by pigment cells. Scarabaeid beetles examined are similar but lack the distal rhabdomere always found in the above groups. All have a basal retinula cell with rhabdomere. In the scarabaeids the retinula cell columns have a content of solids greater than the surrounding cells, suggesting that they act as light guides across the clear zone.

Alternatives to superposition images in clear-zone compound eyes

The clear zone between the cones and the receptor layer in dark -adapted eyes of insects that are active in dim light has formerly been explained as a space to allow formation of a superposition image. Although erect images have been seen in Ephestia (Lepidoptera) and Hydrophilus (Coleoptera), new experiments show that they are accompanied by scattered light and that the angular sensitivity of individual receptors must be wide in the dark-adapted state. Alternatives to the superposition theory are examined, and it is concluded that in eyes with crystalline cones the clear zone (in general, in the numerous shapes and sizes of eyes of nocturnally active insects) enables light entering by many facets to sum upon individual receptors on the far side of the clear zone. In addition to the scattered light effect, light is carried across the clear zone in crystalline tracts or retinula cell columns, which provide a separate optical pathway for each ommatidium also in the light-adapted state.

Fine structure of the compound eyes and interfacetal mechanoreceptors of Cicindela tranquebarica Herbst (Coleoptera: Cicindelidae)

The structure of the compound eyes of adult Cicindela tranquebarica Herbst was examined by use of light, scanning, and transmission electron microscopy. Each ommatidium of these photopic eyes is eucone and has a "subcorneal layer" situated between the corneal lens and crystalline cone. A distal rhabdomere consisting only of microvilli from retinula cell seven, a more proximal, rectangular, fused rhabdom formed from six retinula cells, and a basal, eighth retinula cell with a spherical rhabdomere comprise the light sensitive portions of the ommatidium. The "subcorneal layer" consists of lamellae of endocuticular microfibrils and, in surface view, shows 11 concave polygons. Proximal extensions of the crystalline thread form inter-retinular fibres containing microtubules between retinula cells 1/2, 3/4, 5/6, and 7/1. The primary pigment cells are devoid of pigment granules, but are rich in rough endoplasmic reticulum. Proximal to each retinula cell nucleus are two basal bodies, one perpendicular to the other. The more proximal basal body extends two fibrillar feet proximally which fuse to form a horizontally-banded ciliary rootlet extending the length of the retinula peripheral to the rhabdom. Each ommatidium is surrounded by 16 secondary pigment cells. Interfacetal mechanoreceptors between some adjacent lenses each have a single bipolar neuron, with a dendritic sheath, tubular body, cilium, outer and inner sheath cells, and an axon surrounded by a neurilemma sheath cell.

The ultrastructural organization of the visual system of the wax moth, Galleria mellonella: the retina

The ultrastructural organization of ommatidial components of the retina of the moth. Galleria mellonella are described from electron microscopic observations. Each ommatidium is composed of 12 common retinula cells and one basal eccentric cell. The retinula cells are connected together by a desmosomal strip along their length. The rhabdom occupies the basal thirty percent of the ommatidium and can be divided into nine segments of parallel microvilli. Several cells may contribute to an individual rhabdomere. The rhabdomeres are arranged in a cross with single cell rhabdomeres lying between the arms of the cross. Thin sections of ommatidium absorb polarized light differentially. The total amount of plane polarized light absorbed varies with angle of rotation for an entire ommatidium but there are also differences between the amount of absorption of adjacent rhabdomeric segments. Galleria appears to be the only lepidopteran in which the possibility of the polarized light reception has been reported.

Ultrastructure of the compound eye of the diploid female beetle, Xyleborus ferrugineus

The compound eye of female (diploid) Xyleborus ferrugineus beetles was examined with scanning and transmission electron microscopy. The eye is emarginate, and externally consists of roughly 70-100 facets. Each ommatidium is composed of a thickly biconvex lenslet with about 50 electron dense and rare layers. The lens facet overlies a crystalline cone of the acone type which is roughly hourglass-shaped. Pigment cells envelop the entire ommatidium, and pigment granules also are abundant throughout the cytoplasm of the 8 retinular cells. The rhabdomeres of 2 centrally situated photoreceptor cells effectively fuse into a rhabdom that extends from the base of the crystalline cone deeply into the ommatidium. Six distal peripheral retinular cells encircle the 2 central cells, and their rhabdomeres join laterally to form a rhabdomeric ring around the central rhabdom. The rhabdom and rhabdomeric ring are effectively separated by the cytoplasm of the two central retinular cells which contains the usual organelles and an abundance of shielding pigment granules. Eight axons per ommatidium gather in a tracheae-less fascicle before exiting the eye through the fenestrate basement membrane. No tracheation was observed among the retinular cells. Each Semper cell of each observed crystalline cone contained an abundance of virus-like particles near the cell nucleus. The insect is laboratory reared, and the visual system seems very amenable to photoreceptor investigations.