A pair of compound eyes is the principle visual organs of most insects; they are found in nearly all adults and in many immatures of ametabolous and hemimetabolous orders. As the name suggests, compound eyes are composed of many similar, closely-packed facets (called ommatidia) which are the structural and functional units of vision. The number of ommatidia varies considerably from species to species: some worker ants have fewer than six while some dragonflies may have more than 25,000.
Externally, each ommatidium is marked by a convex thickening of transparent cuticle, the corneal lens. Beneath the lens, there is often a crystalline cone secreted by a pair of semper cells. Together, the lens and the crystalline cone form a dioptric apparatus that refracts incoming light down into a receptor region containing visual pigment. The light-sensitive part of an ommatidium is called the rhabdom. It is a rod-like structure, secreted by an array of 6-8 specialized neurons ( retinula cells), and centered on the optical axis just below the crystalline cone. The rhabdom contains an array of closely packed microtubules where light-sensitive pigments (e.g. rhodopsin, etc.) are stored. These pigments absorb certain wavelengths of incident light and generate nerve impulses through a photochemical process similar to that of vertebrates. The whole ommatidium is surrounded by a curtain of pigmented cell; the “primary iris cells” covering the crystalline cone, and the “secondary iris cells” investing both the primary iris cells and the retinula. The proximal extremities of the ommatidium rest on a fenestrated basement membrane through which the fibres and tracheae run; the nerve fibres pass to the perioptic or outermost tract of the optic lobe of the brain.Most diurnal insects have pigment cells surrounding each ommatidium. These cells limit a facet's field of view by absorbing light that enters through adjacent corneas. Each facet points toward a slightly different part of the visual field -- in composite, they render a mosaic-like impression of the environment. Nocturnal and crepuscular insects have pigment cells that do not completely isolate each facet. Their ommatidia are stimulated by light from larger fields of view. This produces a brighter but theoretically less distinct mosaic image.
The rod like rhabdom is of highly reffractile material; light entering it will therefore be totally reflected from the walls, and thus it forms an ideal structure to receive and conduct light withought loss, even in an eye like that of Simulium in which the rhabdom are curved. This property will also reduce the effect of light reaching the rhabdom obliquely. In the region of the basement membrane there are, in addition, tracheal branches the rhabdom, and the nervous elements are doubly stimulated. The tracheal tapetum is particularly well developed in Noctuids and other nocturnal insects; in these the tracheae divide up at the base of the retina into innumerable fine branches running a parallel course between the retinulae. Although the rhabdom is described as though it were composed of homogenous material it is in fact a complex structure. It is formed by the juxtaposition of the ‘rhabdomeres’ of the retinula cells. The rhobdomere represents the microvilli, or ‘honey-comb border’, which cover the free surface of the retinula cell. The most wide spread photoreceptor substance in the animal kingdom is rhodopsin, formed by the union of ‘retinene’ with protein. Retinene is carotinoid pigment, the aldehyde of vitamin A alcohol. Retinene, partly bound to protein, has been isolated from the head of Apis and other insects: Musca, Locusta, Uropetala(Odonata), Cicindela and Nyctemera(Lepidoptera).
Optical mechanism of the compound eye:
As was originally suggested by Johannes Muller(1829) in his so called ‘mosaic theory’, each ommatidium receives the impression of a luminous area corresponding to its projection on the visual field; and it is the Juxtaposition of all these little luminous areas, varying in the intensity and quality of the light composing them, which gives rise to total erect image perceived by the insect. Since insects cannot form a true (i.e. focused) image of the environment, their visual acuity is relatively poor compared to that of vertebrates. On the other hand, their ability to sense movement, by tracking objects from ommatidium to ommatidium, is superior to most other animals. Temporal resolution of flicker is as high as 200 images/second in some bees and flies (in humans, still images blur into constant motion at about 30 images/second). They can detect polarization patterns in sunlight, and discriminate wavelengths in a range from ultraviolet to yellow (but not red).
The Dioptric apparatus:
The cornea and crystalline cone have rather special optic properties. They have a laminated structure turned inwards and can be pictured as a system made up of series cones with their apices turned inwards superimposed on one another. Examination with microrefrctometer shows that in this system the refractive index is at a maximum at the axis and decreased progressively towards the periphery. From optical stand point the system can be represented by a series of superimposed cylinders whose refringence increases towards the axis, a system called a “lens-cylider”.
Formation of images by Apposition:
In most diurnal insects, Hymenoptera, Diptera, Odonata, many Coleoptera , the cones are surrounded by pigment up to their posterior extremities; they allow the light emerge only at the central point; and the retinulae are short and placed immediately behind the cones. A reversed image of small part of the visual field is formed where the retinula comes in contact with the apex of the cone and has been observed in various insects. But this image has no physiological significance, it merely impresses the retina as a simple luminious point, the apposition of all such points as perceived by the different ommatidia forming the erect image perceived by the compound eye as a whole.
Formation of images by Superposition:
In many nocturnal insects, Lampyridae and other beetles, Noctuidae and other Lepidoptera, the ommatidia are greatly elongated. The retinulae do not lie immediately behind and in contact with the cones but are separated from them by a long interval in contact with the cones, but are separated from them by a long interval occupied by a non-reffractile transparent medium; and the pigment is the iris cells may be concentrated in front between the crystalline cones. The optical system is same as the “lens-cylinder” twice as long as its focal length. Hence the given rhabdom receives rays of light not only through its own facet but also through neighbouring facet. The formation of image in eye is due to a group of bundles of light, each coming through an adjacent facet. So that as focus is lowered the image splits up into number of luminous points. As many as 30 neighbouring ommatidia may unite to concentrate the light in this way upon a single rhabdom. The Image projected will be erect, but there will be no physiological significance. The rhabdom receives only a visual stimulus which is presumably the mean of the components of this image. Since each of these elemental images is formed by the superposition of the light from number of adjacent facets, the compound image as received by the entire retina is termed a “Superposition image”.
In the excised eye of locusta, besides the first (apposition) image, second and third images are formed by the diffraction can be seen in the deeper layers of the retina. These images are of the superposition type; they are formed by the diffraction of light received from the group of adjacent facets. It seems that the rays that contribute to them are not excluded by the investigating pigment of the apposition eye. Indeed there is evidence that Calliphora the wave-lengthof the red end of the spectrum can increase sensitivity. It is exceedingly sensitive to very small movements in the surroundings.
Other type of compound eye:
There are doubtless many intermediate stages between these two types of image formation. In butterflies, for example, which form retinal images of the apposition type, the image formed at the apex of each cone is erect; so that in some eyes of this type the lens and cone may evidently act together as a lens-cylinder of twice its own focal length. This mainly applies to “eucone” and “exocone” eyes with well developed cone. But in certain insects the cones are purely cellular and non refringed e.g. acone eyes of Tipulids, Forficula, Hemiptera, various Coleoptera).Or the cones may be represented by the mass of liquid secreted by the crystalline cells.
Pigment movement in compound eye:
The iris cells contain black pigment which absorbs light, and pale or coloured granules which reflects the light. On account this reflection by the coloured granules which the structure is sometimes termed the ‘iris tapetum’ unlike a true tapetum, however, its function is not to increase the illumination of retina, but to prevent the entry of oblique rays. In butterflies, it shows a central dark spot surrounded by six smaller spots jointed to the first by radial dark lines, and sometimes twelve still smaller peripheral spots in eye. This appearance is termed the ‘pseudopupil’. The central black spot results from the light being absorbed where it falls on the rhabdom or on the black pigment in the retina, while it is reflected from the iris tapetum. In many butterflies, in the middle of the central black spot there is a small luminous spot, due to the light which falls on the rhabdom not being absorbed but reflected from the tracheal tapetum behind. In certain woodland species of butterflies this central spot disappears when the eye is brightly illuminated. This change is due to the movement of pigment in the cells around the basement membrane. It takes place very rapidly and may be complete within 6 seconds. Perhaps it serves to light illumination of the eye approximately constant with rapidly changing light intensities. Pigment movement may also occur also in the apposition eyes of Hemiptera (Notonecta and Corixa).
Migration of pigments are much more striking and important in the superposition eyes if nocturnal forms. Most of be active also in the day-time, and their eyes poises an arrangement comparable with the mammalian iris by which they can be adapted to different light intensities.