In each compound eye, each ommatidia consists basically of a system of lenses that collect and focus the luminance rays; and a sensorial portion that transforms the luminance energy into an electric signal.  The optical portion of the ommatidia has two elements, the cornea and the crystalline cone.  In a superficial view, these lenses are seen to be packed to form a typical hexagonal pattern.  the cornea is a biconcave transparent and colorless cuticular lens, which is produced by the corneagenous cells.  Later on, these cells are displaced to the sides of the ommatidia to form the primary pigmenting cells.  Under the cornea are for Semper cells.  In the eyes with eucone shape, which are present in the majority of the insects, these cells produce a true intercellular crystalline cone.  In the eyes with acone shape, the Semper cells do not produce a crystalline cone, but the clear cytoplasm of these cells take on the role of the lens.  The crystalline cone, or the place occupied by the Semper cells, is laterally surrounded by the primary pigmenting cells.  Under the crystalline cone of each ommatidia, is found the retinula, which is usually made up of eight retinular cells.

          The rhabdom is the photosensitive portion of each ommatidia.  The rhabdom is formed by the microscopic hairs which are packed densely and correspond to each retinular cell.  These hairs are located on the internal face of each retinular cell, in the 90 degree angle with relation to the longitudinal axis of the cell.  In the majority of insects, the rhabdomeres of the neighboring cells are placed in a tight position and interlaced to form a "fused rhabdom;" while the Diptera, Dermaptera, Hemiptera and other Coleoptera are separate, forming an "open rhabdom."  In the majority of compound eyes, the retinular cells of each ommatidia are surrounded by 12-18 secondary pigmented cells.  These cells, just like the primary pigmented cells, contain screen pigments.

          The compound eyes can be grouped basically into two big categories: eyes of apposition and eyes of superposition.  In the eyes of apposition, each ommatidia is optically isolated from its neighbors, meaning that each rhabdom receives only the rays that enter through its own focus system, while the rays that enter through the neighboring facets are absorbed by the pigments of the secondary pigmenting cells.  In the eyes of superposition there is not such optical isolation.  The rays that enter through the corresponding lenses to its different ommatidia are focused in a unique rhabdom.  This is possible because  these eyes have a "clear zone," free of pigmentation, between the crystalline cone and the rhabdom; and because the rhabdom is restrained from the proximal portion of the retinular cells.

          As a general rule, we can say that the eyes of apposition are common in the arthropods that are active in very illuminated environments.  The eyes of superposition, typically, are found on the insects and crustaceans of nocturnal habits; or on those that live in poorly illuminated environments.  However, both cases have more than few exceptions.

          Both types of eyes present variations in structure as well as in the size of the components of the ommatidia.  This has given room for a great diversity of designs for compound eyes: of simple apposition, open rhabdom, afocal, with neural superposition, of refraction superposition, reflection and parabolic.

          The term adaptation, in a broad sense, denotes those events that through a change in the structure, form, function or behavior of organisms, result in a better adjustment to the environmental conditions.  These adjustments, besides happening in an evolutionary sense, happen in individual organisms.  The annual rhythms of migration, Diapause, etc are adaptations to the cycles of weather variations; while the daily rhythms are examples of the adaptation phenomena that happen in a few hours.  But the environmental conditions can change in much shorter periods and are unpredictable.  The sensory organs are capable of adapting very quickly to the changing conditions of the environment.

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