Almost all the arthropods live in environments that experience great fluctuations in the intensity of light.  These variations are the product of (1) the unpredictable variations of the environment (clouds, shade) and (2) the daily exposure to the light-dark cycle.  The visual system, however, adapts to the luminance condition in the environment.  In the arthropods, there are basically two classes of adaptation mechanisms:

1. Regulation of the light flux that reaches the rhabdom: movements of screening pigments located in pigmenting and/or retinular cells, morphological changes in the dioptrical apparatus, changes in the structure, position and/or size of the rhabdom.

2. Changes in the sensitivity of the photoreceptor cells: transformations of the visual pigments (photochemical adaptation) and changes in the sensitivity of the membranes (neural adaptation).

          The visual sensitivity may also be adjusted through changes in the synapse between retinular cells and interneurons in the lamina.  This mechanism, while it's not completely understood, probably involves a regulation in the quantity of neurotransmitter that are released in the synapse terminals.

          The screening pigments are found in the eyes of many arthropods.  In insects, these pigments are located in the primary pigmenting cells, in a variable number of secondary pigmenting cells, and in some cases retinular cells as well.  The most important functions of these pigments are to: optically isolate the ommatidia, control the light flux in the photoreceptors, control the ommatidia opening and the receiving angle of the photoreceptors, and protect the photosensitive pigments from an excessive photoconversion to their inactive state (metarhodopsin).

          The changes in the distribution of the screening pigments and in the form and/or size of the pigmenting cells, which are in response to the light conditions, are one of the most common events in the compound eyes of the arthropods.  The changes in the position of these pigments give place, in many cases, to the phenomena of the eye glow and various classes of "pseudopupils."  These phenomena can be observed, among others, in the intact eye of moths, butterflies, flies and crustaceans; and have been utilized as indicating tools for the adaptation state of the eye.  In some cases, these changes are under the control of an endogenous oscillator.

Objective

          To analyze the morphological differences that are present in the compound eyes and the ocelli of insects.  To recognize the different types of eyes, according to the disposition of the rhabdom and distribution of pigments.

          To study the changes produced in the morphology of the eyes, according to their adaptation to light or darkness.

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