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The diurnal changes of rhabdom structures under a regular light cycle (L:D=12:12) have been examined in the eyes of the grapsid carb, . Night rhabdoms are larger in diameter than noontime rhabdoms by more than a factor of 2. The rhabdomeric membranes are internalized around dawn as coated vesicles. The coated vesicles may be transferred to the parent cytoplasm following transformation into sickle-shaped vesicles or double membrane-bounded vesicles. Conversely, the increase of rhabdom size from noon to midnight may be largely due to the assembly of vacuoles into rhabdomeric membranes. In addition to the diurnal change in rhabdom size, adjacent rhabdomerlc microvilli appear to easily fuse to each other to form thick microvilli, which re-divide again into normal ones. Moreover, extracellular shedding of microvillar membranes occurs during the dark period. Observed changes of rhabdomeric microvilli are discussed in relation to turnover of the photosensitive membranes. [ABSTRACT FROM PUBLISHER]