Each ommatidium in the mosquito compound eye consists of a dioptric apparatus and a layer of retinula cells. The dioptric apparatus includes a cuticular cornea, a biconvex lens, and four cone cells. The cone cells have processes which extend into the receptor layer and terminate below the rhabdom as pigment-filled sacs. Two primary pigment cells and a n undetermined number of secondary pigment cells surround the dioptric apparatus. In the receptor layer eight retinula cells are found in each ommatidium. Six are peripherally arranged around a central one, whereas the remaining retinula cell is sandwiched between and somewhat external to two of the peripheral retinula cells. Each of the eight retinula cells possesses a well developed rhabdomere, along with the usual array of other intracellular organelles characteristic of arthropod photoreceptor cells.

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Mosquito rhodopsin is a digitonin-soluble membrane protein of molecular weight 39,000 daltons, as determined by sodium dodecyl sulfate gel electrophoresis. The rhodopsin undergoes a spectral transition from R515-520 to M480 after orange illumination. The visual pigment apoprotein, opsin, is the major membrane protein in the eye. Protein synthesis in the photoreceptor cells occurs in the perinuclear cytoplasm and the newly made protein is transported to the rhabdom. Light adaptation increases the rate of turnover of this rhabdomal protein. The turnover of electrophoretically isolated opsin is also stimulated by light adaptation. The changes observed in protein metabolism, biochemically, are consistent with previous morphological observations of photoreceptor membrane turnover. The results agree with the hypothesis that the newly synthesized rhabdomal protein is opsin.

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The volume of the rhabdom in compound eyes of mosquitoes decreases upon illumination. This decrease is probably mediated by a bleaching of the visual pigment, since blue light is most effective in producing the change and red light is least effective. The reduction in rhabdom volume appears to be a result of rhabdomal membrane loss to coated vesicles and multivesicular bodies. These organelles were seen most frequently in blue adapted eyes, markedly less frequently in red adapted eyes, and only rarely in dark adapted eyes.