Larvae of the mosquito Aedes aegypti have a cluster of four ocelli on each side of the head. The visual pigment of each ocellus of mosquitoes reared in darkness was characterized by microspectrophotometry, and found to be the same. Larval mosquito rhodopsin (λmax = 515 nm) upon short irradiation bleaches to a stable photoequilibrium with metarhodopsin (λmax = 480 nm). On long irradiation of glutaraldehyde-fixed tissues or in the presence of potassium borohydride, bleaching goes further, and potassium borohydride reduces the product, retinal, to retinol (vitamin A1). In the presence of hydroxylamine, the rhodopsin bleaches rapidly, with conversion of the chromophore to retinaldehyde oxime (λmax about 365 nm).
The spectral sensitivity of lateral ocelli in both wild-type and white-eyed larvae of the yellow fever mosquito Aedes aegypti L. (reared in darkness) was measured by means of the electroretinogram. The spectral sensitivity is maximal at about 520 nm, with a small secondary peak near 370 nm. When allowance is made for some screening and filtering by the eye tissues, the spectral sensitivity is in reasonable agreement with the absorption spectrum of ocellar rhodopsin (λmax = 515 nm).
Though it had been supposed earlier that the bullfrog undergoes a virtually complete metamorphosis of visual systems from vitamin A2 and porphyropsin in the tadpole to vitamin A1 and rhodopsin in the adult, the present observations show that the retina of the adult frog may contain as much as 30–40% porphyropsin, all of it segregated in the dorsal zone. The most dorsal quarter of the adult retina may contain 81–89% porphyropsin mixed with a minor amount of rhodopsin; the ventral half contains only rhodopsin. Further, the dorsal zone contains a two to three times higher concentration of visual pigments than the ventral retina. The pigment epithelium underlying the retina contains a corresponding distribution of vitamins A1 and A2, predominantly vitamin A2 in the dorsal pigment epithelium, exclusively vitamin A1 in the ventral zone. The retina accepts whatever vitamin A the pigment epithelium provides it with, and turns it into the corresponding visual pigment. Thus, a piece of light-adapted dorsal retina laid back on ventral pigment epithelium regenerates rhodopsin, whereas a piece of light-adapted ventral retina laid back on dorsal pigment epithelium regenerates predominantly porphyropsin. Vitamin A2 must be made from vitamin A1, by dehydrogenation at the 3,4-bond in the ring. This conversion must occur in the pigment epithelium, presumably through the action of a vitamin A-3,4-dehydrogenase. The essential change at metamorphosis is to make much less of this dehydrogenase, and to sequester it in the dorsal pigment epithelium. Some adult bullfrogs, perhaps characteristically taken in the summer, contain very little porphyropsin—only perhaps 5%—still sequestered in the dorsal retina. The gradient of light over the retinal surface has little if any effect on this distribution. The greater density of visual pigments in the dorsal retina, and perhaps also—although this is less clear—the presence of porphyropsin in this zone, has some ecological importance in increasing the retinal sensitivity to the dimmer and, on occasion, redder light received from below.