Fly photoreceptors. I. Physical separation of two visual pigments in Calliphora retinula cells 1-6

The two peaks of the spectral sensitivity curves of Calliphora correspond to two visual pigments. The peak sensitivity to polarized light for the u. v. sensitive pigment is at an angle to that for the green-sensitive pigment. The change in angle of the maximum polarization sensitivity as a function of wavelength occurs near 400 nm; in this transition the curves do not follow a cos 2 function. The angle between the two maxima is different for each retinula cell. The only explanation of this phenomenon is that there are two visual pigments in different parts of the receptor, and one part is twisted relative to the other. There are therefore two metarhodopsins and the adaptation mechanisms are partially separate for the two peaks of the spectral sensitivity. The inference of two separated pigments modifies the interpretation of much previous work on fly photoreceptors.

Fly photoreceptors. II. Spectral and polarized light sensitivity in the drone fly Eristalis

Eristalis tenax , the honeybee mimic, has photoreceptors mainly with double peaks as in typical flies, but the peaks are near 350 and 450 nm. Other cell types with peaks at 350 or 450 or 520 nm were encountered but not commonly. Measurements of the polarization sensitivity lead to the conclusion, as in Calliphora , that where there are two visual pigments they are separated in proximal and distal parts of the rhabdomere, with a twist between the two parts. Therefore there must also be two corresponding metarhodopsins. Receptors with a single spectral peak do not show this effect. Self-absorption can be excluded as an influence on spectral or polarization sensitivity. In its colour vision the drone fly is more like a typical fly than a bee but it has less green sensitive receptors and more blue sensitive ones than calliphora .