Localization of spectral receptors in the ommatidium of butterfly compound eye determined by polarization sensitivity

A butterfly Papilio has 5 types of spectral receptors in the compound eye. The spectral sensitivity of each type peaks in the UV, violet, blue, green, and red wavelengths, respectively. The green type contains two subtypes with and without a UV secondary peak. Here we studied the localization of these spectral receptors within the ommatidium. An ommatidium contains 9 photoreceptors (R1-9), each of which is one of the 5 spectral receptor types. The photoreceptors bear parallel microvilli to form a non-twisted rhabdom, and thereby the photoreceptors are polarization sensitive. We first examined the microvillar orientation by electron microscopy. The microvilli of R1, 2, and 9 are oriented dorso-ventrally (0 degrees), whereas those of R3 and 4 are parallel to the antero-posterior axis (90 degrees). The R5-8 bear microvilli diagonally: 45 degrees for R6 and R8, 135 degrees for R5 and R7. We then recorded spectral and polarization sensitivities from single photoreceptors. The peak angle of the polarization sensitivity of the UV, violet, and blue receptors were around 0 degrees, whereas that of the green receptors was around 90 degrees. In the double-peaked green receptors, the peak angle of the polarization sensitivity at UV was also around 90 degrees. The red receptors showed a peak angle of the polarization sensitivity at around 35 degrees. The polarization sensitivity ratio (PSmax/PSmin) of the double-peaked green receptors measured at UV was around 4, whereas the ratio of other receptors was around 2. We conclude that R1 and R2 are either UV, violet, or blue receptors whereas R3 and R4 are green receptors. Some R6 and R8 are red receptors. We also conclude that the UV secondary peak in the double-peaked green receptor is not simply attributable to the coupling with UV receptors.