Intracellular optical physiology of the bee's eye

Summary

As shown in our earlier paper, the non-invasive technique of intracellular optical physiology permits one to measure pupillary responses from only a single spectral type of photoreceptor. The purpose of this paper is to extend the technique to measurements of the polarizational sensitivity of each type of photoreceptor. We characterize the polarizational sensitivity function, which we measured using intracellular optical physiology, byP _max andΦ _max just as the polarizational sensitivity function of a single cell, measured using intracellular electrophysiology, is conventionally characterized by PS andΦ _max. We demonstrate that the polarizational sensitivity function of the UV-pupil (see Table 1) is of the same form as expected for the polarizational sensitivity function of a single photoreceptor.

Based on our measurements of the spectral characteristics of the dark-adapted pupils of UV-, blue-, and green-receptors when they are stimulated with linearly polarized light, and based on the known spectral sensitivity functions of these receptors, we arrive at the following conclusions: Only the UV-receptors drive threshold pupillary responses to polarized light at 350 nm, and only the green-receptors drive responses at 530 nm. However, at 430 nm threshold responses are dominated by blue-receptors only when the stimulus is polarized perpendicular to the z-axis of the corneal array.

The UV-pupil exhibits high sensitivity to polarized light (P _max=8).P _max for the blue-pupil must exceed 4. However, the green-receptors exhibit no polarizational sensitivity at all (P _max<1.3). The direction for maximal sensitivity for both UV-receptors and blue-receptors is when the polarizational angle is perpendicular to the z-axis.

Based on these results, the most likely hypothesis is that receptors number 1 and 5 are UV-receptors, number 2 and 6 are blue-receptors, and the remaining four are all green-receptors. This statement applies to both dorsal and ventral parts of the eye (Fig. 3).

This demonstration of polarizational sensitivity of pupillary responses raises the question of whether or not the rhabdoms of the bee's retina are really twisted. Thus, both hypotheses are evaluated in light of known anatomical and electrophysiological results as well as our new data on the optical physiology of the bee's retina.