Polarized light is tough for us to comprehend. It’s all around us, but we cannot see it without the aid of technology (Actually, that is not quite true. There is an extremely weak visual phenomenon known as Haidinger’s brush which is the extent of polarization perception in humans. I actually had not been able to see the effect until writing this article).
Regardless, our meager polarization perception is the exception rather than the rule in the animal kingdom. Whereas Haidinger’s brush is a useless side-effect of the optics in our eyes, other animals use polarized light vision for navigation, communication, and contrast-enhancement. It can be thought of as an additional channel of visual perception, akin to color vision. We are functionally polarization-blind. Arthropods on the other hand, are masters of perceiving the polarized world.
What follows is an explanation of polarized light, and the mechanism arthropods have evolved in order to perceive it.
Light can be defined both as quanta (photons) and waves. The wavelengths of visible light perceptible to humans are a small part of the electromagnetic spectrum, ranging from about 400 to 700 nano-meters (nm) . As a single a photon travels through space as a wave (yes, I realize how counter intuitive this is), it’s electrical field typically oscillates in a “two-dimensional” manner. Therefore, single photons are all polarized on their own. Here is a representation of three photons polarized at three different angles.
Natural light, produced by stars, rarely reaches the observer as a single photon. Instead it is a sum of many photons, all polarized at different angles (left frame below). Natural light becomes polarized by refracting or reflecting off certain molecules or structures. This polarized light is typically semi-polarized (right frame below), where the majority of the photons are polarized at approximately the same angle. This is detectable by polarization vision systems.
Polarized light is produced a variety of ways in nature. The atmosphere and water both refract un-polarized sunlight, producing a polarized light pattern. This creates a striking pattern in the sky that many animals use as a navigational compass. Also, arthropods are known to have structures on their bodies that reflect polarized light. These polarized light signals can serve as a secretive mode of inter-specific communication. If you can see it.
All animal visual systems use a visual pigment, a specialized transmembrane receptor protein conjugated to a vitamin-A derived light sensitive molecule. This light sensitive molecule, called a chromophore, absorbs light preferentially based on the light’s polarization orientation. If all the visual pigments in a photoreceptor are aligned in the same orientation, that cell will be preferentially sensitive to a particular orientation of polarized light. This is what we see in many arthropod photoreceptors.
Below each facet of an arthropod compound eye there is a barrel shaped array of photoreceptors (see below). Light enters the corneal facet and travels down the center of the photoreceptor array, called the rhabdomere. The rhabdomere is filled with microvilli, bristle-like membrane projections from the photoreceptor cells. Imagine a tooth brush head, with the bristles being the microvilli. Microvilli are a membrane adaptation to increase membrane surface area, and thus increase the amount of visual pigments in the cell. All the microvilli from a particular photoreceptor cell are parallel. Furthermore, all of the visual pigments within each microvilli are oriented down the axis of the microvilli. The net effect of these structures is a photoreceptor that is preferentially sensitive to polarized light oscillating at the angle parallel to the axis of the microvilli.
By comparing the signals from multiple polarization sensitive photoreceptors, an arthropod can discriminate different polarization orientations of light. In crustaceans for example, there are seven photoreceptors with microvilli projected into the light path in each rhabdom. A subset of these receptors project their microvilli in parallel, while the other subset of receptors project their microvilli perpendicular to the first subset. This gives each rhabdomere two channels of perpendicular polarization comparison. By this mechanism arthropods are able to sense polarized light.
Stay tuned for posts about the ecological significance of polarization sensitivity in arthropods.