Humans and insects typically perceive color through 3 types of photoreceptors present in the retina of the eye. In humans, each photoreceptor type is maximally sesitive to a different wavelength of light in the red, green and blue portions of the spectrum. The brain integrates signals from the 3 receptors types and produces the sensation of color. Many insects use a similar process but with the wavelength of light sensed shifted to detect green, blue and ultraviolet. The grasshopper, Phlaeoba, can discriminate between between red and green and other colors, even though the eye has only one type of photoreceptor pigment. How does Phlaeoba discriminate color?
The compound eyes of Phlaeoba have some ommatidia that are clear and others that contain a screening pigment. The screening pigment filters some wavelengths of light more than others. Receptors without the pigment are maximally sensitive to light at 525 nm wavelength. Those receptors with screening pigment are maximally sensitive to light at 545 nm wavelength. Thus, a single photoreceptor pigment can function as if it were 2 distinct pigments with different sensitivities and enable the grasshopper to discriminate color.