The chemical basis of vision depends upon the absorption of light (electromagnetic radiation) by special pigments in the eye. These pigments are transducers: they convert electromagnetic energy into the chemical energy that can initate (stimulate) an impulse within a nerve cell (neuron). In human eyes, the visual pigments are manufactured and stored in the rods and cones of the retina. In insects, all visual pigments are manufactured by retinula cells; they are stored in the rhabdoms of the compound eyes and ocelli.
Not all visual pigments are alike. Rhodopsin, the pigment found in human rods, absorbs a wide range of electromagnetic frequencies (colors) from blue to yellow. There are three types of cone pigments (iodopsins) that are somewhat more selective -- each has an absorbance maximum that correspond to one of the three "primary" colors: blue, green, or red. Our ability to discriminate a full range of color is based on the additive response of all three receptor types. (Figure #1).
Colorblindness occurs when one or more of the cone pigments is defective. A colorblind person still has a range of color perception, but lacks the ability to discriminate a complete spectrum of colors.
- Purple is a color sensation produced by simultaneous stimulation of blue and red color receptors. If a colorblind person lacks blue-absorbing pigments, then the color purple would appear to be the same as the color:
- If a man lacked red-absorbing pigments (see Figure 1), he would be unable to discriminate between:
- Red and Blue
- Green and Purple
- Red and Green
- Blue and Purple
Most insects have only two types of visual pigments (Figure 2). One pigment absorbs green and yellow light (550 nm); the other absorbs blue and ultraviolet light (<480 nm). Insects cannot see red. (Does this mean they don't get angry?) These insects have only limited color vision -- much like that of colorblind humans but with their frequency response shifted into the ultraviolet.
Bichromatic insects (those with only two types of color pigment receptors) are often unable to discriminate pure colors from mixtures of colors. For example, light at 500 nm (blue-green) would be absorbed equally by both the yellow and UV pigments and stimulate an equivalent response in both receptor types. Yet both receptors could also be equally stimulated by mixtures of light (say 450 nm and 550 nm) -- the insect could not discriminate between this mixture and a single color at 500 nm.
- Which wavelengths of light would "mimic" the color green (530 nm) to an insect with bichromatic color vision?
- 525 nm + 625 nm
- 370 nm + 600 nm
- 620 nm + 420 nm
- 400 nm + 500 nm
Honeybees, bumblebees, and many diurnal butterflies have true color vision. They have three visual pigments (Figure 3) with absorption maxima at 360 nm (ultraviolet), 440 nm (blue-violet), and 588 nm (yellow). These trichromatic insects can perceive a complete spectrum of colors (within the range of their spectral sensitivity) and can also discriminate between single colors and mixtures of colors.
A combination of UV and yellow (opposite ends of the insect's visual spectrum) would look "blue-green" to a bichromatic insect because both receptor types are stimulated. But that same color combination would be distinctive to a trichromatic insect because the "blue-violet" receptor is NOT stimulated. Behavioral studies confirm that bees perceive the UV-yellow combination as a unique "color". So what do you call a "color" that is a UV-yellow combination? It is the equivalent of "purple" in the human color scheme, so we call it "bee-purple" in the bee's color scheme. Similarly, wavelengths that stimulate the bee's UV and blue-violet receptors (but not the yellow receptors) would produce a unique stimulus known as "bee violet" in the bee's color scheme.