Flies navigate using a visual system fundamentally different from the human eye, perceiving a distinct range of the electromagnetic spectrum. While humans see a spectrum from violet to red, a fly’s visual world is shifted toward shorter wavelengths. This difference in perception explains why flies are drawn to certain lights and colors that humans find unremarkable. Understanding this insect-specific vision helps determine which colors are essentially invisible to them.
The Basics of Fly Vision
A fly’s vision is based on a pair of large, compound eyes, each made up of hundreds or even thousands of individual units called ommatidia. Each ommatidium acts as a single, tiny photoreceptor, gathering light from a small part of the field of view, which the brain combines into a mosaic-like image. This structure is specialized for quickly detecting motion, which makes flies difficult to swat.
The photoreceptor cells within the ommatidia are tuned to specific light wavelengths, defining the colors a fly can see. Most flies possess photoreceptors sensitive to the ultraviolet (UV), blue, and green parts of the spectrum. The ability to see UV light, which is invisible to humans, allows flies to detect patterns on flowers and surfaces used for navigation or finding food.
The Color Flies Cannot Detect
The color flies have a significant “blind spot” for is red, along with deep orange hues. This is because their photoreceptors are not equipped to respond effectively to the long-wavelength end of the visible spectrum. The visual pigments, or opsins, in a fly’s eye lack the necessary sensitivity to absorb and react to light waves longer than about 600 nanometers.
Objects that appear bright red to the human eye, such as a red apple or a red light, often appear dark or simply black to a fly. This lack of photoreceptor sensitivity means the color fails to register as a distinct hue, making red an ineffective visual stimulus.
Utilizing Color Knowledge for Pest Control
This knowledge of the fly’s color deficiency and preference is directly applied in pest control strategies. Commercial fly traps, particularly non-chemical light traps, exploit the fly’s attraction to short-wavelength light. These traps typically use UV-A light or a combination of blue and yellow colors, which are highly visible and attractive to the insects.
The yellow color, often used on sticky traps, is particularly effective because its bright appearance in the green-to-yellow range strongly stimulates the fly’s color-sensitive photoreceptors.
Conversely, the fly’s inability to perceive red light is used to minimize disturbance in certain environments. Using red light in areas like darkrooms or laboratories can keep the environment visually dark for the insects, reducing their activity. In agriculture or food processing, red surfaces or red-tinted lighting may be used to make certain areas less visually appealing to flies, reducing their presence.